Proposed Rule2024-07397
Review of the Secondary National Ambient Air Quality Standards for Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter
Primary source
Metadata and text below are from the Federal Register, a public-domain U.S. government work. Always verify the official published version before relying on it for any legal matter.
Published
April 15, 2024
Issuing agencies
Environmental Protection Agency
Abstract
Based on the Environmental Protection Agency's (EPA's) review of the air quality criteria and national ambient air quality standards (NAAQS) for oxides of nitrogen (N oxides), oxides of sulfur (SO<INF>X</INF>), and particulate matter (PM), the Environmental Protection Agency (EPA) proposes to revise the existing secondary sulfur dioxide (SO<INF>2</INF>) standard to an annual average, averaged over three consecutive years, with a level within the range from 10 to 15 parts per billion (ppb). Additionally, the Agency proposes to retain the existing secondary standards for N oxides and PM, without revision. The EPA also proposes revisions to the data handling requirements for the proposed secondary SO<INF>2</INF> NAAQS.
Full Text
<html>
<head>
<title>Federal Register, Volume 89 Issue 73 (Monday, April 15, 2024)</title>
</head>
<body><pre>
[Federal Register Volume 89, Number 73 (Monday, April 15, 2024)]
[Proposed Rules]
[Pages 26620-26701]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2024-07397]
[[Page 26619]]
Vol. 89
Monday,
No. 73
April 15, 2024
Part IV
Environmental Protection Agency
-----------------------------------------------------------------------
40 CFR Part 50
Review of the Secondary National Ambient Air Quality Standards for
Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter; Proposed
Rule
Federal Register / Vol. 89 , No. 73 / Monday, April 15, 2024 /
Proposed Rules
[[Page 26620]]
-----------------------------------------------------------------------
ENVIRONMENTAL PROTECTION AGENCY
40 CFR Part 50
[EPA-HQ-OAR-2014-0128; FRL-5788-02-OAR]
RIN 2060-AS35
Review of the Secondary National Ambient Air Quality Standards
for Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter
AGENCY: Environmental Protection Agency (EPA).
ACTION: Proposed rule.
-----------------------------------------------------------------------
SUMMARY: Based on the Environmental Protection Agency's (EPA's) review
of the air quality criteria and national ambient air quality standards
(NAAQS) for oxides of nitrogen (N oxides), oxides of sulfur
(SO<INF>X</INF>), and particulate matter (PM), the Environmental
Protection Agency (EPA) proposes to revise the existing secondary
sulfur dioxide (SO<INF>2</INF>) standard to an annual average, averaged
over three consecutive years, with a level within the range from 10 to
15 parts per billion (ppb). Additionally, the Agency proposes to retain
the existing secondary standards for N oxides and PM, without revision.
The EPA also proposes revisions to the data handling requirements for
the proposed secondary SO<INF>2</INF> NAAQS.
DATES: Comments must be received on or before June 14, 2024.
Public Hearings: The EPA will hold a virtual public hearing on this
proposed rule. This hearing will be announced in a separate Federal
Register notice that provides details, including specific dates, times,
and contact information for these hearings.
ADDRESSES: You may submit comments, identified by Docket ID No. EPA-HQ-
OAR-2014-0128, by any of the following means:
<bullet> Federal eRulemaking Portal: <a href="https://www.regulations.gov/">https://www.regulations.gov/</a>
(our preferred method). Follow the online instructions for submitting
comments.
<bullet> Email: <a href="/cdn-cgi/l/email-protection#7d1c501c1319500f5039121e1618093d180d1c531a120b"><span class="__cf_email__" data-cfemail="fd9cd09c9399d08fd0b9929e969889bd988d9cd39a928b">[email protected]</span></a>. Include the Docket ID No.
EPA-HQ-OAR-2014-0128 in the subject line of the message.
<bullet> Mail: U.S. Environmental Protection Agency, EPA Docket
Center, Air and Radiation Docket, Mail Code 28221T, 1200 Pennsylvania
Avenue NW, Washington, DC 20460.
<bullet> Hand Delivery or Courier (by scheduled appointment only):
EPA Docket Center, WJC West Building, Room 3334, 1301 Constitution
Avenue NW, Washington, DC 20004. The Docket Center's hours of
operations are 8:30 a.m.-4:30 p.m., Monday-Friday (except Federal
Holidays).
Instructions: All submissions received must include the Docket ID
No. for this document. Comments received may be posted without change
to <a href="https://www.regulations.gov">https://www.regulations.gov</a>, including any personal information
provided. For detailed instructions on sending comments and additional
information on the rulemaking process, see the SUPPLEMENTARY
INFORMATION section of this document.
FOR FURTHER INFORMATION CONTACT: Ms. Ginger Tennant, Health and
Environmental Impacts Division, Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Mail Code C539-04,
Research Triangle Park, NC 27711; telephone: (919) 541-4072; email:
<a href="/cdn-cgi/l/email-protection#f581909b9b949b81db929c9b929087b5908594db929a83"><span class="__cf_email__" data-cfemail="7004151e1e111e045e17191e171502301500115e171f06">[email protected]</span></a>.
SUPPLEMENTARY INFORMATION:
General Information
Preparing Comments for the EPA
Follow the online instructions for submitting comments. Once
submitted to the Federal eRulemaking Portal, comments cannot be edited
or withdrawn. The EPA may publish any comment received to its public
docket. Do not submit electronically any information you consider to be
Confidential Business Information (CBI) or other information whose
disclosure is restricted by statute. Multimedia submissions (audio,
video, etc.) must be accompanied by a written submission. The written
comment is considered the official comment and should include
discussion of all points you wish to make. The EPA will generally not
consider comments or comment contents located outside of the primary
submission (i.e., on the web, the cloud, or other file sharing system).
For additional submission methods, the full EPA public comment policy,
information about CBI or multimedia submissions, and general guidance
on making effective comments, please visit <a href="https://www.epa.gov/dockets/commenting-epa-dockets">https://www.epa.gov/dockets/commenting-epa-dockets</a>.
When submitting comments, remember to:
<bullet> Identify the action by docket number and other identifying
information (subject heading, Federal Register date and page number).
<bullet> Explain why you agree or disagree, suggest alternatives,
and substitute language for your requested changes.
<bullet> Describe any assumptions and provide any technical
information and/or data that you used.
<bullet> Provide specific examples to illustrate your concerns and
suggest alternatives.
<bullet> Explain your views as clearly as possible, avoiding the
use of profanity or personal threats.
<bullet> Make sure to submit your comments by the comment period
deadline identified.
Availability of Information Related to This Action
All documents in the dockets pertaining to this action are listed
on the <a href="http://www.regulations.gov">www.regulations.gov</a> website. This includes documents in the
docket for the proposed decision (Docket ID No. EPA-HQ-OAR-2014-0128)
and a separate docket, established for the Integrated Science
Assessment (ISA) (Docket ID No. EPA-HQ-ORD-2013-0620) that has been
incorporated by reference into the docket for this proposed decision.
Although listed in the index, some information is not publicly
available, e.g., CBI or other information whose disclosure is
restricted by statute. Certain other material, such as copyrighted
material, is not placed on the internet and may be viewed with prior
arrangement with the EPA Docket Center. Additionally, a number of the
documents that are relevant to this proposed decision are available
through the EPA's website at <a href="https://www.epa.gov/naaqs/">https://www.epa.gov/naaqs/</a>. These
documents include the Integrated Science Assessment for Oxides of
Nitrogen, Oxides of Sulfur and Particulate Matter Ecological Criteria
(U.S. EPA, 2020a), available at <a href="https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=349473">https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=349473</a>, and the Policy Assessment for the Review
of the Secondary National Ambient Air Quality Standards for Oxides of
Nitrogen, Oxides of Sulfur, and Particulate Matter, (U.S. EPA, 2024),
available at <a href="https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards">https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards</a>.
Table of Contents
The following topics are discussed in this preamble:
Executive Summary
I. Background
A. Legislative Requirements
B. Related Control Programs
C. History of the Secondary Standards for N Oxides,
SO<INF>X</INF> and PM
1. N Oxides
2. SO<INF>X</INF>
3. Related Actions Addressing Acid Deposition
4. Most Recent Review of the Secondary Standards for N Oxides
and SO<INF>X</INF>
5. PM
D. Current Review
II. Rationale for Proposed Decisions
A. Introduction
1. Basis for Existing Secondary Standards
[[Page 26621]]
2. Prior Review of Deposition-Related Effects
3. General Approach for This Review
B. Air Quality and Deposition
1. Sources, Emissions and Atmospheric Processes Affecting
SO<INF>X</INF>, N Oxides and PM
2. Recent Trends in Emissions, Concentrations and Deposition
3. Relationships Between Concentrations and Deposition
C. Welfare Effects Evidence
1. Nature of Effects
a. Direct Effects of SO<INF>X</INF> and N Oxides
b. Acid Deposition-Related Ecological Effects
c. Nitrogen Enrichment and Associated Ecological Effects
d. Other Deposition-Related Effects
2. Public Welfare Implications
3. Exposure Conditions and Deposition-Related Metrics
a. Acidification and Nitrogen Enrichment in Aquatic Ecosystems
b. Deposition-Related Effects in Terrestrial Ecosystems
c. Direct Effects of N Oxides, SO<INF>X</INF> and PM in Ambient
Air
D. Quantitative Exposure and Risk Assessment for Aquatic
Acidification
1. Key Design Aspects
2. Key Limitations and Uncertainties
3. Summary of Results
E. Proposed Conclusions
1. Evidence and Exposure/Risk-Based Considerations in the Policy
Assessment
a. Direct Effects on Biota
b. Evidence of Ecosystem Effects of S and N Deposition
c. Sulfur Deposition and SO<INF>X</INF>
d. Nitrogen Deposition and N Oxides and PM
2. CASAC Advice and Public Comments
3. Administrator's Proposed Conclusions
F. Proposed Decision on the Secondary Standards
III. Interpretation of the Secondary SO<INF>2</INF> Standard
A. Background
B. Interpretation of the Secondary SO<INF>2</INF> Standard
IV. Ambient Air Monitoring Network for SO<INF>2</INF>
V. Clean Air Act Implementation Requirements for Proposed Secondary
SO<INF>2</INF> Standard
A. Designation of Areas
B. Section 110(a)(1) and (2) Infrastructure SIP Requirements
C. Prevention of Significant Deterioration and Nonattainment New
Source Review Programs for the Proposed Secondary SO<INF>2</INF>
Standard
D. Alternative PSD Compliance Demonstration Approach for the
Proposed Secondary SO<INF>2</INF> Standard
E. Transportation Conformity Program
F. General Conformity Program
VI. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
B. Paperwork Reduction Act (PRA)
C. Regulatory Flexibility Act (RFA)
D. Unfunded Mandates Reform Act (UMRA)
E. Executive Order 13132: Federalism
F. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
G. Executive Order 13045: Protection of Children From
Environmental Health Risks and Safety Risks
H. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution or Use
I. National Technology Transfer and Advancement Act (NTTAA)
J. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations and Executive Order 14096: Revitalizing Our Nation's
Commitment to Environmental Justice for All
References
Executive Summary
This document presents the Administrator's proposed decisions in
the current review of the secondary NAAQS for SO<INF>X</INF>, N oxides,
and PM. The existing secondary standards are: for SO<INF>2</INF>, 0.5
ppm as a 3-hour average not to be exceeded more than once in a year;
for NO<INF>2</INF>, 53 ppb as an annual average; for PM<INF>2.5</INF>,
15.0 [micro]g/m\3\ as the 3-year average of annual averages, and 35
[micro]g/m\3\ as the 3-year average of annual 98th percentile 24-hour
averages; and, for PM<INF>10</INF>, 150 [micro]g/m\3\ as a 24-hour
average, not to be exceeded more than once per year on average over
three years. Sections 108 and 109 of the Clean Air Act (CAA, the Act)
require the EPA to periodically review the air quality criteria--the
science upon which the standards are based--and the standards
themselves. Under section 109(b)(2) of the Act, a secondary standard
must ``specify a level of air quality the attainment and maintenance of
which in the judgment of the Administrator, based on such criteria, is
requisite to protect the public welfare from any known or anticipated
adverse effects associated with the presence of [the] pollutant in the
ambient air.'' In conducting this review of the secondary
SO<INF>X</INF>, N oxides, and PM NAAQS, the EPA has carefully evaluated
the currently available scientific literature on the ecological effects
of SO<INF>X</INF>, N oxides, and PM,\1\ focusing particularly on the
new literature available since the conclusion of the previous reviews
in 2012 and 2013, respectively, as described in the Integrated Science
Assessment (ISA). The ecological effects addressed in this review
include direct effects of N oxides and SO<INF>X</INF>, and PM loading,
on vegetation surfaces, as well as ecological effects related to
atmospheric deposition of S and N compounds in sensitive ecosystems.
---------------------------------------------------------------------------
\1\ The ecological effects of PM that are the focus of this
action were not considered in EPA's recently completed
reconsideration of the primary and secondary NAAQS for PM. In the
review of the PM secondary standards completed in 2020, and
reconsidered more recently, the EPA considered effects on visibility
and climate and materials damage, but did not consider the
ecological effects that are addressed here (89 FR 16202, March 6,
2024).
---------------------------------------------------------------------------
Sulfur oxides and N oxides, their transformation products (which
include particulate compounds), and N- and S-containing components of
PM in ambient air can contribute to atmospheric deposition of S and N
compounds. Emissions of SO<INF>X</INF>, N oxides, PM and PM precursors
have declined dramatically over the past two decades, continuing a
longer-term trend. In response to the reductions in S- and N-containing
compounds, levels of S and N deposition have also been reduced,
although the declining trend in N deposition in the last decade has
slowed and, in some areas, reversed, due to increasing ammonia
emissions.
The Administrator's proposed decision in this review is to revise
the existing secondary SO<INF>2</INF> standard and to retain the
existing secondary standards for N oxides and PM. In this document, the
EPA summarizes the background and rationale for the Administrator's
proposed decisions in this review. The EPA solicits comment on the
proposed decisions described here and on a number of alternate options,
and requests commenters also provide the rationales supporting the
views articulated in submitted comments.
The Administrator's proposed decisions are based on his
consideration of the characterization of the available scientific
evidence in the ISA, quantitative and policy analyses presented in the
Policy Assessment (PA), and advice from the Clean Air Scientific
Advisory Committee (CASAC).\2\ In conveying its advice in this review,
the CASAC provided consensus advice that the existing SO<INF>2</INF>
and NO<INF>2</INF> secondary standards were adequate to protect against
direct effects of S and N oxides on plants and lichens. With regard to
deposition-related effects and SO<INF>2</INF>, the majority of CASAC
recommended an annual secondary standard of 10-15 ppb, and the minority
recommended a secondary standard identical to the existing primary
standard. In consideration of deposition-related effects and the
NO<INF>2</INF> and PM<INF>2.5</INF> secondary standards, the
[[Page 26622]]
CASAC majority recommended revision of the levels of the existing
annual NO<INF>2</INF> and PM<INF>2.5</INF> secondary standards, and the
minority recommended adopting secondary standards identical to the
existing annual NO<INF>2</INF> and PM<INF>2.5</INF> primary standards.
---------------------------------------------------------------------------
\2\ Over the course of this review, the EPA developed planning
documents, an ISA and a PA, drafts of which were made available for
public comment and reviewed by the CASAC Oxides of Nitrogen, Oxides
of Sulfide and Particulate Matter Secondary NAAQS Panel (<a href="https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards">https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards</a>).
---------------------------------------------------------------------------
Based on his consideration of the ecological effects evidence in
the ISA, the evaluations and quantitative information in the PA,
including the quantitative REA for aquatic acidification, and advice
from the CASAC, the Administrator is proposing that the current short-
term secondary SO<INF>2</INF> standard is not requisite to protect the
public welfare from known or anticipated adverse effects associated
with the presence of SO<INF>X</INF> in ambient air, including
particularly deposition-related effects, and that it should be revised
to also provide such protection against effects related to deposition
of sulfur (S) compounds to ecosystems. Specifically, the EPA is
proposing to revise the existing standard to be an annual average
standard, averaged over three years, with a level within the range from
10 to 15 parts per billion (ppb) based on the Administrator's proposed
judgment that a standard in this range would provide protection for
both direct effects on vegetation surfaces and ecosystem deposition-
related effects. The EPA solicits comments on this proposal, including
the averaging time, form and range of levels for the revised standard.
The EPA also solicits comments on a number of alternative options for a
new secondary SO<INF>2</INF> standard. The EPA solicits comment on
setting the level for a new annual average standard (averaged over
three years) in the range from 5 to 10 ppb, and on revising the
existing secondary standard to be identical to the existing primary
standard in all respects. Further, the EPA solicits comments on
retaining the existing 3-hour standard, in addition to establishing a
new annual secondary standard.
The Administrator is also proposing to retain the secondary
nitrogen dioxide (NO<INF>2</INF>) and PM standards, without revision.
With regard to the secondary NO<INF>2</INF> standard, the Administrator
finds that the evidence related to the N oxides, NO<INF>2</INF> and
nitrogen oxide (NO), does not clearly call into question the adequacy
of protection provided by the existing standard for direct effects or
for deposition-related effects (in light of the currently diminished
role of N oxides in total N deposition, particularly in areas with
highest deposition), such that revision is not warranted. The EPA
solicits comment on the proposed decision to retain the existing
secondary NO<INF>2</INF> standard, without revision, and also on the
alternative approach of revising the form of the existing standard to a
3-year average and the level to a value within the range from 35 to 40
ppb.
With regard to PM, the Administrator proposes to conclude that the
current evidence does not call into question the adequacy of the
existing PM standards with regard to direct effects and deposition of
pollutants other than S and N compounds. Further, he judges that
protection of sensitive ecosystems from S deposition is more
effectively achieved through a revised SO<INF>2</INF> standard than a
PM standard, and that a revised PM standard is not warranted to provide
public welfare protection against adverse effects related to S or N
deposition. The Administrator additionally proposes to conclude that
PM<INF>2.5</INF> is not an appropriate indicator for a secondary
standard intended to provide protection of the public welfare from
adverse effects related to S or N deposition. Thus, based on
consideration of the PA analyses and conclusions, and consideration of
CASAC advice, the Administrator proposes to conclude that no change to
the annual PM<INF>2.5</INF> secondary standard is warranted and he
proposes to retain the existing PM<INF>2.5</INF> secondary standard,
without revision. The EPA solicits comment on the proposed decision.
Additionally, in recognizing that there may be alternate views with
regard to whether and to what extent a standard with a PM<INF>2.5</INF>
indicator might be expected to provide control of N deposition, and in
light of the rationale provided by the CASAC minority, the EPA also
solicits comment on the alternative approach of revising the secondary
PM<INF>2.5</INF> (with PM<INF>2.5</INF> referring to particles with a
nominal mean aerodynamic diameter less than or equal to 2.5
micrometers) annual standard to a level of 12 micrograms per meter
cubed ([micro]g/m\3\). With regard to other PM secondary standards,
based on evaluations and conclusions of the PA, including consideration
of recommendations from the CASAC, the Administrator proposes to retain
the existing 24-hour secondary PM<INF>2.5</INF> standard, without
revision. Further, based on the lack of evidence calling into question
the adequacy of the secondary PM<INF>10</INF> standards for protection
of ecological effects, he also proposes to retain the secondary
PM<INF>10</INF> standards without revision.
This document additionally includes proposed revisions related to
implementation of the proposed secondary SO<INF>2</INF> annual
standard. Specifically, the EPA is proposing revisions to the data
handling requirements in appendix T of 40 CFR part 50 to include
specifications needed for the proposed new annual average standard.
This document also describes the SO<INF>2</INF> monitoring network and
its adequacy for surveillance for the proposed annual standard. Lastly,
the document discusses implementation processes pertinent to
implementation of the proposed new standard.
I. Background
A. Legislative Requirements
Two sections of the CAA govern the establishment and revision of
the NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to
identify and list certain air pollutants and then to issue air quality
criteria for those pollutants. The Administrator is to list those
pollutants ``emissions of which, in his judgment, cause or contribute
to air pollution which may reasonably be anticipated to endanger public
health or welfare''; ``the presence of which in the ambient air results
from numerous or diverse mobile or stationary sources''; and for which
he ``plans to issue air quality criteria. . . .'' (42 U.S.C.
7408(a)(1)). Air quality criteria are intended to ``accurately reflect
the latest scientific knowledge useful in indicating the kind and
extent of all identifiable effects on public health or welfare which
may be expected from the presence of [a] pollutant in the ambient air.
. . .'' 42 U.S.C. 7408(a)(2).
Section 109 of the Act (42 U.S.C. 7409) directs the Administrator
to propose and promulgate ``primary'' and ``secondary'' NAAQS for
pollutants for which air quality criteria are issued [42 U.S.C.
7409(a)]. Under section 109(b)(2), a secondary standard must ``specify
a level of air quality the attainment and maintenance of which, in the
judgment of the Administrator, based on such criteria, is requisite to
protect the public welfare from any known or anticipated adverse
effects associated with the presence of [the] pollutant in the ambient
air.'' \3\
---------------------------------------------------------------------------
\3\ Under CAA section 302(h) (42 U.S.C. 7602(h)), effects on
welfare include, but are not limited to, ``effects on soils, water,
crops, vegetation, manmade materials, animals, wildlife, weather,
visibility, and climate, damage to and deterioration of property,
and hazards to transportation, as well as effects on economic values
and on personal comfort and well-being.''
---------------------------------------------------------------------------
In setting primary and secondary standards that are ``requisite''
to protect public health and welfare, respectively, as provided in
section 109(b), the EPA's task is to establish standards that are
neither more nor less stringent than necessary. In so doing, the EPA
may not consider the costs of implementing the standards. See
generally, Whitman v.
[[Page 26623]]
American Trucking Ass'ns, 531 U.S. 457, 465-472, 475-76 (2001).
Likewise, ``[a]ttainability and technological feasibility are not
relevant considerations in the promulgation of national ambient air
quality standards'' (American Petroleum Institute v. Costle, 665 F.2d
1176, 1185 [D.C. Cir. 1981]). However, courts have clarified that in
deciding how to revise the NAAQS in the context of considering standard
levels within the range of reasonable values supported by the air
quality criteria and judgments of the Administrator, EPA may consider
``relative proximity to peak background . . . concentrations'' as a
factor (American Trucking Ass'ns, v. EPA, 283 F.3d 355, 379 [D.C. Cir.
2002]).
Section 109(d)(1) of the Act requires periodic review and, if
appropriate, revision of existing air quality criteria to reflect
advances in scientific knowledge on the effects of the pollutant on
public health and welfare. Under the same provision, the EPA is also to
periodically review and, if appropriate, revise the NAAQS, based on the
revised air quality criteria.\4\
---------------------------------------------------------------------------
\4\ This section of the Act requires the Administrator to
complete these reviews and make any revisions that may be
appropriate ``at five-year intervals.''
---------------------------------------------------------------------------
Section 109(d)(2) addresses the appointment and advisory functions
of an independent scientific review committee. Section 109(d)(2)(A)
requires the Administrator to appoint this committee, which is to be
composed of ``seven members including at least one member of the
National Academy of Sciences, one physician, and one person
representing State air pollution control agencies.'' Section
109(d)(2)(B) provides that the independent scientific review committee
``shall complete a review of the criteria . . . and the national
primary and secondary ambient air quality standards . . . and shall
recommend to the Administrator any new . . . standards and revisions of
existing criteria and standards as may be appropriate. . . .'' Since
the early 1980s, this independent review function has been performed by
the CASAC of the EPA's Science Advisory Board.
Section 109(b)(2) specifies that ``[a]ny national secondary ambient
air quality standard prescribed under subsection (a) shall specify a
level of air quality the attainment and maintenance of which in the
judgment of the Administrator, based on such criteria, is requisite to
protect the public welfare from any known or anticipated adverse
effects associated with the presence of such air pollutant in the
ambient air.'' Consistent with this statutory direction, EPA has always
understood the goal of the NAAQS is to identify a requisite level of
air quality, and the means of achieving a specific level of air quality
is to set a standard expressed as a concentration of a pollutant in the
air, such as in terms of parts per million (ppm), parts per billion
(ppb), or micrograms per cubic meter ([mu]g/m\3\). Thus, while
deposition-related effects are included within the ``adverse effects
associated with the presence of such air pollutant in the ambient
air,'' EPA has never found a standard that quantifies atmospheric
deposition onto surfaces to constitute a national secondary ambient air
quality standard.
B. Related Control Programs
States are primarily responsible for ensuring attainment and
maintenance of ambient air quality standards once the EPA has
established them. Under CAA sections 110 and part D, subparts 1, 5, and
6 for nitrogen and sulfur oxides, and subparts 1, 4, and 6 for PM, and
related provisions and regulations, States are to submit, for the EPA's
approval, State implementation plans (SIPs) that provide for the
attainment and maintenance of such standards through control programs
directed to sources of the pollutants involved. The States, in
conjunction with the EPA, also administer the prevention of significant
deterioration of air quality program that covers these pollutants. See
42 U.S.C. 7470-7479. In addition, Federal programs provide for or
result in nationwide reductions in emissions of N oxides,
SO<INF>X,</INF> PM and other air pollutants under title II of the Act,
42 U.S.C. 7521-7574, which involves controls for motor vehicles,
nonroad engines and equipment, and the new source performance standards
under section 111 of the Act, 42 U.S.C. 7411.
C. History of the Secondary Standards for N Oxides, SOX and PM
Secondary NAAQS were first established for N oxides, SO<INF>X</INF>
and PM in 1971 (36 FR 8186, April 30, 1971). Since that time, the EPA
has periodically reviewed the air quality criteria and secondary
standards for these pollutants, with the most recent reviews that
considered the evidence for ecological effects of these pollutants
being completed in 2012 and 2013 (77 FR 20218, April 3, 2012; 78 FR
3086, January 15, 2013). The subsections below summarize key
proceedings from the initial standard setting in 1971 to the last
reviews in 2012-2013.\5\
---------------------------------------------------------------------------
\5\ Since the late 1970s, each review of the air quality
criteria and standards has generally involved the development of an
Air Quality Criteria Document or ISA and a Staff Paper or staff
Policy Assessment, which is often accompanied by or includes a
quantitative exposure or risk assessment, prior to the regulatory
decision-making phase.
---------------------------------------------------------------------------
1. N Oxides
The EPA first promulgated NAAQS for N oxides in April 1971 after
reviewing the relevant science on the public health and welfare effects
in the 1971 Air Quality Criteria for Nitrogen Oxides (air quality
criteria document or AQCD).\6\ With regard to welfare effects, the 1971
AQCD described effects of NO<INF>2</INF> on vegetation and corrosion of
electrical components linked to particulate nitrate (U.S. EPA, 1971).
The primary and secondary standards were both set at 0.053 parts per
million (ppm) NO<INF>2</INF> as an annual average (36 FR 8186, April
30, 1971). In 1982, the EPA published an updated AQCD (U.S. EPA,
1982a). Based on the 1982 AQCD, the EPA proposed to retain the existing
standards in February 1984 (49 FR 6866, February 23, 1984). After
considering public comments, the EPA published the final decision to
retain these standards in June 1985 (50 FR 25532, June 19, 1985).
---------------------------------------------------------------------------
\6\ In reviews initiated prior to 2007, the AQCD provided the
scientific foundation (i.e., the air quality criteria) for the
NAAQS. Since that time, the ISA has replaced the AQCD.
---------------------------------------------------------------------------
The EPA began a second review of the primary and secondary
standards for oxides of nitrogen in 1987 (52 FR 27580, July 22, 1987).
In November 1991, the EPA released an updated draft AQCD for CASAC and
public review and comment (56 FR 59285, November 25, 1991). The CASAC
reviewed the draft document at a meeting held on July 1, 1993, and
concluded in a closure letter to the Administrator that the document
provided ``an adequate basis'' for EPA's decision-making in the review
(Wolff, 1993). The final AQCD was released later in 1993 (U.S. EPA,
1993). Based on the 1993 AQCD, the EPA's Office of Air Quality Planning
and Standards (OAQPS) prepared a Staff Paper,\7\ drafts of which were
reviewed by the CASAC (Wolff, 1995; U.S. EPA, 1995a). In October 1995,
the EPA proposed not to revise the secondary NO<INF>2</INF> NAAQS (60
FR 52874; October 11, 1995). After consideration of the comments
received on the proposal, the Administrator finalized the decision not
to revise the
[[Page 26624]]
NO<INF>2</INF> NAAQS (61 FR 52852; October 8, 1996). The subsequent
(and most recent) review of the N oxides secondary standard was a joint
review with the secondary standard for SO<INF>X</INF>, which was
completed in 2012 (see subsection 4 below).
---------------------------------------------------------------------------
\7\ Prior to reviews initiated in 2007, the Staff Paper
summarized and integrated key studies and the scientific evidence,
and from the 1990s onward, it also assessed potential exposures and
associated risk. The Staff Paper also presented the EPA staff's
considerations and conclusions regarding the adequacy of existing
NAAQS and, when appropriate, the potential alternative standards
that could be supported by the evidence and information. More recent
reviews present this information in the Policy Assessment.
---------------------------------------------------------------------------
2. SO<INF>X</INF>
The EPA first promulgated secondary NAAQS for sulfur oxides in
April 1971 based on the scientific evidence evaluated in the 1969 AQCD
(U.S. DHEW, 1969a [1969 AQCD]; 36 FR 8186, April 30, 1971). These
standards, which were established on the basis of evidence of adverse
effects on vegetation, included an annual arithmetic mean standard, set
at 0.02 ppm SO<INF>2</INF>,\8\ and a 3-hour average standard set at 0.5
ppm SO<INF>2</INF>, not to be exceeded more than once per year. In
1973, based on information indicating there to be insufficient data to
support the finding of a study in the 1969 AQCD concerning vegetation
injury associated with SO<INF>2</INF> exposure over the growing season,
rather than from short-term peak concentrations, the EPA proposed to
revoke the annual mean secondary standard (38 FR 11355, May 7, 1973).
Based on consideration of public comments and external scientific
review, the EPA released a revised chapter of the AQCD and published
its final decision to revoke the annual mean secondary standard (U.S.
EPA, 1973; 38 FR 25678, September 14, 1973). At that time, the EPA
additionally noted that injury to vegetation was the only type of
SO<INF>2</INF> welfare effect for which the evidence base supported a
quantitative relationship, stating that although data were not
available at that time to establish a quantitative relationship between
SO<INF>2</INF> concentrations and other public welfare effects,
including effects on materials, visibility, soils, and water, the
SO<INF>2</INF> primary standards and the 3-hour secondary standard may
to some extent mitigate such effects. The EPA also stated it was not
clear that any such effects, if occurring below the current standards,
were adverse to the public welfare (38 FR 25679, September 14, 1973).
---------------------------------------------------------------------------
\8\ Established with the annual standard as a guide to be used
in assessing implementation plans to achieve the annual standard was
a maximum 24-hour average concentration not to be exceeded more than
once per year (36 FR 8187, April 30, 1971).
---------------------------------------------------------------------------
In 1979, the EPA announced initiation of a concurrent review of the
air quality criteria for oxides of sulfur and PM and plans for
development of a combined AQCD for these pollutants (44 FR 56730,
October 2, 1979). The EPA subsequently released three drafts of a
combined AQCD for CASAC review and public comment. In these reviews,
and in guidance provided at the August 20-22, 1980, public meeting of
the CASAC on the first draft AQCD, the CASAC concluded that acidic
deposition was a topic of extreme scientific complexity because of the
difficulty in establishing firm quantitative relationships among
emissions of relevant pollutants, formation of acidic wet and dry
deposition products, and effects on terrestrial and aquatic ecosystems
(53 FR 14935, April 26, 1988). The CASAC also noted that a fundamental
problem of addressing acid deposition in a criteria document is that
acid deposition is produced by several different criteria pollutants:
oxides of sulfur, oxides of nitrogen, and the fine particulate fraction
of suspended particles (U.S. EPA, 1982b, pp. 125-126). The CASAC also
felt that any document on this subject should address both wet and dry
deposition, since dry deposition was believed to account for a
substantial portion of the total acid deposition problem (53 FR 14936,
April 26, 1988; Lippman, 1987). For these reasons, CASAC recommended
that, in addition to including a summary discussion of acid deposition
in the final AQCD, a separate, comprehensive document on acid
deposition be prepared prior to any consideration of using the NAAQS as
a regulatory mechanism for the control of acid deposition.
Following CASAC closure on the AQCD for oxides of sulfur in
December 1981, the EPA released a final AQCD (U.S. EPA, 1982b), and the
EPA's OAQPS prepared a Staff Paper that was released in November 1982
(U.S. EPA, 1982c). The issue of acidic deposition was not, however,
assessed directly in the OAQPS Staff Paper because the EPA followed the
guidance given by the CASAC, subsequently preparing the following
documents to address acid deposition: The Acidic Deposition Phenomenon
and Its Effects: Critical Assessment Review Papers, Volumes I and II
(U.S. EPA, 1984a, b) and The Acidic Deposition Phenomenon and Its
Effects: Critical Assessment Document (U.S. EPA, 1985) (53 FR 14935-36,
April 26, 1988). Although these documents were not considered criteria
documents and had not undergone CASAC review, they represented the most
comprehensive summary of scientific information relevant to acid
deposition completed by the EPA at that point.
In April 1988, the EPA proposed not to revise the existing
secondary standards for SO<INF>2</INF> (53 FR 14926, April 26, 1988).
This proposed decision with regard to the secondary SO<INF>2</INF>
NAAQS was due to the Administrator's conclusions that: (1) based upon
the then-current scientific understanding of the acid deposition
problem, it would be premature and unwise to prescribe any regulatory
control program at that time; and (2) when the fundamental scientific
uncertainties had been decreased through ongoing research efforts, the
EPA would draft and support an appropriate set of control measures (53
FR 14926, April 26, 1988). This review of the secondary standard for
SO<INF>X</INF> was concluded in 1993, subsequent to the Clean Air Act
Amendments of 1990 (see section I.C.3). The EPA decided not to revise
the secondary standard, concluding that revisions to the standard to
address acidic deposition and related SO<INF>2</INF> welfare effects
were not appropriate at that time (58 FR 21351, April 21, 1993). In
describing the decision, the EPA recognized the significant reductions
in SO<INF>2</INF> emissions, ambient air SO<INF>2</INF> concentrations,
and ultimately deposition expected to result from implementation of the
title IV program, which was expected to significantly decrease the
acidification of water bodies and damage to forest ecosystems and to
permit much of the existing damage to be reversed with time (58 FR
21357, April 21, 1993). While recognizing that further action might be
needed to address acidic deposition in the longer term, the EPA judged
it prudent to await the results of the studies and research programs
then underway, including those assessing the comparative merits of
secondary standards, acidic deposition standards and other approaches
to controlling acidic deposition and related effects, and then to
determine whether additional control measures should be adopted or
recommended to Congress (58 FR 21358, April 21, 1993).
3. Related Actions Addressing Acid Deposition
In 1980, Congress created the National Acid Precipitation
Assessment Program (NAPAP). During the 10-year course of this program,
the program issued a series of reports, including a final report in
1990 (NAPAP, 1991). On November 15, 1990, Amendments to the CAA were
passed by Congress and signed into law by the President. In title IV of
these Amendments, Congress included a statement of findings including
the following:
(1) the presence of acidic compounds and their precursors in the
atmosphere and in deposition from the atmosphere represents a threat
to natural resources, ecosystems, materials, visibility, and public
health; . . . (3) the problem of acid deposition is of national and
international significance; . . .
[[Page 26625]]
(5) current and future generations of Americans will be adversely
affected by delaying measures to remedy the problem[.]
The goal of title IV was to reduce emissions of SO<INF>2</INF> by
10 million tons and N oxides emissions by 2 million tons from 1980
emission levels in order to achieve reductions over broad geographic
regions/areas. In envisioning that further action might be necessary in
the long term, Congress included section 404 of the 1990 Amendments.
This section requires the EPA to conduct a study on the feasibility and
effectiveness of an acid deposition standard or standards to protect
``sensitive and critically sensitive aquatic and terrestrial
resources'' and at the conclusion of the study, submit a report to
Congress. Five years later, the EPA submitted to Congress its report
titled Acid Deposition Standard Feasibility Study: Report to Congress
(U.S. EPA, 1995b) in fulfillment of this requirement. The Report to
Congress concluded that establishing acid deposition standards for Sand
N deposition might at some point in the future be technically feasible
although appropriate deposition loads for these acidifying chemicals
could not be defined with reasonable certainty at that time.
The 1990 Amendments also added new language to sections of the CAA
pertaining to ecosystem effects of criteria pollutants, such as acid
deposition. For example, a new section 108(g) was inserted, stating
that ``[t]he Administrator may assess the risks to ecosystems from
exposure to criteria air pollutants (as identified by the Administrator
in the Administrator's sole discretion).'' The definition of welfare in
CAA section 302(h) was expanded to indicate that welfare effects
include those listed therein, ``whether caused by transformation,
conversion, or combination with other air pollutants.'' Additionally,
in response to legislative initiatives such as the 1990 Amendments, the
EPA and other Federal agencies continued research on the causes and
effects of acidic deposition and related welfare effects of
SO<INF>2</INF> and implemented an enhanced monitoring program to track
progress (58 FR 21357, April 21, 1993).
4. Most Recent Review of the Secondary Standards for N Oxides and
SO<INF>X</INF>
In December 2005, the EPA initiated a joint review \9\ of the air
quality criteria for oxides of nitrogen and sulfur and the secondary
NAAQS for NO<INF>2</INF> and SO<INF>2</INF> (70 FR 73236, December 9,
2005).\10\ The review focused on the evaluation of the protection
provided by the secondary standards for oxides of nitrogen and oxides
of sulfur for two general types of effects: (1) direct effects on
vegetation of exposure to gaseous oxides of nitrogen and sulfur, which
are the type of effects that the existing NO<INF>2</INF> and
SO<INF>2</INF> secondary standards were developed to protect against,
and (2) effects associated with the deposition of oxides of nitrogen
and sulfur to sensitive aquatic and terrestrial ecosystems (77 FR
20218, April 3, 2012).
---------------------------------------------------------------------------
\9\ Although the EPA has historically adopted separate secondary
standards for oxides of nitrogen and oxides of sulfur, the EPA
conducted a joint review of these standards because oxides of
nitrogen and sulfur and their associated transformation products are
linked from an atmospheric chemistry perspective, as well as from an
environmental effects perspective. The joint review was also
responsive to the National Research Council (NRC) recommendation for
the EPA to consider multiple pollutants, as appropriate, in forming
the scientific basis for the NAAQS (NRC, 2004).
\10\ The review was conducted under a schedule specified by
consent decree entered into by the EPA with the Center for
Biological Diversity and four other plaintiffs. The schedule, which
was revised on October 22, 2009, provided that the EPA sign notices
of proposed and final rulemaking concerning its review of the oxides
of nitrogen and oxides of sulfur NAAQS no later than July 12, 2011,
and March 20, 2012, respectively.
---------------------------------------------------------------------------
The Integrated Review Plan (IRP) for the review was released in
December 2007, after review of a draft IRP by the public and CASAC (72
FR 57570, October 10, 2007; Russell, 2007; U.S. EPA, 2007). The first
and second drafts of the ISA were released in December 2007 and August
2008, respectively, for the CASAC and public review (72 FR 72719,
December 21, 2007; 73 FR 10243, February 26, 2008; Russell and
Henderson, 2008; 73 FR 46908, August 12, 2008; 73 FR 53242, September
15, 2008; Russell and Samet, 2008a). The final ISA (referred to as 2008
ISA below) was released in December 2008 (73 FR 75716, December 12,
2008; U.S. EPA, 2008a). Based on the scientific information in the ISA,
the EPA planned and developed a quantitative Risk and Exposure
Assessment (REA),\11\ two drafts of which were made available for
public comment and reviewed by the CASAC (73 FR 10243, February 26,
2008; 73 FR 50965, August 29, 2008; Russell and Samet, 2008b; 73 FR
53242, September 15, 2008; 74 FR 28698, June 17, 2009; Russell and
Samet, 2009). The final REA was released in September 2009 (U.S. EPA,
2009a; 74 FR 48543; September 23, 2009).
---------------------------------------------------------------------------
\11\ Although the REA for the 2012 review was presented in its
own separate document, the REA for a NAAQS review may be presented
in its own separate document or as one or more appendices in the PA
(e.g., U.S. EPA 2020b, 2020c, and PA for current review [U.S. EPA,
2024]).
---------------------------------------------------------------------------
Drawing on the information in the final REA and ISA, the EPA OAQPS
prepared a PA, two drafts of which were made available for public
comment and review by the CASAC (75 FR 10479, March 8, 2010; 75 FR
11877, March 12, 2010; Russell and Samet, 2010b; 75 FR 57463, September
21, 2010; 75 FR 65480, October 25, 2010; Russell and Samet, 2010a). The
final PA was released in January 2011 (U.S. EPA, 2011). Based on
additional discussion subsequent to release of the final PA, the CASAC
provided additional advice and recommendations on the multipollutant,
deposition-based standard described in the 2011 PA (76 FR 4109, January
24, 2011; 76 FR 16768, March 25, 2011; Russell and Samet, 2011).
For the purpose of protection against the direct effects on
vegetation of exposure to gaseous oxides of nitrogen and sulfur, the PA
concluded that consideration should be given to retaining the current
standards. With respect to the effects associated with the deposition
of oxides of nitrogen and oxides of sulfur to sensitive aquatic and
terrestrial ecosystems, the 2011 PA focused on the acidifying effects
of nitrogen and sulfur deposition on sensitive aquatic ecosystems.
Based on the information in the ISA, the assessments in the REA, and
the CASAC advice, the 2011 PA concluded that consideration should be
given to a new multipollutant standard intended to address deposition-
related effects.
On August 1, 2011, the EPA published a proposed decision to retain
the existing annual average NO<INF>2</INF> and 3-hour average
SO<INF>2</INF> secondary standards, recognizing the protection they
provided from direct effects on vegetation (76 FR 46084, August 1,
2011). Further, after considering the multipollutant approach to
establishing secondary standards that was described in the 2011 PA, the
Administrator proposed not to set such a new multipollutant secondary
standard in light of a number of uncertainties. Additionally, the
Administrator proposed to revise the secondary standards by adding
secondary NO<INF>2</INF> and SO<INF>2</INF> standards identical to the
1-hour primary NO<INF>2</INF> and SO<INF>2</INF> standards, both set in
2010, noting that these new primary standards \12\ would result in
reductions in oxides of nitrogen and sulfur that would likely reduce
nitrogen and sulfur deposition to sensitive
[[Page 26626]]
ecosystems (76 FR 46084, August 1, 2011). After consideration of public
comments, the final decision in the review was to retain the existing
standards to address the direct effects on vegetation of exposure to
gaseous oxides of nitrogen and sulfur and to not set additional
standards particular to effects associated with deposition of oxides of
nitrogen and sulfur on sensitive aquatic and terrestrial ecosystems at
that time (77 FR 20218, April 3, 2012).
---------------------------------------------------------------------------
\12\ The 1-hour primary standards set in 2010 included the
NO<INF>2</INF> standard of 100 ppb, as the 98th percentile of 1-hour
daily maximum concentrations, averaged over three years, and the
SO<INF>2</INF> standard of 75 ppb, as the 99th percentile of 1-hour
daily maximum concentrations, averaged over three years (75 FR 6474,
February 9, 2010; 75 FR 35520, June 22, 2010).
---------------------------------------------------------------------------
The EPA's 2012 decision was challenged by the Center for Biological
Diversity and other environmental groups. The petitioners argued that
having decided that the existing standards were not adequate to protect
against adverse public welfare effects such as damage to sensitive
ecosystems, the Administrator was required to identify the requisite
level of protection for the public welfare and to issue a NAAQS to
achieve and maintain that level of protection. The District of Columbia
Circuit (D.C. Circuit) disagreed, finding that the EPA acted
appropriately in not setting a secondary standard given the EPA's
conclusions that ``the available information was insufficient to permit
a reasoned judgment about whether any proposed standard would be
`requisite to protect the public welfare . . .'.'' \13\ In reaching
this decision, the court noted that the EPA had ``explained in great
detail'' the profound uncertainties associated with setting a secondary
NAAQS to protect against aquatic acidification.\14\
---------------------------------------------------------------------------
\13\ Center for Biological Diversity, et al. v. EPA, 749 F.3d
1079, 1087 (2014).
\14\ Id. at 1088.
---------------------------------------------------------------------------
5. PM
The EPA first established a secondary standard for PM in 1971 (36
FR 8186, April 30, 1971), based on the original AQCD, which described
the evidence as to effects of PM on visibility, materials, light
absorption, and vegetation (U.S. DHEW, 1969b). To provide protection
generally from visibility effects and materials damage, the secondary
standard was set at 150 [micro]g/m\3\, as a 24-hour average, from total
suspended particles (TSP), not to be exceeded more than once per year
(36 FR 8187; April 30, 1971).\15\
---------------------------------------------------------------------------
\15\ Additionally, a guide to be used in assessing
implementation plans to achieve the 24-hour standard was set at 60
[micro]g/m\3\, as an annual geometric mean (36 FR 8187; April 30,
1971).
---------------------------------------------------------------------------
In October 1979, the EPA announced the first periodic review of the
air quality criteria and NAAQS for PM (44 FR 56730, October 2, 1979).
As summarized in subsection 2 above, the EPA developed a new AQCD for
PM and SO<INF>X</INF>, drafts of which were reviewed by the CASAC (U.S.
EPA, 1982b). Subsequently, the EPA OAQPS developed a Staff Paper (U.S.
EPA, 1982d), two drafts of which were reviewed by the CASAC
(Friedlander, 1982). Further, the EPA OAQPS prepared an Addendum to the
1982 Staff Paper, which also received CASAC review (Lippman, 1986; U.S.
EPA, 1986). After consideration of public comments on a proposed
decision, the final decision in that review revised the indicator for
PM NAAQS from TSP to particulate matter with mass median diameter of 10
microns (PM<INF>10</INF>) (49 FR 10408, March 20, 1984; 52 FR 24634,
July 1, 1987). With an indicator of PM<INF>10</INF>, two secondary
standards were established to be the same as the primary standards. A
24-hour secondary standard was set at 150 [micro]g/m\3\, with the form
of one expected exceedance per year, on average over three years.
Additionally, an annual secondary standard was set at 50 [micro]g/m\3\,
with a form of annual arithmetic mean, averaged over three years (52 FR
24634, July 1, 1987).
In April 1994, the EPA initiated the second periodic review of the
air quality criteria and NAAQS for PM. In developing the AQCD, the
Agency made available three external review drafts for public and CASAC
review; the final AQCD was released in 1996 (U.S. EPA, 1996). The EPA's
OAQPS prepared a Staff Paper that was released in November 1997, after
CASAC and public review of two drafts (U.S. EPA, 1996; Wolff, 1996).
Revisions to the PM standards were proposed in 1996, and in 1997 the
EPA promulgated final revisions (61 FR 65738; December 13, 1996; 62 FR
38652, July 18, 1997). With the 1997 decision, the EPA added new
standards, using PM<INF>2.5</INF> as the indicator for fine particles.
The new secondary standards were set equal to the primary standards, in
all respects, as follows: (1) an annual standard with a level of 15.0
[micro]g/m\3\, based on the 3-year average of annual arithmetic mean
PM<INF>2.5</INF> concentrations from single or multiple community-
oriented monitors; \16\ and (2) a 24-hour standard with a level of 65
[micro]g/m\3\, based on the 3-year average of the 98th percentile of
24-hour PM<INF>2.5</INF> concentrations at each monitor within an area.
Further, the EPA retained the annual PM<INF>10</INF> standard, without
revision, and revised the form of the 24-hour PM<INF>10</INF> standard
to be based on the 99th percentile of 24-hour PM<INF>10</INF>
concentrations at each monitor in an area.
---------------------------------------------------------------------------
\16\ The 1997 annual PM<INF>2.5</INF> standard was compared with
measurements made at the community-oriented monitoring site
recording the highest concentration or, if specific constraints were
met, measurements from multiple community-oriented monitoring sites
could be averaged (i.e., ``spatial averaging''). In the last review
(completed in 2012) the EPA replaced the term ``community-oriented''
monitor with the term ``area-wide'' monitor. Area-wide monitors are
those sited at the neighborhood scale or larger, as well as those
monitors sited at micro- or middle-scales that are representative of
many such locations in the same core-based statistical area (CBSA)
(78 FR 3236, January 15, 2013).
---------------------------------------------------------------------------
Following promulgation of the 1997 p.m. NAAQS, several parties
filed petitions for review, raising a broad range of issues. In May
1999, the U.S. Court of Appeals for the D.C. Circuit upheld the EPA's
decision to establish fine particle standards, (American Trucking
Ass'ns, Inc. v. EPA, 175 F. 3d 1027, 1055-56 [D.C. Cir. 1999]). The
D.C. Circuit also found ``ample support'' for the EPA's decision to
regulate coarse particle pollution, but vacated the 1997
PM<INF>10</INF> standards, concluding that the EPA had not provided a
reasonable explanation justifying use of PM<INF>10</INF> as an
indicator for coarse particles (Id at 1054-55). Pursuant to the D.C.
Circuit's decision, the EPA removed the vacated 1997 PM<INF>10</INF>
standards, leaving the pre-existing 1987 PM<INF>10</INF> standards in
place (65 FR 80776, December 22, 2000). The D.C. Circuit also upheld
the EPA's determination not to establish more stringent secondary
standards for fine particles to address effects on visibility (Id at
1027). The D.C. Circuit also addressed more general issues related to
the NAAQS, including issues related to the consideration of costs in
setting NAAQS and the EPA's approach to establishing the levels of
NAAQS.
In October 1997, the EPA initiated the third periodic review of the
air quality criteria and NAAQS for PM (62 FR 55201, October 23, 1997).
After the CASAC and public review of several drafts of the AQCD, the
EPA released the final AQCD in October 2004 (U.S. EPA, 2004a, b). The
EPA's OAQPS finalized the Staff Paper in December 2005 (U.S. EPA,
2005). On December 20, 2005, the EPA announced its proposed decision to
revise the NAAQS for PM and solicited public comment on a broad range
of options (71 FR 2620, January 17, 2006). On September 21, 2006, the
EPA announced its final decisions to revise the PM NAAQS to provide
increased protection of public health and welfare (71 FR 61144, October
17, 2006). Revisions to the secondary standards were identical to those
for the primary standards, with the decision describing the protection
provided specifically for visibility and non-visibility related welfare
effects (71 FR 61203-61210, October 17, 2006). With regard to the
standards for fine particles, the EPA revised the level of
[[Page 26627]]
the 24-hour PM<INF>2.5</INF> standards to 35 [micro]g/m\3\, retained
the level of the annual PM<INF>2.5</INF> standards at 15.0 [micro]g/
m\3\, and revised the form of the annual PM<INF>2.5</INF> standards by
narrowing the constraints on the optional use of spatial averaging.
With regard to the standards for PM<INF>10</INF>, the EPA retained the
24-hour standards, with levels at 150 [micro]g/m\3\, and revoked the
annual standards.
Several parties filed petitions for review of the EPA's 2006 p.m.
NAAQS decision. One of these petitions raised the issue of setting the
secondary PM<INF>2.5</INF> standards identical to the primary
standards. On February 24, 2009, the D.C. Circuit issued its opinion in
American Farm Bureau Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009)
and remanded the standards to the EPA because the Agency failed to
adequately explain why setting the secondary PM standards identical to
the primary standards provided the required protection for public
welfare, including protection from visibility impairment (Id. at 528-
32). The EPA responded to the court's remands as part of the subsequent
review of the PM NAAQS, which was initiated in 2007.
In June 2007, the EPA initiated the fourth periodic review of the
air quality criteria and the PM NAAQS (72 FR 35462, June 28, 2007).
Based on the NAAQS review process, as revised in 2008 and again in
2009, the EPA held science/policy issue workshops on the primary and
secondary PM NAAQS (72 FR 34003, June 20, 2007; 72 FR 34005, June 20,
2007), and prepared and released the planning and assessment documents
that are part of the review process (i.e., IRP [U.S. EPA, 2008b], ISA
[U.S. EPA, 2009b], REA planning document for welfare [U.S. EPA, 2009c],
and an urban-focused visibility assessment [U.S. EPA, 2010], and PA
[U.S. EPA, 2011]). In June 2012, the EPA announced its proposed
decision to revise the NAAQS for PM (77 FR 38890, June 29, 2012). In
December 2012, the EPA announced its final decisions to revise the
primary and secondary PM<INF>2.5</INF> annual standards (78 FR 3086,
January 15, 2013). With regard to the secondary standards, the EPA
retained the 24-hour PM<INF>2.5</INF> and PM<INF>10</INF> standards,
with a revision to the form of the 24-hour PM<INF>2.5</INF>, to
eliminate the option for spatial averaging (78 FR 3086, January 15,
2013). Petitioners challenged the EPA's final rule. On judicial review,
the revised standards and monitoring requirements were upheld in all
respects (National Association of Manufacturers v. EPA, 750 F.3d 921,
[D.C. Cir. 2014]).
The subsequent review of the PM secondary standards, completed in
2020, and its subsequent reconsideration, focused on consideration of
protection provided from visibility effects, materials damage, and
climate effects (85 FR 82684, December 18, 2020; 89 FR 16202, March 6,
2024). Those effects--visibility effects, materials damage and climate
effects--are not addressed in this review. The evidence for ecological
effects of PM is addressed in the review of the air quality criteria
and standards described in the PA for this review.
D. Current Review
In August 2013, the EPA issued a call for information in the
Federal Register for information related to the newly initiated review
of the air quality criteria for oxides of sulfur and oxides of nitrogen
and announced a public workshop to discuss policy-relevant scientific
information to inform the review (78 FR 53452, August 29, 2013). Based
in part on the information received in response to the call for
information, the EPA developed a draft IRP, which was made available
for consultation with the CASAC and for public comment (80 FR 69220,
November 9, 2015). Comments from the CASAC and the public on the draft
IRP were considered in preparing the final IRP (Diez Roux and
Fernandez, 2016; U.S. EPA, 2017). In developing the final IRP, the EPA
expanded the review to also include review of the criteria and
standards related to ecological effects of PM in recognition of
linkages between these pollutants (oxides of nitrogen, oxides of sulfur
and PM) with respect to atmospheric transformation of N and S oxides
into particulate compounds, deposition of N and S compounds and
associated ecological effects (U.S. EPA, 2017). Addressing the
pollutants together enables a comprehensive consideration of the nature
and interactions of the pollutants, which is important for ensuring
thorough evaluation of the scientific information relevant to
ecological effects of N and S deposition.
In March 2017, the EPA released the first external review draft of
the Integrated Science Assessment (ISA) for Oxides of Nitrogen, Oxides
of Sulfur, and Particulate Matter Ecological Criteria (82 FR 15702,
March 30, 2017), which was then reviewed by the CASAC at a public
meeting on May 24-25, 2017 (82 FR 15701, March 30, 2017) and August 31,
2017 (82 FR 35200, July 28, 2017; Diez Roux and Fernandez, 2017). With
consideration of comments from the CASAC and the public, the EPA
released a second external review draft (83 FR 29786, June 26, 2018),
which the CASAC reviewed at public meetings on September 5-6, 2018 (83
FR 2018; July 9, 2018) and April 27, 2020 (85 FR 16093, March 30, 2020;
Cox, Kendall, and Fernandez, 2020a).\17\ The EPA released the final ISA
in October 2020 (85 FR 66327, October 19, 2020; U.S. EPA, 2020a). In
planning for quantitative aquatic acidification exposure/risk analyses
for consideration in the PA, the EPA solicited public comment and
consulted with the CASAC (83 FR 31755, July 9, 2018; Cox, Kendall, and
Fernandez, 2020b; U.S. EPA, 2018; 83 FR 42497, August 22, 2018).
---------------------------------------------------------------------------
\17\ A change in CASAC membership contributed to an extended
time period between the two public meetings.
---------------------------------------------------------------------------
The draft PA, including the REA for aquatic acidification as an
appendix, was completed in May 2023 and was made available for review
by the CASAC and for public comment (88 FR 34852, May 31, 2023). The
CASAC review was conducted at public meetings held on June 28-29, 2023
(88 FR 17572, March 23, 2023), and September 5-6, 2023 (88 FR 45414,
July 17, 2023). The CASAC conveyed advice on the standards and comments
on the draft PA in its September 27, 2023, letter to the Administrator
(Sheppard, 2023). The final PA was completed in January 2024 (89 FR
2223, January 12, 2024).
Materials upon which this proposed decision is based, including the
documents described above, are available to the public in the docket
for this review.\18\ The timeline for the remainder of this review is
governed by a consent decree that requires the EPA to sign a notice of
proposed decision by April 9, 2024, and a final decision notice by
December 10, 2024 (Center for Biological Diversity v. Regan [No. 4:22-
cv-02285-HSG (N.D. Cal.]).
---------------------------------------------------------------------------
\18\ The docket for this review, EPA-HQ-OAR-2014-0128, has
incorporated the ISA docket (EPA-HQ-ORD-2013-0620) by reference.
Both are publicly accessible at <a href="http://www.regulations.gov">www.regulations.gov</a>.
---------------------------------------------------------------------------
II. Rationale for Proposed Decisions
This section presents the rationale for the Administrator's
proposed decisions in the review of the secondary standards for the
ecological effects of SO<INF>X</INF>, N oxides and PM. This rationale
is based on a thorough review of the full evidence base, including the
scientific information available since the last review of the secondary
standards for N oxides and SO<INF>X</INF>, which is generally published
between January 2008 and May 2017 (and considered in the ISA), as well
as more recent studies identified during peer review or by public
[[Page 26628]]
comments (ISA, section IS.1.2),\19\ integrated with the information and
conclusions from previous assessments and presented in the ISA, on
ecological effects associated with SO<INF>X</INF>, N oxides and PM and
pertaining to their presence in ambient air. The Administrator's
rationale also takes into account: (1) the PA evaluation of the policy-
relevant information in the ISA and presentation of quantitative
analyses of air quality, exposure and aquatic acidification risks; (2)
CASAC advice and recommendations, as reflected in discussions of drafts
of the ISA and PA at public meetings and in the CASAC's letters to the
Administrator; and (3) public comments received during the development
of these documents.
---------------------------------------------------------------------------
\19\ In addition to the review's opening ``Call for
Information'' (78 FR 53452, August 29, 2013), multiple search
methodologies were applied to identify relevant scientific findings
that have emerged since the 2008 ISA. Search techniques for the
current ISA identified and evaluated studies and reports that have
undergone scientific peer review and were published or accepted for
publication between January 2008 (providing some overlap with the
cutoff date for the last ISA) and May 2017. Studies published after
the literature cutoff date for this ISA were also considered if they
were submitted in response to the Call for Information or identified
in subsequent phases of ISA development, particularly to the extent
that they provide new information that affects key scientific
conclusions. References that are cited in the ISA, the references
that were considered for inclusion but not cited, and electronic
links to bibliographic information and abstracts can be found at:
<a href="https://hero.epa.gov/hero/index.cfm/project/page/project_id/2965">https://hero.epa.gov/hero/index.cfm/project/page/project_id/2965</a>
(ISA, section IS.1.2).
---------------------------------------------------------------------------
In presenting the rationale for the Administrator's proposed
decisions and their foundations, section II.A provides background on
the general approach in this review, including a summary of the basis
for the existing standards (section II.A.1), a summary of the prior
review of the SO<INF>X</INF> and N oxides standards for deposition-
related effects (section II.A.2) and the general approach for the
current review (section II.A.3).
Section II.B summarizes air quality information and analyses
relating S and N deposition to concentrations of SO<INF>X</INF>, N
oxides and PM. Section II.C summarizes the currently available
ecological effects evidence as summarized in the ISA, focusing on
consideration of key policy-relevant aspects. Section II.D summarizes
the exposure and risk information for this review, drawing on the
quantitative analyses of aquatic acidification risk, presented in the
PA. Section II.E presents the Administrator's proposed conclusions on
the current standards and potential alternatives (section II.E.3),
drawing on both evidence-based and exposure/risk-based considerations
from the PA (section II.E.1) and advice from the CASAC (section
II.E.2).
A. Introduction
As is the case for all such reviews, this review is based, most
fundamentally, on using the Agency's assessments of the current
scientific evidence and associated quantitative analyses to inform the
Administrator's judgment regarding secondary standards for
SO<INF>X</INF>, N oxides and PM that are requisite to protect the
public welfare from known or anticipated adverse effects associated
with that pollutant's presence in the ambient air. The EPA's
assessments are primarily documented in the ISA and PA, both of which
have received CASAC review and public comment (82 FR 15702, March 30,
2017; 82 FR 15701, March 30, 2018; 83 FR 29786; June 26, 2018; 83 FR
31755, July 9, 2018; 85 FR 16093; March 20, 2020; 88 FR 34852, May 31,
2023; 88 FR 17572, March 23, 2023; 88 FR 45414, July 17, 2023). In
bridging the gap between the scientific assessments of the ISA and the
judgments required of the Administrator in his decisions on the current
standard, the PA evaluates policy implications of the assessment of the
current evidence in the ISA and the quantitative exposure and risk
analyses and information documented in the PA. In evaluating the public
welfare protection afforded by the current standard, the four basic
elements of the NAAQS (indicator, averaging time, level, and form) are
considered collectively.\20\
---------------------------------------------------------------------------
\20\ The indicator defines the chemical species or mixture to be
measured in the ambient air for the purpose of determining whether
an area attains the standard. The averaging time defines the period
over which air quality measurements are to be averaged or otherwise
analyzed. The form of a standard defines the air quality statistic
that is to be compared to the level of the standard in determining
whether an area attains the standard. For example, the form of the
annual NAAQS for fine particulate matter (PM<INF>2.5</INF>) is the
average of annual mean concentrations for three consecutive years,
while the form of the 3-hour secondary NAAQS for SO<INF>2</INF> is
the second highest 3-hour average in a year. The level of the
standard defines the air quality concentration used for that
purpose.
---------------------------------------------------------------------------
The Agency's approach in its review of secondary standards is
consistent with the requirements of the provisions of the CAA related
to the review of NAAQS and with how the EPA and the courts have
historically interpreted the CAA. These provisions require the
Administrator to establish secondary standards that, in the
Administrator's judgment, are requisite (i.e., neither more nor less
stringent than necessary) to protect the public welfare from known or
anticipated adverse effects associated with the presence of the
pollutant in the ambient air. In so doing, the Administrator considers
advice from the CASAC and public comment. This approach is based on a
recognition that the available welfare effects evidence generally
reflects a range of effects that include ambient air exposure
circumstances for which scientists generally agree that effects are
likely to occur as well as lower levels at which the likelihood and
magnitude of response become increasingly uncertain. The CAA does not
require that standards be set at a zero-risk level, but rather at a
level that reduces risk sufficiently so as to protect the public
welfare from known or anticipated adverse effects.
The Agency's decisions on the adequacy of the current secondary
standards and, as appropriate, on any potential alternative standards
considered in a review, are largely public welfare policy judgments
made by the Administrator based on the Administrator's informed
assessment of what constitutes requisite protection against adverse
effects to public welfare. A public welfare policy decision draws upon
scientific information and analyses about welfare effects, exposure and
risks, as well as judgments about the appropriate response to the range
of uncertainties that are inherent in the scientific evidence and
analyses. The ultimate determination as to what level of damage to
ecosystems and the services provided by those ecosystems is adverse to
public welfare is not wholly a scientific question, although it is
informed by scientific studies linking ecosystem damage to losses in
ecosystem services and information on the value of those losses of
ecosystem services. In reaching such decisions, the Administrator seeks
to establish standards that are neither more nor less stringent than
necessary for this purpose.
Thus, in general, conclusions reached by the Administrator in
secondary NAAQS reviews on the amount of public welfare protection from
the presence of the pollutant(s) in ambient air that is appropriate to
be afforded by a secondary standard take into account a number of
considerations. Among these considerations are the nature and degree of
effects of the pollutant, including the Administrator's judgments on
what constitutes an adverse effect to the public welfare as well as the
strengths and limitations of the available and relevant information,
with its associated uncertainties. Across reviews, it is generally
recognized that such judgments should neither overstate nor understate
the strengths and limitations of the evidence and information nor the
appropriate inferences to be drawn as to risks to public welfare, and
that the choice of
[[Page 26629]]
the appropriate level of protection is a public welfare policy judgment
entrusted to the Administrator under the CAA taking into account both
the available evidence and the uncertainties (80 FR 65404-05, October
26, 2015). Thus, the Administrator's final decisions in such reviews
draw upon the scientific information and analyses about welfare
effects, environmental exposures and risks, and associated public
welfare significance, as well as judgments about how to consider the
range and magnitude of uncertainties that are inherent in the
scientific evidence and quantitative analyses.
1. Basis for Existing Secondary Standards
In the last review of the secondary standards for SO<INF>X</INF>
and N oxides, completed in 2012, the EPA retained the existing 3-hour
SO<INF>2</INF> standard, with its level of 0.5 ppm, and the annual
NO<INF>2</INF> standard, with its level of 0.053 ppm (77 FR 20218,
April 3, 2012). Both of these secondary standards were established in
1971 (36 FR 8186, April 30, 1971). The basis for both the existing
SO<INF>2</INF> and NO<INF>2</INF> secondary standard is to provide
protection to the public welfare related to direct effects on
vegetation (U.S. DHEW, 1969a; U.S. EPA, 1971).
The welfare effects evidence for SO<INF>X</INF> in previous reviews
indicates a relationship between short- and long-term SO<INF>2</INF>
exposures and foliar damage to cultivated plants, reductions in
productivity, species richness, and diversity (U.S. DHEW, 1969a; U.S.
EPA, 1982c; U.S. EPA, 2008). At the time the standard was set,
concentrations of SO<INF>2</INF> in the ambient air were also
associated with other welfare effects, including effects on materials
and visibility related to sulfate, a particulate transformation product
of SO<INF>2</INF> (U.S. DHEW, 1969a). However, the available data were
not sufficient to establish a quantitative relationship between
specific SO<INF>2</INF> concentrations and such effects (38 FR 25679,
September 14, 1973). Accordingly, direct effects of SO<INF>X</INF> in
ambient air on vegetation is the basis for the existing secondary
standard for SO<INF>X</INF>. Effects on materials and visibility (which
relate to particles in air, including sulfates) have more recently been
considered in the PM secondary NAAQS reviews (e.g., 85 FR 82684,
December 18, 2020).
The welfare effects evidence for N oxides in previous reviews
includes foliar injury, leaf drop, and reduced yield of some crops
(U.S. EPA, 1971; U.S. EPA, 1982c; U.S. EPA, 1993; U.S. EPA, 2008a).
Since it was established in 1971, the secondary standard for N oxides
has been reviewed three times, in 1985, 1996, and 2012 (50 FR 25532,
June 19, 1985; 61 FR 52852; October 8, 1996; 77 FR 20218, April 3,
2012). Although those reviews identified additional effects related to
N deposition, they all have concluded that the existing NO<INF>2</INF>
secondary standard provided adequate protection related to the
``direct'' effects of airborne N oxides on vegetation on which the
standard is based).
In the last review of the secondary PM standards with regard to
protection from ecological effects, completed in 2013, the EPA retained
the 24-hour PM<INF>2.5</INF> standard, with its level of 35 [micro]g/
m\3\, and the 24-hour PM<INF>10</INF> standard, with its level of 150
[micro]g/m\3\ (78 FR 3228, January 15, 2013). With regard to the annual
PM<INF>2.5</INF> standard, the EPA retained the averaging time and
level, set at 15 [micro]g/m\3\, while revising the form to remove the
option for spatial averaging consistent with this change to the primary
annual PM<INF>2.5</INF> standard (78 FR 3225, January 15, 2013). The
2013 review considered the PM standards with regard to protection for
an array of effects that include effects on visibility, materials
damage, and climate effects, as well as ecological effects, and the EPA
concluded that those standards provided protection for ecological
effects (e.g., 78 FR 3225-3226, 3228, January 15, 2013). In reaching
this conclusion, it was noted that the PA for the review explicitly
excluded discussion of the effects associated with deposited PM
components of N oxides and SO<INF>X</INF> and their transformation
products, which were being addressed in the joint review of the
secondary NO<INF>2</INF> and SO<INF>2</INF> NAAQS (78 FR 3202, January
15, 2013). The ecological effects of PM considered include direct
effects on plant foliage as well as effects of the ecosystem loading of
PM constituents such as metals or organic compounds (2009 ISA, section
2.5.3). For all of these effects, the 2013 decision recognized an
absence of information that would support any different standards and
concluded the existing standards, with the revision to the form of the
annual PM<INF>2.5</INF> standard, provided the requisite protection (78
FR 3086, January 15, 2013).
2. Prior Review of Deposition-Related Effects
In the 2012 review of the NO<INF>2</INF> and SO<INF>2</INF>
secondary standards, the EPA recognized that a significant increase in
understanding of the effects of N oxides and SO<INF>X</INF> had
occurred since the prior secondary standards reviews for those
pollutants (77 FR 20236, April 3, 2012). Considering the extensive
evidence available at that time, the Agency concluded that the most
significant risks of adverse effects of N oxides and SO<INF>X</INF> to
public welfare were those related to deposition of N and S compounds to
both terrestrial and aquatic ecosystems (77 FR 20236, April 3, 2012).
Accordingly, in addition to evaluating the protection provided by the
secondary standards for N oxides and SO<INF>X</INF> from effects
associated with the airborne pollutants, the 2012 review also included
extensive analyses of the welfare effects associated with nitrogen and
sulfur deposition to sensitive aquatic and terrestrial ecosystems (77
FR 20218, April 3, 2012).
Based on the available evidence, the risks of atmospheric
deposition analyzed in the 2009 REA related to two categories of
ecosystem effects: acidification and nutrient enrichment (U.S. EPA,
2009a). The analyses included assessment of risks of both types of
effects in both terrestrial and aquatic ecosystems. While the available
evidence supported conclusions regarding the role of atmospheric
deposition of S and N compounds in acidification and nutrient
enrichment of aquatic and terrestrial ecosystems, there was variation
in the strength of the evidence and of the information supporting
multiple quantitative linkages between pollutants in ambient air and
ecosystem responses and potential public welfare implications.
While there is extensive evidence of deleterious effects of
excessive nitrogen loadings to terrestrial and aquatic ecosystems,
consideration of the nutrient enrichment-related effects of atmospheric
N and S deposition with regard to identification of options to provide
protection for deposition-related effects was limited by several
factors. For example, the co-stressors affecting forests, including
other air pollutants such as ozone, and limiting factors such as
moisture and other nutrients, confound the assessment of marginal
changes in any one stressor or nutrient in a forest ecosystem, limiting
the information on the effects of changes in N deposition on
forestlands and other terrestrial ecosystems (2011 PA, section 6.3.2).
Further, only a fraction of the deposited N was reported to be taken up
by the forests, with most of the N retained in the soils, such that
forest management practices can significantly affect the nitrogen
cycling within a forest ecosystem (2008 ISA section 3.3.2.1 and Annex
C, section C.6.3). Factors affecting consideration of aquatic
eutrophication effects included the appreciable contributions of non-
atmospheric sources to waterbody nutrient loading, which affected our
attribution of specific effects to
[[Page 26630]]
atmospheric sources of N, and limitations in the ability of the
available data and models to characterize incremental adverse impacts
of atmospheric N deposition (2011 PA, section 6.3.2).
The linkages between terrestrial acidification and atmospheric
deposition of N and S compounds were also limited by the sparseness of
available data for identifying appropriate assessment levels for
terrestrial acidification indicators and uncertainties with regard to
empirical case studies in the ISA (e.g., the potential for other
stressors to confound relationships between deposition and terrestrial
acidification effects). However, the evidence in the 2008 ISA and the
REA analyses of aquatic acidification provided strong support to the
evidence for a relationship between atmospheric deposition of N and S
compounds and loss of acid neutralizing capacity (ANC) in sensitive
ecosystems, with associated aquatic acidification effects.
In light of the evidence and findings of these analyses and advice
from the CASAC, the PA concluded it was appropriate to place greatest
confidence in findings related to the aquatic acidification-related
effects of N oxides and SO<INF>X</INF> relative to other deposition-
related effects. Therefore, the PA focused on aquatic acidification
effects from deposition of N and S compounds in identifying policy
options for providing public welfare protection from deposition-related
effects of N oxides and SO<INF>X</INF>, concluding that the available
information and assessments were only sufficient at that time to
support development of a standard to address aquatic acidification.
Consistent with this, the PA concluded it was appropriate to consider a
secondary standard in the form of an aquatic acidification index (AAI)
and identified a range of AAI values (which correspond to minimum ANC
levels) for consideration (2011 PA, section 7.6.2). Conceptually, the
AAI is an index that uses the results of ecosystem and air quality
modeling to estimate waterbody ANC. The standard level for an AAI-based
standard was conceptually envisioned to be a national minimum target
ANC for waterbodies in the ecoregions of the U.S. for which data were
considered adequate for these purposes (2011 PA, section 7.6.2).
While the NAAQS have historically been set in terms of an ambient
air concentration, an AAI-based standard was envisioned to have a
single value established for the AAI, but the concentrations of
SO<INF>X</INF> and N oxides would be specific to each ecoregion, taking
into account variation in several factors that influence waterbody ANC,
and consequently could vary across the U.S. The factors, specific to
each ecoregion (``F factors''), which it was envisioned would be
established as part of the standard, include: surface water runoff
rates and so-called ``transference ratios,'' which are factors applied
to back-calculate or estimate the concentrations of SO<INF>X</INF> and
N oxides corresponding to target deposition values that would meet the
AAI-based standard level, which is also the target minimum ANC (2011
PA, Chapter 7).\21\ The ecoregion-specific values for these factors
would be specified based on then available data and simulations of the
Community Multiscale Air Quality (CMAQ) model, and codified as part of
such a standard. As part of the standard, these factors would be
reviewed in the context of each periodic review of the NAAQS.
---------------------------------------------------------------------------
\21\ These were among the ecoregion-specific factors that
comprised the parameters F1 through F4 in the AAI equation (2011 PA,
p. 7-37). The parameter F2 represented the ecoregion-specific
estimate of acidifying deposition associated with reduced forms of
nitrogen, NH<INF>X</INF> (2011 PA, p. 7-28 and ES-8 to ES-9). The
2011 PA suggested that this factor could be specified based on a
2005 CMAQ model simulation over 12-km grid cells or monitoring might
involve the use of monitoring data for NH<INF>X</INF> applied in dry
deposition modeling. It was recognized that appreciable spatial
variability, as well as overall uncertainty, were associated with
this factor.
---------------------------------------------------------------------------
After consideration of the PA conclusions, the Administrator
concluded that while the conceptual basis for the AAI was supported by
the available scientific information, there were limitations in the
available relevant data and uncertainties associated with specifying
the elements of the AAI, specifically those based on modeled factors,
that posed obstacles to establishing such a standard under the CAA. It
was recognized that the general structure of an AAI-based standard
addressed the potential for contributions to acid deposition from both
N oxides and SO<INF>X</INF> and quantitatively described linkages
between ambient air concentrations, deposition, and aquatic
acidification, considering variations in factors affecting these
linkages across the country. However, the Administrator judged that the
limitations and uncertainties in the available information were too
great to support establishment of a new standard that could be
concluded to provide the requisite protection for such effects under
the Act (77 FR 20218, April 3, 2012). These uncertainties generally
related to the quantification of the various elements of the standard
(the ``F factors''), and their representativeness at an ecoregion
scale. These uncertainties and the complexities in this approach were
recognized to be unique to the 2012 review of the NAAQS for N and S
oxides and were concluded to preclude the characterization and degree
of protectiveness that would be afforded by an AAI-based standard,
within the ranges of levels and forms identified in the PA, and the
representativeness of F factors in the AAI equation described in the
2011 PA (77 FR 20261, April 3, 2012).
. . . the Administrator recognizes that characterization of the
uncertainties in the AAI equation as a whole represents a unique
challenge in this review primarily as a result of the complexity in
the structure of an AAI based standard. In this case, the very
nature of some of the uncertainties is fundamentally different than
uncertainties that have been relevant in other NAAQS reviews. She
notes, for example, some of the uncertainties uniquely associated
with the quantification of various elements of the AAI result from
limitations in the extent to which ecological and atmospheric
models, which have not been used to define other NAAQS, have been
evaluated. Another important type of uncertainty relates to
limitations in the extent to which the representativeness of various
factors can be determined at an ecoregion scale, which has not been
a consideration in other NAAQS.'' [77 FR 20261, April 3, 2012]
The Administrator concluded that while the existing secondary
standards were not adequate to provide protection against potentially
adverse deposition-related effects associated with N oxides and
SO<INF>X</INF>, it was not appropriate under section 109 to set any new
or additional standards at that time to address effects associated with
deposition of N and S compounds on sensitive aquatic and terrestrial
ecosystems (77 FR 20262-20263, April 3, 2012).
3. General Approach for This Review
As is the case for all NAAQS reviews, this secondary standards
review uses the Agency's assessment of the current scientific evidence
and associated quantitative analyses as a foundation to inform the
Administrator's judgments regarding secondary standards that are
requisite to protect the public welfare from known or anticipated
adverse effects. The approach for this review of the secondary
SO<INF>X</INF>, N oxides, and PM standards builds on the last reviews
of those pollutants, including the substantial assessments and
evaluations performed over the course of those reviews, and considering
the more recent scientific information and air quality data now
available to inform understanding of the key policy-relevant issues in
the current review.
[[Page 26631]]
This review of the secondary standards for SO<INF>X</INF>, N
oxides, and PM assesses the protection provided by the standards from
two categories of effects: direct effects of the airborne pollutants
and indirect effects of the associated S- and N-containing compounds
(in gaseous and particulate form) deposited in ecosystems. In so doing,
the review draws on the currently available evidence as assessed in the
ISA (and prior assessments) and quantitative exposure, risk, and air
quality information in the PA, including the REA for aquatic
acidification.
With regard to direct effects, we draw on the currently available
evidence as assessed in the ISA, including the determinations regarding
the causal nature of relationships between the airborne pollutants and
ecological effects, which focus most prominently on vegetation, and
quantitative exposure and air quality information. Based on this
information, we consider the policy implications, most specifically
whether the evidence supports the retention or revision of the current
NO<INF>2</INF> and SO<INF>2</INF> secondary standards. With regard to
the effects of PM, we take a similar approach, based on the evidence
presented in the current ISA and conclusions from the review of the PM
NAAQS concluded in 2013 (in which ecological effects were last
considered) to assess the effectiveness of the current PM standard to
protect against these types of impacts.
With regard to deposition-related effects, we consider the evidence
for the array of effects identified in the ISA (and summarized in
section II.B below), including both terrestrial and aquatic effects;
and the limitations in the evidence and associated uncertainties; as
well as the public welfare implications of such effects. The overall
approach takes into account the nature of the welfare effects and the
exposure conditions associated with effects in identifying S and N
deposition levels appropriate to consider in the context of public
welfare protection. To identify and evaluate metrics relevant to air
quality standards (and their elements), we have assessed relationships
developed from air quality measurements near pollutant sources and
deposition estimates nearby and in downwind ecoregions. In so doing,
the available quantitative information both on deposition and effects,
and on ambient air concentrations and deposition, has been assessed
with regard to the existence of linkages between SO<INF>X</INF>, N
oxides, and PM in ambient air and deposition-related effects. These
assessments then inform judgments on the likelihood of occurrence of
deposition-related effects under air quality that meets the existing
standards for these pollutants, or potential alternatives.
In considering the information on deposition and effects, we
recognize that the impacts from the dramatically higher deposition
rates of the past century can affect how ecosystems and biota respond
to more recent, lower deposition rates, complicating interpretation of
impacts related to more recent, lower deposition levels. This
complexity is illustrated by findings of studies that compared soil
chemistry across 15-30-year intervals (1984-2001 and 1967-1997) and
reported that although atmospheric deposition in the Northeast declined
across those intervals, soil acidity increased (ISA, Appendix 4,
section 4.6.1). As noted in the ISA, ``[i]n areas where N and S
deposition has decreased, chemical recovery must first create physical
and chemical conditions favorable for growth, survival, and
reproduction'' (ISA, Appendix 4, section 4.6.1). Thus, the extent to
which S and N compounds (once deposited) are retained in soil matrices
(with potential effects on soil chemistry) influences the dynamics of
the response of the various environmental pathways to changes in air
quality, in addition to the influences of emissions, ambient air
concentrations and associated deposition.
The two-pronged approach to this review's consideration of
deposition-related effects based on the available information in the
ISA (summarized in section II.C and II.D below) includes the
consideration of deposition levels that may be associated with
ecological effects of potential concern. In this step, we consider and
strive to focus on effects for which the evidence is most robust with
regard to established quantitative relationships between deposition and
ecosystem effects. The information for terrestrial ecosystems is
derived primarily from analysis of the evidence presented in the ISA.
For aquatic ecosystems, primary focus is given to effects related to
aquatic acidification, for which we have conducted quantitative risk
and exposure analyses based on available modeling applications that
relate acid deposition and acid buffering capability in U.S.
waterbodies, as summarized in section II.D below (PA, section 5.1 and
Appendix 5A).
In parallel fashion to identification of deposition levels for
consideration, air quality and deposition analyses have been employed
to inform an understanding of relationships between ambient air
concentrations near pollutant sources in terms of metrics relevant to
air quality standards (and their elements) and ecosystem deposition
estimates. As described in section II.B below, several different types
of analyses have been performed in this review for this purpose.
Interpretation of findings from these analyses, in combination with the
identified deposition levels of interest, and related policy judgments
regarding limitations and associated uncertainties of the underlying
information, inform the Administrator's proposed conclusions on the
extent to which existing standards, or potential alternative standards,
might be expected to provide protection from these levels.
In summary, our approach to evaluating the standards with regard to
protection from ecological effects related to ecosystem deposition of N
and S compounds (presented in the sections that follow) involves
multiple components: (1) review of the scientific evidence to identify
the ecological effects associated with the three pollutants, both those
related to direct pollutant contact and to ecosystem deposition; (2)
assessment of the evidence and characterization of the REA results to
identify deposition levels related to categories of ecosystem effects;
(3) analysis of relationships between ambient air concentrations of the
three pollutants and deposition of N and S compounds to understand key
aspects of these relationships that can inform the Administrator's
decisions on policy options for ambient air standards to protect
against air concentrations associated with direct effects and with
deposition-related effects that are judged adverse to the public
welfare. As is described in sections II.B and II.E, for two of the
pollutants, N oxides and PM, relating ambient air concentrations to
deposition (of N compounds) is particularly complex because N
deposition also results from an additional air pollutant that is not
controlled by NAAQS for N oxides and PM. Thus, separate from the
evaluation of standards for SO<INF>X</INF>, the evaluation for N oxides
and PM also considers current information (e.g., spatial and temporal
trends) related to the additional air pollutant, ammonia
(NH<INF>3</INF>), that contributes to N deposition and to PM components
that do not contribute to N deposition. Evaluation of all of this
information, together, is considered by the Administrator in reaching
his proposed decision, as summarized in section II.E.
B. Air Quality and Deposition
The three criteria pollutants that are the focus of this review
(SO<INF>X</INF>, N oxides, and PM) include both gases and
[[Page 26632]]
particles. Both their physical state and chemical properties, as well
as other factors, influence their deposition as N- or S-containing
compounds. The complex pathway from emissions of these pollutants and
their precursors to eventual deposition varies by pollutant and is
influenced by a series of atmospheric processes and chemical
transformations that occur at multiple spatial and temporal scales (PA,
Chapters 2 and 6).
A complication in the consideration of the influence of these
criteria pollutants on N deposition (and associated ecological effects)
is posed by the contribution of other, non-criteria, pollutants in
ambient air, specifically NH<INF>3</INF>. As summarized below, although
there is a decreasing temporal trend in emissions of N oxides, the
coincident increasing trend in NH<INF>3</INF> emissions has reduced the
influence of N oxides on N deposition (PA, sections 6.2.1, 6.4.2 and
7.2.3.3). Variability and temporal changes in the composition of PM,
including with regard to N- (and S-) containing compounds, is another
factor affecting decisions in this review (as discussed in sections
II.1.d(3)) and II.3 below).
This section includes a brief summary of the major emission sources
of SO<INF>X</INF>, N oxides, and PM (section II.B.1). This is followed
by a description of how those emissions are transported and transformed
within the atmosphere to eventually contribute to S and N deposition
(section II.B.1). Available information on current levels of emissions
and air concentrations of these three pollutants across the U.S. and
their trends is summarized in section II.B.2, accompanied by a
description of estimated deposition levels across the U.S. and how they
have changed over the past two decades. Finally, while many of the
ecological effects examined in this review are associated with
deposition of N and S, the NAAQS are set in terms of pollutant
concentrations. To that end, section II.B.3 discusses the findings of
analyses performed to relate ambient air concentrations of the relevant
pollutants and S or N deposition, over a range of conditions (e.g.,
pollutant, region, time period), and summarizes key observations that
may inform the Administrator's judgments in this review.
1. Sources, Emissions and Atmospheric Processes Affecting
SO<INF>X</INF>, N Oxides and PM
Sulfur dioxide is one of a small group of highly reactive gases
collectively known as SO<INF>X</INF>. Sulfur dioxide is generally
present at higher concentrations in the ambient air than the other
gaseous SO<INF>X</INF> species (ISA, Appendix 2, section 2.1) and, as a
result, SO<INF>2</INF> is the indicator for the existing NAAQS for
SO<INF>X</INF>. The main anthropogenic source of SO<INF>2</INF>
emissions is fossil fuel combustion (PA, section 2.2.2). Based on the
2020 National Emissions Inventory (NEI), the top three emission sources
of SO<INF>2</INF> in the U.S. are: coal-fired electrical generating
units (48% of total), industrial processes (27%), and other stationary
source fuel combustion (9%).
Once emitted to the atmosphere, the atmospheric lifetime of
SO<INF>2</INF> is typically less than 1-2 days; it can either remain in
the gas phase or be oxidized to form sulfate particles
(SO<INF>4</INF><SUP>2-</SUP>). Modeling studies suggest that oxidation
accounts for more than half of SO<INF>2</INF> removal on a national
basis (PA, section 2.1.1). The rate of SO<INF>2</INF> oxidation
accelerates with greater availability of oxidants. Oxidants are
generally depleted near source stacks, so that more SO<INF>2</INF> is
oxidized to SO<INF>4</INF><SUP>2-</SUP> in cleaner air downwind of
SO<INF>X</INF> sources (2008 ISA, section 2.6.3.1). The atmospheric
lifetime of SO<INF>4</INF><SUP>2-</SUP> particles is longer, ranging
from 2 to 10 days. As SO<INF>4</INF><SUP>2-</SUP> particles are
generally within the fine particle size range, they are a component of
PM<INF>2.5</INF> (PA, section 2.1.1). The spatial distribution of both
SO<INF>2</INF> and SO<INF>4</INF><SUP>2-</SUP> deposition reflects the
distribution of SO<INF>X</INF> emissions (i.e., most S deposition is in
the eastern U.S.; PA, section 2.5.3) and wind patterns. Precipitation
variability also modulates the spatial distribution of S wet
deposition. In sum, both SO<INF>2</INF>, and the
SO<INF>4</INF><SUP>2-</SUP> particles converted from SO<INF>2</INF>,
contribute to S deposition but do so over different time and geographic
scales, with dry deposition of SO<INF>2</INF> typically occurring near
the source, and wet deposition of sulfate particles being more regional
in nature.
The term N oxides refers to all forms of oxidized nitrogen
compounds, including nitric oxide (NO), NO<INF>2</INF>, nitric acid
(HNO<INF>3</INF>), and particulate nitrate
(NO<INF>3</INF><SUP>-</SUP>). Most N oxides enter the atmosphere as
either NO or NO<INF>2</INF>, which are collectively referred to as
NO<INF>X</INF> (PA, section 2.1.2). Anthropogenic sources account for
the majority of NO<INF>X</INF> emissions in the U.S., per the 2020 NEI,
with highway vehicles (26% of total), stationary fuel combustion which
includes electric generating units (25%), and non-road mobile sources
(19%) identified as the largest contributors to total emissions. Other
anthropogenic NO<INF>X</INF> sources include agricultural field
burning, prescribed fires, and various industrial processes such as
cement manufacturing and oil and gas production (PA, section 2.2.1).
Once emitted into the atmosphere, NO<INF>X</INF> can deposit to the
surface or be chemically converted to other gaseous N oxides, including
HNO<INF>3</INF>, as well as to particulate NO<INF>3</INF><SUP>-</SUP>.
Unlike particulate SO<INF>4</INF><SUP>2-</SUP>, which exists almost
entirely in the fine particle range, NO<INF>3</INF><SUP>-</SUP>
particles may occur either in the fine or coarse size range, such that
not all particulate NO<INF>3</INF><SUP>-</SUP> is a component of
PM<INF>2.5</INF>. Each form of oxidized N is removed from the
atmosphere at different rates by both dry and wet deposition. As a
general rule, the gas phase species tend to have shorter atmospheric
lifetimes, either dry depositing (e.g., as HNO<INF>3</INF>) or quickly
converting to particulate NO<INF>3</INF><SUP>-</SUP>. Particulate
NO<INF>3</INF><SUP>-</SUP> is more efficiently removed by precipitation
(wet deposition) and has a similar atmospheric lifetime as particulate
SO<INF>4</INF><SUP>2-</SUP> (2-10 days).
In addition to N oxides, there is another category of nitrogen
pollutants, referred to as reduced nitrogen, which is distinct from N
oxides but also contributes to nitrogen deposition. The most common
form of reduced N in the air is ammonia gas (NH<INF>3</INF>). Sources
of NH<INF>3</INF> emissions include livestock waste (49% of total in
2020 NEI), fertilizer application (33%) and aggregate fires (11%).
Ammonia tends to dry deposit near sources (PA, section 2.1.3). It can
also be converted to particle form, as ammonium (NH<INF>4</INF>\+\),
which can be transported farther distances and is most efficiently
removed by precipitation (PA, section 2.1.3). Ammonia, unlike N oxides
or PM<INF>2.5</INF>, is not a criteria pollutant and is not directly
regulated under CAA section 109.
In sum, particulate matter is both emitted to the atmosphere and
can be formed in the atmosphere from precursor chemical gases (such as
is the case for NO<INF>X</INF> and SO<INF>X</INF>). The components of
PM<INF>2.5</INF> mass that contribute to S and N deposition are
secondary products formed in the atmosphere after being emitted (e.g.,
particulate sulfate, particulate NO<INF>3</INF><SUP>-</SUP>,
NH<INF>4</INF>\+\). There are other components of PM<INF>2.5</INF> mass
that do not contribute to S and N deposition, e.g., black carbon,
organic carbon, dust (PA, section 2.4.3).
2. Recent Trends in Emissions, Concentrations, and Deposition
Emissions of SO<INF>X</INF>, oxides of N, and PM have declined
dramatically over the past two decades, continuing a longer-term trend
(PA, section 2.2). NEI data indicate an 87% decrease in total
SO<INF>2</INF> emissions between 2002 and 2022, including reductions of
91% in emissions from electricity generating units and 96% in emissions
from mobile
[[Page 26633]]
sources. Total anthropogenic NO<INF>X</INF> emissions have also trended
downward across the U.S. between 2002 and 2022 at only slightly smaller
percentages than SO<INF>2</INF>. Nationwide estimates indicate a 70%
decrease in anthropogenic NO<INF>X</INF> emissions over this time
period, driven in part by large emission reductions in the highway
vehicle sector (84%) and from stationary fuel combustion (68%) (PA,
section 2.2.1). In contrast with these declining 20-year trends in
NO<INF>X</INF> and SO<INF>X</INF> emissions, the annual rate of
NH<INF>3</INF> emissions has increased by over 20 percent since 2002
(PA, section 2.2.3). The two largest contributors are emissions from
livestock waste and fertilizer application, which have increased by 11%
and 44%, respectively, from 2002 to 2022. These trends in emissions
have had ramifications for N deposition patterns across the U.S., as
described further below.
As expected, the large reductions in SO<INF>X</INF> and
NO<INF>X</INF> emissions have resulted in substantially lower ambient
air concentrations in recent years relative to what was observed in
previous periods. The State and Local Air Monitoring Stations (SLAMS)
network supports the implementation of the NAAQS. In 2021, all ambient
monitoring sites with valid SO<INF>2</INF> design values (n=333) \22\
are less than the level of the existing secondary standard (500 ppb)
\23\ and more than 75 percent of the sites have design values less than
20 ppb (PA, section 2.4.2). These values reflect a downward trend over
the past two decades with median 3-hour secondary SO<INF>2</INF> values
down substantially from 2000 levels (from ~50 ppb to ~10 ppb).
---------------------------------------------------------------------------
\22\ A design value is a statistic that summarizes the air
quality data for a given area in terms of the indicator, averaging
time, and form of the standard. Design values can be compared to the
level of the standard and are typically used to designate areas as
meeting or not meeting the standard and assess progress towards
meeting the NAAQS. Design values are computed and published annually
by EPA (<a href="https://www.epa.gov/air-trends/air-quality-designvalues">https://www.epa.gov/air-trends/air-quality-designvalues</a>).
\23\ The existing secondary standard for SO<INF>2</INF> is 0.5
ppm (500 ppb), as a 3-hour average, not to be exceeded more than
once per year.
---------------------------------------------------------------------------
Similar trends are evident in the data for the primary
SO<INF>2</INF> standard (annual 99th percentile of 1-hour daily maximum
concentrations, averaged over 3 years with a level of 75 ppb). In the
2019-2021 period, the maximum design value for the primary
SO<INF>2</INF> standard was 376 ppb at a monitoring site near an
industrial park in southeast Missouri. It is important to note that
peak and mean SO<INF>2</INF> concentrations are higher at source-
oriented sites than monitoring locations that are not source-oriented.
Additionally, it is not uncommon for there to be high SO<INF>2</INF>
values in areas with recurring volcanic eruptions (e.g., Hawaii). In
the mid-1990s, the median value of all sites with valid 1-hour
SO<INF>2</INF> design values often exceeded 75 ppb (PA, Figure 2-26).
Since then, the entire distribution of values (including source-
oriented sites) has continued to decline such that the median value
across the network of sites is now between 5 and 10 ppb (PA, Figure 2-
26). The EPA also evaluated trends in annual average SO<INF>2</INF>
data from 2000-2021 and observed improving trends of similar magnitude
with the longer-term (annual) averaging time. It is important to note
that both peak and mean SO<INF>2</INF> concentrations are higher at
source-oriented sites than monitoring locations that are not source-
oriented.
Regarding NO<INF>2</INF>, design values at all 399 sites with valid
secondary NO<INF>2</INF> design values (annual average concentrations)
in 2021 are less than the 53 ppb level of the existing secondary
standard,\24\ and the majority of sites (98 percent) have design values
that are less than 20 ppb. In 2021, the maximum was 30 ppb,\25\ and the
median was 7 ppb. As with SO<INF>2</INF>, the more recent
NO<INF>2</INF> design values also reflect a downward trend over the
past two decades. Median annual NO<INF>2</INF> design values across the
U.S. decreased by ~50% between 2000 and 2021 (15 ppb to 7 ppb).
---------------------------------------------------------------------------
\24\ Sites in the contiguous U.S. have met the existing
NO<INF>2</INF> secondary standard since around 1991 (PA, Figure 2-
22).
\25\ The maximum annual average NO<INF>2</INF> concentrations
has been at, slightly above or slightly below 30 ppb since about
2008, with the highest 3-year average value just above 30 ppb (PA,
Figures 2-22 and 7-9).
---------------------------------------------------------------------------
Likewise, the median of the annual average PM<INF>2.5</INF>
concentrations decreased substantially from 2000 to 2021 (from 12.8
[micro]g/m\3\ to 8 [micro]g/m\3\). The median of the annual 98th
percentile 24-hour PM<INF>2.5</INF> concentrations at the more than
1000 sites monitored also decreased, from 32 [micro]g/m\3\ in 2000 to
21 [micro]g/m\3\ in 2021. Although both the annual average and 98th
percentile 24-hour PM<INF>2.5</INF> concentrations decreased steadily
from the early 2000s until 2016, these values have fluctuated in recent
years due to large-scale wildfire events (PA, section 2.4.3; U.S. EPA,
2023, Figures 23 and 24).
These emission reductions and subsequent downward trends in air
concentrations have also contributed to a nationwide decrease in N and
S deposition (PA, sections 2.5.3 and 6.2.1). Total S deposition and N
deposition declined by 68% and 15%, respectively, calculated as a
nationwide, three-year average between 2000-2002 and 2019-2021 (PA,
section 6.2.1). The trend in S deposition is more robust than for N
because of the offsetting influence of increasing emissions of reduced
forms of nitrogen over the same timeframe. The largest reductions in
total S and N deposition are seen in regions downwind of point sources
and transportation corridors related to emission reductions from
electricity generating units and mobile sources.
3. Relationships Between Concentrations and Deposition
As the NAAQS are set in terms of pollutant concentrations, analyses
in the PA evaluated relationships between criteria pollutant
concentrations in ambient air and ecosystem deposition across the U.S.
We examined these relationships over a range of conditions (e.g.,
pollutant, region, time period), and considered deposition both near
sources and at distance (allowing for pollutant transport and
associated transformation). The findings of these analyses, described
in detail in Chapter 6 and Appendix 6A of the PA, have informed
consideration of indicators and levels for potential secondary
standards based on consideration of deposition-related effects (PA,
Chapter 7).
As is evident from the air quality-deposition analyses, relating
ecosystem deposition to ambient air concentrations is not
straightforward. Deposition rates vary across ecosystems nationally,
and there is not a simple one-to-one relationship between ambient air
concentrations of any one indicator and S or N deposition. As discussed
above, the atmospheric processes that lead from pollutant emissions
loading to eventual deposition to the earth's surface are complex.
Multiple chemicals, both gaseous and particulate, from multiple types
of sources contribute to S and N deposition. Further, both criteria
pollutants and non-criteria pollutants contribute to N deposition.
There are also multiple deposition pathways (i.e., dry deposition and
wet deposition) that can influence the spatial and temporal scales at
which deposition occurs, which vary by pollutant and pollutant phase.
In light of these challenges, the PA employed five different
approaches for considering relationships between S and N deposition
rates and ambient air concentrations. First, as part of a ``real-world
experiment,'' the PA analyses leveraged the recent downward trends in
NO<INF>X</INF> and SO<INF>X</INF> emissions and corresponding air
quality concentrations as well as the trends in deposition estimates
(TDep or total deposition) to examine the correlation between
[[Page 26634]]
observed decreases in emissions and concentration and observed changes
in deposition over the past two decades (PA, section 6.2.1). The TDep
estimates used in these analyses are based on a hybrid approach that
involves a fusion of measured and modeled values, where measured values
are given more weight at the monitoring locations and modeled data are
used to fill in spatial gaps and provide information on chemical
species that are not measured by routine monitoring networks (Schwede
and Lear, 2014).\26\ For the second approach, we assessed how air
quality concentrations and associated deposition levels are related
within a chemical-transport model (CMAQ \27\) both nationally and then
at certain Class I areas \28\ (PA, section 6.2.2.1) where additional
monitoring data are collected as part of the Clean Air Status and
Trends Network (CASTNET) and the Interagency Monitoring of Protected
Visual Environments (IMPROVE) networks. As a third approach, we
analyzed the relationships across a limited number of monitoring
locations (in Class I areas) where both air quality data (CASTNET and
IMPROVE) and wet deposition of S and N was measured to evaluate the
associations between concentrations and deposition at a local scale
(PA, section 6.2.2.2 and 6.2.2.3). The fourth approach also considered
the local associations between the two terms at the local scale but did
so using a broader set of ambient air concentration measurements (i.e.,
all valid SO<INF>2</INF>, NO<INF>2</INF>, and PM<INF>2.5</INF>
measurements at SLAMS across the U.S.) and a hybrid set of deposition
estimates (TDep) (PA, section 6.2.3).
---------------------------------------------------------------------------
\26\ Other than the estimates associated with the CMAQ analysis
(second approach referenced above), the deposition estimates used in
these analyses are those provided by the National Atmospheric
Deposition Program, TDep Science Committee. One of the outputs of
this effort are annual datasets of total deposition estimates in the
U.S., which are referred to as the TDep datasets (technical updates
available from NADP, 2021; ISA, appendix 2, section 2.6).
\27\ The CMAQ is a state of the science photochemical air
quality model that relies on scientific first principles to simulate
the concentration of airborne gases and particles and the deposition
of these pollutants back to Earth's surface under user-prescribed
scenarios. See <a href="https://www.epa.gov/cmaq">https://www.epa.gov/cmaq</a> for more detail.
\28\ Areas designated as Class I include all international
parks, national wilderness areas which exceed 5,000 acres in size,
national memorial parks which exceed 5,000 acres in size, and
national parks which exceed 6,000 acres in size, provided the park
or wilderness area was in existence on August 7, 1977. Other areas
may also be Class I if designated as Class I consistent with the
CAA.
---------------------------------------------------------------------------
Finally, in recognition of the fact that air quality at upwind
locations can also influence downwind deposition, the fifth approach
used a trajectory model (HYSPLIT--The Hybrid Single-Particle Lagrangian
Integrated Trajectory model) to identify upwind areas where emissions
might be expected to influence deposition at downwind ecoregions (PA,
section 6.2.4 and Appendix 6A).\29\ Once those potential zones of
influence were established, we evaluated the relationships between air
quality metrics for the three pollutants \30\ at sites within those
zones with deposition estimates in the downwind ecoregion, as 3-year
averages for five periods: 2001-2003, 2006-2008, 2010-2012, 2014-2016
and 2018-2020. The metrics, Ecoregion Air Quality Metrics (EAQMs),
include a weighted-average (EAQM-weighted) and a maximum metric (EAQM-
max). The EAQM-max is the maximum metric value among the sites linked
to the downwind ecoregion and, for the EAQM-weighted, the value of each
site linked to the downwind ecoregion was weighted by how often the
forward HYSPLIT trajectory crossed into the ecoregion, i.e., sites with
more frequent trajectory intersections with the ecoregion are weighted
higher (PA, section 6.2.4.1).
---------------------------------------------------------------------------
\29\ Upwind sites of influence were identified for all 84
ecoregions (level III categorization) in the contiguous U.S.
\30\ For SO<INF>2</INF>, there were two sets of metrics: one
based on an annual average and one based on the 2nd highest 3-hour
maximum concentration in the year. Both the NO<INF>2</INF> and
PM<INF>2.5</INF> metrics are annual averages. For relating to 3-year
average deposition, all are averaged across three years.
---------------------------------------------------------------------------
As with any assessment, there are uncertainties and limitations
associated with the analyses summarized above. These are more fully
discussed in the PA (PA, sections 6.3 and 6.4). The evaluation of
measured air quality concentrations (SO<INF>2</INF>, NO<INF>2</INF>,
and PM<INF>2.5</INF>) and TDep estimates of deposition at all SLAMS
(generally composed of sites that use either a Federal Reference Method
[FRM] or a Federal Equivalence Method [FEM]) is a robust analysis
(i.e., large number of monitors distributed across the U.S.) and
particularly relevant given that compliance with the current standards
(both primary and secondary) is judged using design value metrics based
on measurements at the current SO<INF>2</INF>, NO<INF>2</INF> and
PM<INF>2.5</INF> monitors. However, these site-based comparisons do not
account for deposition associated with the transport of pollutants
emitted some distance upwind. Each of the other analyses completed to
bolster this analysis have their own limitations ranging from model
uncertainty to limited geographical scope.
The full set of quantitative results of the characterization of air
quality and deposition relationships are discussed more thoroughly in
Chapter 6 and Appendix 6A of the PA. In combination, these analyses
supported the PA conclusion of a strong association between
SO<INF>2</INF> and S deposition. Regarding N oxides and PM, however,
the results, and associated information, indicated more variable
relationships between NO<INF>2</INF> concentrations and N deposition,
and PM<INF>2.5</INF> concentrations with either S or N deposition.
For SO<INF>2</INF>, annual monitored SO<INF>2</INF> concentrations,
at existing monitors within the SLAMS network, averaged over 3 years at
the national scale were highly correlated to S deposition estimates in
the TDep dataset at the local scale (correlation coefficient of
0.70),\31\ especially in the earlier periods of the record and across
the eastern U.S. (PA, section 6.2.3). This association was confirmed by
the relationships between SO<INF>2</INF> annual values at the
identified upwind sites of influence and S deposition estimates from
TDep in downwind ecoregions, especially in those locations where the
annual average SO<INF>2</INF> concentrations are greater than 5 ppb
(PA, section 6.2.4.2). Finally, we note that the observed declines in
national levels of S deposition over the past two decades have occurred
during a period in which emissions of SO<INF>2</INF> have also declined
sharply (PA, sections 6.2.1 and 6.4.1).
---------------------------------------------------------------------------
\31\ The correlation coefficients are based on Spearman's rank
correlation coefficient. These coefficients are generally used to
assess how well the relationship between two variables can be
described via a monotonic function. The term ``r value'' is
sometimes used as shorthand for this correlation coefficient. Higher
values indicate that the two variables are highly associated with
one another (can range from 1.0 to -1.0).
---------------------------------------------------------------------------
Analyses in the PA also investigated relationships between S
deposition and air quality metrics other than the current indicator
species (SO<INF>2</INF>) in a limited number of circumstances. For
example, an evaluation of the association between
SO<INF>4</INF><SUP>2-</SUP> and total S deposition across 27 Class I
areas where data for both parameters were available, concluded that the
correlations between particle sulfate and total sulfate (i.e.,
SO<INF>2</INF> + SO<INF>4</INF><SUP>2-</SUP>) with total S deposition
(correlation coefficients of 0.55 and 0.61, respectively) was lower
than what was exhibited for SO<INF>2</INF> and S deposition at the
SLAMS (PA, section 6.2.2). The analyses also concluded that there was
poor correlation (correlation coefficient of 0.33) between
PM<INF>2.5</INF> mass, as measured at IMPROVE sites, with total S
deposition estimates for those sites (PA, sections 2.3.3 and 6.2.2.3).
While these analyses are based on data at a relatively limited number
of sites, as compared to the SLAMS network, the
[[Page 26635]]
results suggest that there are no clear advantages to considering
PM<INF>2.5</INF> mass, particle sulfate, or total sulfate as an
indicator for a secondary NAAQS, over using SO<INF>2</INF>.
Both NO<INF>2</INF> and certain components of PM<INF>2.5</INF> can
contribute to N deposition. As is the case for SO<INF>2</INF> and S
deposition, there are multiple pathways for N deposition (dry and wet),
and multiple scales of N deposition (local and regional). However,
there are some additional complications in the consideration of how air
quality concentrations (i.e., NO<INF>2</INF> and PM<INF>2.5</INF> mass)
are associated with eventual N deposition. First, not all N deposition
is caused by the criteria pollutants (PA, Chapter 2 and section 6.1.1).
Ammonia emissions also lead to N deposition, especially through dry
deposition at local scales. Second, only certain components of
PM<INF>2.5</INF> mass contribute to N deposition (i.e.,
NO<INF>3</INF><SUP>-</SUP> and NH<INF>4</INF>\+\). As a result of these
two factors, the association between NO<INF>2</INF> concentrations and
N deposition, and PM<INF>2.5</INF> concentrations and N deposition is
less robust than what is observed for SO<INF>2</INF>. Our multi-faceted
approach to evaluating these relationships confirmed this expectation.
For example, when comparing NO<INF>2</INF> observations at SLAMS across
the U.S. against the N deposition estimates from TDep, there are weaker
associations than what is observed in the similar SO<INF>2</INF>
comparisons (PA, section 6.4.2). There is little correlation for N
deposition with concentrations of NO<INF>2</INF>, as evidenced by a
Spearman's correlation coefficient of 0.38, compared to 0.70 for
SO<INF>2</INF> and S deposition (PA, Table 6-6 and Table 6-4). Further,
the trajectory-based analyses of the relationships between
NO<INF>2</INF> annual values in the identified upwind zones of
influence and N deposition estimates from TDep in downwind ecoregions
indicate negative correlations (PA, Table 6-10). These negative
correlations are observed for both the EAQM-weighed and EAQM-max
values. This relative lack of association was confirmed by considering
national trends over the past 20 years, where sharp declines in
NO<INF>2</INF> emissions and concentrations are linked in time with
sharp declines in oxidized N deposition (PA, Table 6-2), but not
associated with recent trends in total or reduced atmospheric N
deposition. Since 2010, NO<INF>2</INF> concentrations have continued to
drop while N deposition has remained steady (PA, section 6.2.1). As
noted for S deposition and S compound metrics above, the PA also
investigated relationships between N deposition and air quality metrics
other than the current indicator species (NO<INF>2</INF>). Across the
27 Class I areas where collocated data were available, the PA evaluated
the relationships between several air quality parameters (e.g., nitric
acid, particulate NO<INF>3</INF><SUP>-</SUP>, and NH<INF>4</INF>\+\)
and, as for S deposition and S compound metrics, the PA concluded there
were no clear advantages over the consideration of NO<INF>2</INF> or
PM<INF>2.5</INF> mass. In sum, the evidence suggests that
NO<INF>2</INF> would be a weak indicator of total atmospheric N
deposition, especially in areas where ammonia is prevalent or where
PM<INF>2.5</INF> mass is dominated by species other than
NO<INF>3</INF><SUP>-</SUP> or NH<INF>4</INF>\+\ (PA, section 6.4.2).
C. Welfare Effects Evidence
The information summarized here is based on our scientific
assessment of the welfare effects evidence available in this review;
this assessment is documented in the ISA \32\ and its policy
implications are further discussed in the PA (and summarized in section
II.E.1 below). More than 3,000 studies are newly available since the
last review and considered in the ISA.\33\ While expanding the evidence
for some effect categories, studies on acid deposition, a key group of
effects from the last review, are largely consistent with the evidence
that was previously available. The subsections below briefly summarize
the following aspects of the evidence: the nature of welfare effects of
S oxides, N oxides and PM (section II.C.1); the potential public
welfare implications (section II.C.2); and exposure concentrations and
deposition-related metrics (section II.C.3).
---------------------------------------------------------------------------
\32\ The ISA builds on evidence and conclusions from previous
assessments, focusing on synthesizing and integrating the newly
available evidence (ISA, section IS.1.1). Past assessments are cited
when providing further details not repeated in newer assessments.
\33\ The study count and citations are available on the project
page for the ISA on the Health & Environmental Research Online
(HERO) website documents these studies (<a href="https://heronet.epa.gov/heronet/index.cfm/project/page/project_id/2965">https://heronet.epa.gov/heronet/index.cfm/project/page/project_id/2965</a>).
---------------------------------------------------------------------------
1. Nature of Effects
This welfare effects evidence base available in the current review
includes decades of extensive research on the ecological effects oxides
of nitrogen, oxides of sulfur and PM. In the sections below we
summarize the nature of the direct effects of gas-phase exposure to
oxides of nitrogen and sulfur (section II.C.1.a), acid deposition-
related ecological effects (section II.C.1.b), N enrichment and
associated effects (section II.C.1.c), and other effects (section
II.C.1.d).
a. Direct Effects of SO<INF>X</INF> and N Oxides
There is a well-established body of scientific evidence that has
shown that acute and chronic exposures to oxides of N and S, such as
SO<INF>2</INF>, NO<INF>2</INF>, NO, HNO<INF>3</INF> and peroxyacetyl
nitrate (PAN) in the air, are associated with negative effects on
vegetation. Such scientific evidence, as was available in 1971, was the
basis for the current secondary NAAQS for oxides of sulfur and oxides
of nitrogen. The current scientific evidence continues to demonstrate
such effects, with the ISA specifically concluding that the evidence is
sufficient to infer a causal relationship between gas-phase
SO<INF>2</INF> and injury to vegetation (ISA, Appendix 3, section
3.6.1), and between gas-phase NO, NO<INF>2</INF> and PAN and injury to
vegetation (ISA, Appendix 3, section 3.6.2). The ISA additionally
concluded the evidence to be sufficient to infer a causal relationship
between exposure to HNO<INF>3</INF> and changes to vegetation, noting
that experimental exposure can damage leaf cuticle of tree seedlings
and HNO<INF>3</INF> concentrations have been reported to have
contributed to declines in lichen species in the Los Angeles basin
(ISA, Appendix 3, section 3.6.3).
Specifically for SO<INF>X</INF>, high concentrations in the first
half of the twentieth century have been blamed for severe damage to
plant foliage that occurred near large ore smelters during that time
(ISA, Appendix 3, section 3.2). In addition to foliar injury, which is
usually a rapid response, SO<INF>2</INF> exposures have also been
documented to reduce plant photosynthesis and growth. The appearance of
foliar injury can vary significantly among species and growth
conditions (which affect stomatal conductance). For lichens, damage
from SO<INF>2</INF> exposure has been observed to include reduction in
metabolic functions that are vital for growth and survival (e.g.,
decreases in photosynthesis and respiration), damage to cellular
integrity (e.g., leakage of electrolytes), and structural changes (ISA,
Appendix 3, section 3.2; Belnap et al., 1993; Farmer et al., 1992,
Hutchinson et al., 1996).
Although there is evidence of plant injury associated with
SO<INF>2</INF> exposures dating back more than a century (ISA, Appendix
3, section 3.2), as exposures have declined in the U.S., some studies
in the eastern U.S. have reported increased growth in some
SO<INF>2</INF>-sensitive tree species (e.g., Thomas et al., 2013).
Although the authors attributed the growth response to reductions in
SO<INF>2</INF>-associated acid deposition, and related recovery from
soil acidification, the relative roles of different pathways are
[[Page 26636]]
unclear as a historical deposition record was not available (ISA,
Appendix 3, section 3.2). Other researchers have suggested that the
observed response was related to the fact that the trees were growing
on a limestone outcrop that could be well buffered from soil
acidification (Schaberg et al., 2014). This seems to suggest a somewhat
faster recovery than might be expected from deposition-related soil
acidification, which may indicate a relatively greater role for changes
in ambient air concentrations of SO<INF>2</INF>, in combination with
changes in other gases, than was previously understood (ISA, Appendix
3, section 3.2 and Appendix 5, section 5.2.1.3).
The evidence base evaluated in the 1993 Air Quality Criteria
Document for Oxides of N included evidence of phytotoxic effects of NO,
NO<INF>2,</INF> and PAN on plants through decreasing photosynthesis and
induction of visible foliar injury (U.S. EPA, 1993 [1993 AQCD]). The
1993 AQCD additionally concluded that concentrations of NO,
NO<INF>2</INF>, and PAN in the atmosphere were rarely high enough to
have phytotoxic effects on vegetation. Little new information is
available since that time on these phytotoxic effects at concentrations
currently observed in the U.S. (ISA, Appendix 3, section 3.3).
The evidence indicates that HNO<INF>3</INF> had a role in observed
declines in lichen species in the 1970s in the Los Angeles basin (ISA,
Appendix 3, section 3.3). A 2008 resampling of areas shown to be
impacted in the past by HNO<INF>3</INF> found community shifts,
declines in the most pollutant-sensitive lichen species, and increases
in abundance of nitrogen-tolerant lichen species compared to 1976-1977,
indicating that these lichen communities have not recovered and had
experienced additional changes (ISA, Appendix 3, section 3.4). The
recently available evidence on this topic also included a study of six
lichen species that reported changes in physiology and functioning
including decreased chlorophyll content and chlorophyll fluorescence,
decreased photosynthesis and respiration, and increased electrolyte
leakage from HNO<INF>3</INF> exposures for 2-11 weeks (daily peak
levels near 50 ppb) in controlled chambers. (ISA, Appendix 3, section
3.4).
b. Acid Deposition-Related Ecological Effects
The connection between SO<INF>X</INF> and N oxide emissions to
ambient air, atmospheric deposition of N and/or S, and the
acidification of acid-sensitive soils and surface waters is well
documented with many decades of evidence, particularly in the eastern
U.S. (ISA, section IS.5; Appendix 8, section 8.1). In the atmosphere,
SO<INF>X</INF> and N oxides undergo reactions to form various acidic
compounds that are removed from the atmosphere through deposition.
Acidifying deposition can affect biogeochemical processes in soils,
with ramifications for terrestrial biota and for the chemistry and
biological functioning of associated surface waters (ISA, Appendix 7,
section 7.1). These effects depend on the magnitude and rate of
deposition, as well as multiple biogeochemical processes that occur in
soils and waterbodies.
Soil acidification is influenced by the deposition of inorganic
acids (HNO<INF>3</INF> and H<INF>2</INF>SO<INF>4</INF>), and by
chemical and biological processes. When NO<INF>3</INF><SUP>-</SUP>, or
SO<INF>4</INF><SUP>2-</SUP> leach from soils to surface waters, an
equivalent number of positive cations, or countercharge, are also
transported. If the countercharge is provided by a base cation (e.g.,
calcium, [Ca\2+\], magnesium [Mg\2+\], sodium [Na\+\], or potassium
[K\+\]), rather than hydrogen ions (H\+\), the leachate is neutralized,
but the soil becomes more acidic from the hydrogen ions left behind and
the base saturation of the soil is reduced by the loss of the base
cation. Depending on the relative rates of soil processes that
contribute to the soil pools of H\+\ and base cations, such as
weathering, continued SO<INF>4</INF><SUP>2-</SUP> or
NO<INF>3</INF><SUP>-</SUP> leaching can deplete the soil base cation
pool, which contributes to increased acidity of the leaching soil
water, and by connection, the surface water. Accordingly, the ability
of a watershed to neutralize acidic deposition is determined by a
variety of biogeophysical factors including weathering rates, bedrock
composition, vegetation and microbial processes, physical and chemical
characteristics of soils, and hydrology (ISA Appendix 4, section 4.3).
(1) Freshwater Ecosystems
As was the case in the last review, the body of evidence available
in this review, including that newly available, is sufficient to infer
a causal relationship between N and S deposition and the alteration of
freshwater biogeochemistry (ISA, section IS.6.1). Additionally, based
on the previously available evidence, the current body of evidence is
also sufficient to conclude that a causal relationship exists between
acidifying deposition and changes in biota, including physiological
impairment and alteration of species richness, community composition,
and biodiversity in freshwater ecosystems (ISA, section IS.6.3).
The effects of acid deposition on aquatic systems depend largely
upon the ability of the system to neutralize additional acidic inputs
from the environment, whether from the atmosphere or from surface
inputs. There is a large amount of variability between freshwater
systems in this regard, which reflects their underlying geology as well
as their history of acidic inputs. Accordingly, different freshwater
systems (e.g., in different geographic regions) respond differently to
similar amounts of acid deposition. The main factor in determining
sensitivity is the underlying geology of an area and its ability to
provide soil base cations through weathering to buffer acidic inputs
(ISA, Appendix 8, section 8.5.1). As noted in the ISA, ``[g]eologic
formations having low base cation supply, due mainly to low soil and
bedrock weathering, generally underlie the watersheds of acid-sensitive
lakes and streams'' (ISA, Appendix 8, p. 8-58).
Longstanding evidence has well characterized the changes in
biogeochemical processes and water chemistry caused by N and S
deposition to surface waters and their watersheds and the ramifications
for biological functioning of freshwater ecosystems (ISA, Appendix 8,
section 8.1). The 2020 ISA found that the newly available scientific
research ``reflects incremental improvements in scientific knowledge of
aquatic biological effects and indicators of acidification as compared
with knowledge summarized in the 2008 ISA'' (ISA, Appendix 8, p. 8-80).
Previously and newly available studies ``indicate that aquatic
organisms in sensitive ecosystems have been affected by acidification
at virtually all trophic levels and that these responses have been well
characterized for several decades'' (ISA, Appendix 8, p. 8-80). For
example, information reported in the previous 2008 ISA ``showed
consistent and coherent evidence for effects on aquatic biota,
especially algae, benthic invertebrates, and fish that are most clearly
linked to chemical indicators of acidification'' (ISA, Appendix 8, p.
8-80). These indicators are surface water pH, base cation ratios, ANC,
and inorganic aluminum (Al) concentration (ISA, Appendix 8, Table 8-9).
The effects of waterbody acidification on fish species are
especially well understood in the scientific literature, and many
species (e.g., brown and brook trout and Atlantic salmon) have been
documented to have experienced adverse effects from acidification (ISA,
Appendix 8, section 8.3). Among these species, the earliest lifestages
are most
[[Page 26637]]
sensitive to acidic conditions. Many effects of acidic surface waters
on fish, particularly effects on gill function or structure, relate to
the combination of low pH and elevated dissolved Al (ISA, Appendix 8,
section 8.3.6.1). In general, biological effects in aquatic ecosystems
are primarily attributable to low pH and high inorganic aluminum
concentration (ISA, p. ES-14). Waterbody pH largely controls the
bioavailability of Al, which is toxic to fish, and aluminum
mobilization is largely confined to waters with a pH below about 5.5,
which the ISA describes as corresponding to an ANC in the range of
about 10 to 30 [micro]eq/L in low to moderate DOC waters of the
Northeast (ISA, Appendix 7, section 7.1.2.6 and Appendix 8, section
8.6.4).
The parameter ANC is an indicator of the buffering capacity of
natural waters against acidification. Although ANC does not directly
affect biota, it is an indicator of acidification that relates to pH
and aluminum levels (ISA, p. ES-14), or watershed characteristics like
base cation weathering (BCw) rate (ISA, Appendix 8, sections 8.1 and
8.3.6.3). Accordingly, ANC is commonly used to describe the potential
sensitivity of a freshwater system to acidification-related effects. It
can be measured in water samples and is also often estimated for use in
water quality modeling, as is done in the aquatic acidification risk
assessment for this review, as summarized in section II.D below. Water
quality models are generally better at estimating ANC than at
estimating other indicators of acidification-related risk, such as pH.
Acid neutralizing capacity is estimated as the molar sum of strong base
cations minus the molar sum of strong acid anions, specifically
including SO<INF>4</INF><SUP>2-</SUP> and NO<INF>3</INF><SUP>-</SUP>
(e.g., Driscoll et al., 1994). Thus, values below zero indicate a
deficit in the ability to buffer acidic inputs, and increasing values
above zero represent increasing buffering capability for acidic inputs.
Further, across waterbodies within impacted areas of Shenandoah
National Park streams and Adirondack Mountain lakes, a positive
relationship has often been observed between ANC and number of fish
species, at least for the ANC range from about zero to 50 [micro]eq/L
(ISA, Appendix 7, section 7.1.2.6; Cosby et al., 2006; Sullivan et al.,
2006, Bulger et al., 1999).
Values of ANC can also be influenced by high concentrations of
naturally occurring organic acids, which can reduce bioavailability of
Al, buffering effects usually associated with low pH and high total Al
concentrations (Waller et al., 2012; ISA, Appendix 8, section 8.3.6.4);
in waters where that occurs, ANC may not be a good indicator of risk to
biota.
In addition to acidity of surface waters quantified over weeks or
months, waterbodies can also experience spikes in acidity in response
to episodic precipitation or rapid snowmelt events. In these events
(hours-days), a surge or pulse of drainage water, containing acidic
compounds, is routed through upper soil horizons rather than the deeper
soil horizons that would usually provide buffering for acidic compounds
(ISA, Appendix 7, section 7.1). While some streams and lakes may have
chronic or base flow chemistry that provides suitable conditions for
aquatic biota, they may experience occasional acidic episodes with the
potential for deleterious consequences to sensitive biota (ISA,
Appendix 8, section 8.5). For example, in some impacted northeastern
waterbodies, ANC levels may dip below zero for hours to days or weeks
in response to such events, while waterbodies labeled chronically
acidic have ANC levels below zero throughout the year (ISA, Appendix 7,
section 7.1.1.2; Driscoll et al., 2001). Accordingly, headwater streams
in both the eastern and western U.S. tend to be more sensitive to such
episodes due to their smaller watersheds and, in the east, their
underlying geology (ISA, Appendix 8, section 8.5.1).
National survey data dating back to the early 1980s through 2004,
that were available for the 2008 ISA, indicated acidifying deposition
had acidified surface waters in the southwestern Adirondacks, New
England uplands, eastern portion of the upper Midwest, forested Mid-
Atlantic highlands, and Mid-Atlantic coastal plain (2008 ISA, section
4.2.2.3; ISA, Appendix 8, section 8.5.1). For example, a survey of
waterbodies in the Adirondacks in 1984-1987 found 27% of streams to
have ANC values below zero, with a minimum value of -134
microequivalents per liter ([mu]eq/L) (Sullivan et al., 2006). Values
of ANC below 20 [mu]eq/L in Shenandoah stream sites were associated
with fewer fish of sensitive species compared to sites with higher ANC
(Bulger et al., 1999). A more recent study of two groups of Adirondack
lakes for which water quality data were available from 1982 and 1992,
respectively, reported significant increases in ANC in the large
majority of those lakes, with the magnitude of the increases varying
across the lakes (Driscoll et al., 2016; ISA, Appendix 7, section
7.1.3.1). As described in the ISA, ``[a]cidic waters were mostly
restricted to northern New York, New England, the Appalachian Mountain
chain, upper Midwest, and Florida'' (ISA, Appendix 8, p. 8-60). Despite
the appreciable reductions in acidifying deposition that have occurred
in the U.S. since the 1960s and 1970s, aquatic ecosystems across the
U.S. are still experiencing effects from historical contributions of N
and S (ISA, Appendix 8, section 8.6).
(2) Terrestrial Ecosystems
There is longstanding evidence that changes in soil biogeochemical
processes caused by acidifying deposition of N and S to terrestrial
systems are linked to changes in terrestrial biota, with associated
impacts on ecosystem characteristics. The currently available evidence,
including that newly available in this review, supports and strengthens
this understanding (ISA, Appendix 5, section 5.1). Consistent with
conclusions in the last review, the current body of evidence is
sufficient to infer a causal relationship between acidifying deposition
and alterations of biogeochemistry in terrestrial ecosystems.
Additionally, and consistent with conclusions in the last review, the
current body of evidence is sufficient to infer a causal relationship
between acidifying N and S deposition and the alteration of the
physiology and growth of terrestrial organisms and the productivity of
terrestrial ecosystems. The current body of evidence is also sufficient
to conclude that a causal relationship exists between acidifying N and
S deposition and alterations of species richness, community
composition, and biodiversity in terrestrial ecosystems (2008 ISA,
sections 4.2.1.1 and 4.2.1.2; 2020 ISA, Appendix 4, section 4.1 and
Appendix 5, sections 5.7.1 and 5.7.2).
Deposition of acidifying compounds to acid-sensitive soils can
cause soil acidification, increased mobilization of Al from soil to
drainage water, and deplete the pool of exchangeable base cations in
the soil (ISA, Appendix 5, section 5.2 and Appendix 4, sections 4.3.4
and 4.3.5). The physiological effects of acidification on terrestrial
biota include slower growth and increased mortality among sensitive
plant species, which are generally attributable to physiological
impairment caused by Al toxicity (related to increased availability of
inorganic Al in soil water) and a reduced ability of plant roots to
take up base cations (ISA, Appendix 4, section 4.3 and Appendix 5,
section 5.2). The U.S. tree species most studied with regard to effects
of acid deposition are red spruce and sugar maple, although there is
also evidence for other tree species such as flowering
[[Page 26638]]
dogwood (ISA, Appendix 5, section 5.2.1).
The physiological effects of acidifying deposition on terrestrial
biota can also result in changes in species composition whereby
sensitive species are replaced by more tolerant species, or the
sensitive species that were dominant in the community become a
minority. For example, increasing soil cation availability (as in
Ca\2+\ addition or gradient experiments) was associated with greater
growth and seedling colonization for sugar maple while American beech
was more prevalent on soils with lower levels of base cations where
sugar maple is less often found (ISA, Appendix 5, section 5.2.1.3.1;
Duchesne and Ouimet, 2009). In a study of understory species
composition, soil acid-base chemistry was found to be a predictor of
understory species composition (ISA, Appendix 5, section 5.2.2.1).
Additionally, limited evidence, including a recent S addition study and
agricultural soil gradient study, indicated that soil acid-base
chemistry predicted and was correlated with diversity and composition
of soil bacteria, fungi, and nematodes (ISA, Appendix 5, section
5.2.4.1).
In addition to Ca\2+\ addition experiments, the recently available
evidence also includes addition or gradient studies evaluating
relationships between soil chemistry indicators of acidification (e.g.,
soil pH, base cation to aluminum (Bc:Al) ratio, base saturation, and
Al) and ecosystem biological endpoints, including physiological and
community responses of trees and other vegetation, lichens, soil biota,
and fauna (ISA, Appendix 5, Tables 5-2 and 5-6). Further, the 2020 ISA
reports on several large observational studies evaluating statistical
associations between tree growth or survival, as assessed at monitoring
sites across the U.S. and estimates of average deposition of S or N
compounds at those sites over time periods on the order of 10 years
(ISA, Appendix 5, section 5.5.2 and Appendix 6, section.6.2.3.1; Dietze
and Moorcroft, 2011; Thomas et al., 2010; Horn et al., 2018). Negative
associations were observed for survival and growth in several species
or species groups with S deposition metrics; positive and negative
associations were reported with N deposition (PA, sections 5.3.2.3 and
5.3.4 and Appendix 5B).
Although there has been no systematic national survey of U.S.
terrestrial ecosystem soils, the forest ecosystems considered the most
sensitive to terrestrial acidification from atmospheric deposition
include forests of the Adirondack Mountains of New York, Green
Mountains of Vermont, White Mountains of New Hampshire, the Allegheny
Plateau of Pennsylvania, and mountain top and ridge forest ecosystems
in the southern Appalachians (2008 ISA, Appendix 3, section 3.2.4.2;
ISA, Appendix 5, section 5.3). Underlying geology is the principal
factor governing the sensitivity of both terrestrial and aquatic
ecosystems to acidification from S and N deposition. Geologic
formations with low base cation supply (e.g., sandstone, quartzite),
due mainly to low weathering rates, generally underlie these acid
sensitive watersheds. Other factors also contribute to the overall
sensitivity of an area to acidifying nitrogen and sulfur deposition,
including topography, soil chemistry, land use, and hydrology (ISA,
Appendix 5, section 5.3). For example, ``[a]cid-sensitive ecosystems
are mostly located in upland mountainous terrain in the eastern and
western U.S. and are underlain by bedrock that is resistant to
weathering, such as granite or quartzite sandstone'' (ISA, Appendix 7,
p. 7-45). Further, as well documented in the evidence, biogeochemical
sensitivity to deposition-driven acidification (and eutrophication [see
section 4.3 below]) is the result of historical loading, geologic/soil
conditions (e.g., mineral weathering and S adsorption), and
nonanthropogenic sources of N and S loading to the system (ISA,
Appendix 7, section 7.1.5).
Recently available evidence includes some studies describing early
stages of recovery from soil acidification in some eastern forests. For
example, studies at the Hubbard Brook Experimental Forest in New
Hampshire reported indications of acidification recovery in soil
solution measurements across the period from 1984 to 2011 (ISA,
Appendix 4, section 4.6.1; Fuss et al., 2015). Another study of 27
sites in eastern Canada and the northeastern U.S. found reductions in
wet deposition SO<INF>4</INF><SUP>2-</SUP> were associated with
increases in soil base saturation and decreases in exchangeable Al
(ISA, Appendix 4, section 4.6.1; Lawrence et al., 2015). Recent
modeling analyses indicate extended timeframes for recovery are likely,
as well as delays or lags related to accumulated pools of S in forest
soils (ISA, Appendix 4, section 4.6.1).
c. Nitrogen Enrichment and Associated Ecological Effects
The numerous ecosystem types that occur across the U.S. have a
broad range of sensitivity to N enrichment. Organisms in their natural
environments are commonly adapted to the nutrient availability in those
environments. Historically, N has been the primary limiting nutrient
for plants in many ecosystems. In such ecosystems, when the limiting
nutrient, N, becomes more available, whether from atmospheric
deposition, runoff, or episodic events, the subset of plant species
able to most effectively use the higher nitrogen levels may out-compete
other species, leading to a shift in the community composition that may
be dominated by a smaller number of species, i.e., a community with
lower diversity (ISA, sections IS.6.1.1.2, IS.6.2.1.1 and IS.7.1.1,
Appendix 6, section 6.2.4 and Appendix 7, section 7.2.6.6). Thus,
change in the availability of nitrogen in nitrogen-limited systems can
affect growth and productivity, with ramifications on relative
abundance of different species of vegetation, and potentially further
and broader ramifications on ecosystem processes, structure, and
function.
Both N oxides and reduced forms of nitrogen (NH<INF>X</INF>) can
contribute to N enrichment. In addition to atmospheric deposition,
other sources of S and N can play relatively greater or lesser roles in
contributing to S and N inputs, depending on location. For example,
many waterbodies receive appreciable amounts of N from agricultural
runoff and municipal or industrial wastewater discharges. For many
terrestrial and freshwater ecosystems, sources of N other than
atmospheric deposition, including fertilizer and waste treatment,
contribute to ecosystem total N with contributions that can be larger
than that from atmospheric deposition (ISA Appendix 7, sections 7.1 and
7.2). Additionally, the impacts of historic deposition in both aquatic
and terrestrial ecosystems pose complications to discerning the
potential effects of more recent lower deposition rates.
(1) Aquatic and Wetland Ecosystems
Nitrogen additions, including from atmospheric deposition, to
freshwater, estuarine and near-coastal ecosystems can contribute to
eutrophication, which typically begins with nutrient-stimulated rapid
algal growth developing into an algal bloom that can, depending on
various site-specific factors, be followed by anoxic conditions
associated with the algal die-off (ISA, ES.5.2). Decomposition of the
plant biomass from the subsequent algal die-off contributes to reduced
waterbody oxygen, which in turn can affect higher-trophic-level
species, e.g., contributing to fish mortality (ISA, p. ES-18).
The extensive body of evidence in this area is sufficient to infer
causal relationships between N deposition and
[[Page 26639]]
the alteration of biogeochemistry in freshwater, estuarine and near-
coastal marine systems (ISA, Appendix 7, sections 7.1 and 7.2).
Further, consistent with findings in the last review, the current body
of evidence is sufficient to infer a causal relationship between N
deposition and changes in biota, including altered growth and
productivity, species richness, community composition, and biodiversity
due to N enrichment in freshwater ecosystems (ISA, Appendix 9, section
9.1). The body of evidence is sufficient to infer a causal relationship
between N deposition and changes in biota, including altered growth,
total primary production, total algal community biomass, species
richness, community composition, and biodiversity due to N enrichment
in estuarine environments (ISA, Appendix 10, section 10.1).
The impact of N additions on wetlands, and whether they may serve
as a source, sink, or transformer of atmospherically deposited N, is
extremely variable and depends on the type of wetland and other
factors, such as physiography, and local hydrology, as well as climate
(ISA, section IS.8.1 and Appendix 11, section 11.1). Studies generally
show N enrichment to decrease the ability of wetlands to retain and
store N, which may diminish the wetland ecosystem service of improving
water quality (ISA, section IS.8.1). Consistent with the evidence
available in the last review, the current body of evidence is
sufficient to infer a causal relationship between N deposition and the
alteration of biogeochemical cycling in wetlands. Newly available
evidence regarding N inputs and plant physiology expands the evidence
base related to species diversity. The currently available evidence,
including that newly available, is sufficient to infer a causal
relationship between N deposition and the alteration of growth and
productivity, species physiology, species richness, community
composition, and biodiversity in wetlands (ISA, Appendix 11, section
11.10).
The relative contribution of atmospheric deposition to total
wetland N loading varies with wetland type, with bogs receiving the
greatest contribution and accordingly being most vulnerable to nutrient
enrichment effects of N deposition (ISA, Appendix 11, section 11.1).
For example, bogs, which receive 70-100% of hydrological input from
rainfall, are more sensitive to N deposition than fens (55-83% as
rainfall), which are more sensitive than coastal wetlands (10-20% as
rainfall) (ISA, Appendix 11, section 11.10). For freshwater fens,
marshes, and swamps, inputs from ground and surface water are often of
similar order of magnitude as that from precipitation, while estuarine
and coastal wetlands receive water from multiple sources, with
precipitation being among the smaller of those sources (ISA, Appendix
11, section 11.1).
Nitrogen loading and other factors contribute to nutrient
enrichment, which contributes to eutrophication. Such nitrogen-driven
eutrophication alters freshwater biogeochemistry and can impact
physiology, survival, and biodiversity of sensitive aquatic biota.
Evidence newly available in this review provides insights regarding N
enrichment and its impacts in several types of aquatic systems,
including freshwater streams and lakes, estuarine and near-coastal
systems, and wetlands. With regard to freshwaters, for example, studies
published since the 2008 ISA augment the evidence base for high-
elevation waterbodies where the main N source is atmospheric
deposition. Recent evidence continues to indicate that N limitation is
common in oligotrophic waters in the western U.S., with shifts in
nutrient limitation, from N limitation, to between N and phosphorus (P)
limitation, or to P limitation, reported in some alpine lake studies
(ISA, Appendix 9, section 9.1.1.3). Small inputs of N in such water
bodies have been reported to increase nutrient availability or alter
the balance of N and P, with the potential to stimulate growth of
primary producers and contribute to changes in species richness,
community composition, and diversity.
Another type of N loading effect in other types of freshwater lakes
includes a role in the composition of freshwater algal blooms and their
toxicity (ISA, Appendix 9, section 9.2.6.1). Information in this
review, including studies in Lake Erie, indicates that growth of some
harmful algal species, including those that produce microcystin, are
favored by increased availability of N and its availability in
dissolved inorganic form (ISA, Appendix 9, p. 9-28; Davis et al., 2015;
Gobler et al., 2016).
The relative contribution of N deposition to total N loading varies
among waterbodies. For example, atmospheric deposition is generally
considered to be the main source of N inputs to most headwater stream,
high-elevation lake, and low-order stream watersheds that are far from
the influence of other N sources like agricultural runoff and
wastewater effluent (ISA, section ES5.2). In other fresh waterbodies,
however, agricultural practices and point source discharges have been
estimated to be larger contributors to total N loading (ISA, Appendix
7, section 7.1.1.1). Since the 2008 ISA, several long-term monitoring
studies in the Appalachian Mountains, the Adirondacks, and the Rocky
Mountains have reported temporal patterns of declines in surface water
NO<INF>3</INF><SUP>-</SUP> concentration corresponding to declines in
atmospheric N deposition (ISA, Appendix 9, section 9.1.1.2). Declines
in basin wide NO<INF>3</INF><SUP>-</SUP> concentrations have also been
reported for the nontidal Potomac River watershed and have been
attributed to declines in atmospheric N deposition (ISA, Appendix 7,
section 7.1.5.1).
Nutrient inputs to coastal and estuarine waters are important
influences on the health of these waterbodies. Continued inputs of N,
the most common limiting nutrient in estuarine and coastal systems,
have resulted in N over-enrichment and subsequent alterations to the
nutrient balance in these systems (ISA, Appendix 10, p. 10-6). For
example, the rate of N delivery to coastal waters is strongly
correlated to changes in primary production and phytoplankton biomass
(ISA, Appendix 10, section 10.1.3). Algal blooms and associated die-
offs can contribute to hypoxic conditions (most common during summer
months), which can contribute to fish kills and associated reductions
in marine populations (ISA, Appendix 10). Further, the prevalence and
health of submerged aquatic vegetation (SAV), which is important
habitat for many aquatic species, has been identified as a biological
indicator for N enrichment in estuarine waters (ISA, Appendix 10,
section 10.2.5). Previously available evidence indicated the role of N
loading in SAV declines in multiple U.S. estuaries through increased
production of macroalgae or other algae, which reduce sunlight
penetration into shallow waters where SAV is found (ISA, Appendix 10,
section 10.2.3). Newly available studies have reported findings of
increased SAV populations in two tributaries of the Chesapeake Bay
corresponding to reduction in total N loading from all sources since
1990 (ISA, Appendix 10, section 10.2.5). The newly available studies
also identify other factors threatening SAV, including increasing
temperature related to climate change (ISA, Appendix 10, section
10.2.5).
The degree to which N enrichment and associated ecosystem impacts
are driven by atmospheric N deposition varies greatly and is largely
unique to the specific ecosystem. Analyses based on data across two to
three decades
[[Page 26640]]
extending from the 1990s through about 2010 estimate that most of the
analyzed estuaries receive 15-40% of their N inputs from atmospheric
sources (ISA, section ES 5.2; ISA, Appendix 7, section 7.2.1), though
for specific estuaries contributions can vary more widely. In areas
along the West Coast, N sources may include coastal upwelling from
oceanic waters, as well as transport from watersheds. Common N inputs
to estuaries include those associated with freshwater inflows
transporting N from agriculture, urban, and wastewater sources, in
addition to atmospheric deposition across the watershed (ISA, section
IS 2.2.2; ISA, Appendix 7, section 7.2.1).
There are estimates of atmospheric N loading to estuaries available
from several recent modeling studies (ISA, Table 7-9). One analysis of
estuaries along the Atlantic Coast and the Gulf of Mexico, which
estimated that 62-81% of N delivered to the eastern U.S. coastal zone
is anthropogenic in source, also reported that atmospheric N deposition
to freshwater that is subsequently transported to estuaries represents
17-21% of the total N loading into the coastal zone (McCrackin et al.,
2013; Moore et al., 2011). In the Gulf of Mexico, 26% of the N
transported to the Gulf in the Mississippi/Atchafalaya River basin was
estimated to be contributed from atmospheric deposition (which may
include volatilized losses from natural, urban, and agricultural
sources) (Robertson and Saad, 2013). Another modeling analysis
identified atmospheric deposition to watersheds as the dominant source
of N to the estuaries of the Connecticut, Kennebec, and Penobscot
rivers. For the entire Northeast and mid-Atlantic coastal region,
however, it was the third largest source (20%), following agriculture
(37%) and sewage and population-related sources (28%) (ISA, Appendix 7,
section 7.2.1). Estimates for West Coast estuaries indicate much
smaller contribution from atmospheric deposition. For example, analyses
for Yaquina Bay, Oregon, estimated direct deposition to contribute only
0.03% of N inputs; estimated N input to the watershed from N fixing red
alder (Alnus rubra) trees was a much larger (8%) source (ISA, Appendix
7, section 7.2.1; Brown and Ozretich, 2009).
Evidence in coastal waters has recognized a role of nutrient
enrichment in acidification of some coastal waters (ISA, Appendix 10,
section 10.5). More specifically, nutrient-driven algal blooms may
contribute to ocean acidification, possibly through increased
decomposition, which lowers dissolved oxygen levels in the water column
and contributes to lower pH. Such nutrient-enhanced acidification can
also be exacerbated by warming (associated with increased microbial
respiration) and changes in buffering capacity (alkalinity) of
freshwater inputs (ISA, Appendix 10, section 10.5).
(2) Terrestrial Ecosystems
It is long established that N enrichment of terrestrial ecosystems
increases plant productivity (ISA, Appendix 6, section 6.1). Building
on this, the currently available evidence, including evidence that is
longstanding, is sufficient to infer a causal relationship between N
deposition and the alteration of the physiology and growth of
terrestrial organisms and the productivity of terrestrial ecosystems
(ISA, Appendix 5, section 5.2 and Appendix 6, section 6.2). Responsive
ecosystems include those that are N limited and/or contain species that
have evolved in nutrient-poor environments. In these ecosystems the N-
enrichment changes in plant physiology and growth rates vary among
species, with species that are adapted to low N supply being readily
outcompeted by species that require more N. In this manner, the
relative representation of different vegetation species may be altered,
and some species may be eliminated altogether, such that community
composition is changed and species diversity declines (ISA, Appendix 6,
sections 6.3.2 and 6.3.8). The currently available evidence in this
area is sufficient to infer a causal relationship between N deposition
and the alteration of species richness, community composition, and
biodiversity in terrestrial ecosystems (ISA, section IS.5.3 and
Appendix 6, section 6.3).
Previously available evidence described the role of N deposition in
changing soil carbon and N pools and fluxes, as well as altering plant
and microbial growth and physiology in an array of terrestrial
ecosystems (ISA, Appendix 6, section 6.2.1). Nitrogen availability is
broadly limiting for productivity in many terrestrial ecosystems (ISA,
Appendix 6, section 6.2.1). Accordingly, N additions contribute to
increased productivity and can alter biodiversity. Eutrophication, one
of the mechanisms by which increased productivity and changes in
biodiversity associated with N addition to terrestrial ecosystems can
occur, comprises multiple effects that include changes to the
physiology of individual organisms, alteration of the relative growth
and abundance of various species, transformation of relationships
between species, and indirect effects on availability of essential
resources other than N, such as light, water, and nutrients (ISA,
Appendix 6, section 6.2.1).
The currently available evidence for the terrestrial ecosystem
effects of N enrichment, including eutrophication, includes studies in
a wide array of systems, including forests (tropical, temperate, and
boreal), grasslands, arid and semi-arid scrublands, and tundra (PA,
section 4.1; ISA, Appendix 6). The organisms affected include trees,
herbs and shrubs, and lichen, as well as fungal, microbial, and
arthropod communities. Lichen communities, which have important roles
in hydrologic cycling, nutrient cycling, and as sources of food and
habitat for other species, are also affected by atmospheric N (PA,
section 4.1; ISA, Appendix 6). The recently available studies on the
biological effects of added N in terrestrial ecosystems include
investigations of plant and microbial physiology, long-term ecosystem-
scale N addition experiments, regional and continental-scale monitoring
studies, and syntheses.
The previously available evidence included N addition studies in
the U.S. and N deposition gradient studies in Europe that reported
associations of N deposition with reduced species richness and altered
community composition for grassland plants, forest understory plants,
and mycorrhizal fungi (soil fungi that have a symbiotic relationship
with plant roots) (ISA, Appendix 6, section 6.3). New evidence for
forest communities in this review indicates that N deposition alters
the physiology and growth of overstory trees, and that N deposition has
the potential to change the community composition of forests (ISA,
Appendix 6, section 6.6). Recent studies on forest trees include
analyses of long-term forest inventory data collected from across the
U.S. and Europe (ISA, Appendix 6, section 6.2.3.1). The recent evidence
also includes findings of variation in forest understory and non-forest
plant communities with atmospheric N deposition gradients in the U.S.
and in Europe. For example, gradient studies in Europe have found
higher N deposition to be associated with forest understory plant
communities with more nutrient-demanding and shade-tolerant plant
species (ISA, Appendix 6, section 6.3.3.2). A recent gradient study in
the U.S. found associations between herb and shrub species richness and
N deposition, that were highly dependent on soil pH (ISA, Appendix 6,
section 6.3.3.2).
Recent evidence includes associations of variation in lichen
community
[[Page 26641]]
composition with N deposition gradients in the U.S. and Europe, (ISA,
Appendix 6, section 6.3.7; Table 6-23). Differences in lichen community
composition have been attributed to atmospheric N pollution in forests
throughout the West Coast, in the Rocky Mountains, and in southeastern
Alaska. Differences in epiphytic lichen growth or physiology have been
observed along atmospheric N deposition gradients in the highly
impacted area of southern California, and also in more remote locations
such as Wyoming and southeastern Alaska (ISA, Appendix 6, section
6.3.7). Historical deposition may play a role in observational studies
of N deposition effects, complicating the disentangling of responses
that may be related to more recent N loading.
Newly available findings from N addition experiments expand on the
understanding of mechanisms linking changes in plant and microbial
community composition to increased N availability. Such experiments in
arid and semi-arid environments indicate that competition for resources
such as water may exacerbate the effects of N addition on diversity
(ISA, Appendix 6, section 6.2.6). The newly available evidence includes
studies in arid and semiarid ecosystems, particularly in southern
California, that have reported changes in plant community composition,
in the context of a long history of significant N deposition, with
fewer observations of plant species loss or changes in plant diversity
(ISA, Appendix 6, section 6.3.6).
Nitrogen limitation in grasslands and the dominance by fast-growing
species that can shift in abundance rapidly (in contrast to forest
trees) contribute to an increased sensitivity of grassland ecosystems
to N inputs (ISA, Appendix 6, section 6.3.6). Studies in southern
California coastal sage scrub communities, including studies of the
long-term history of N deposition, which was appreciably greater in the
past than recent rates, indicate impacts on community composition and
species richness in these ecosystems (ISA, Appendix 6, sections 6.2.6
and 6.3.6). In summary, the ability of atmospheric N deposition to
override the natural spatial heterogeneity in N availability in arid
ecosystems, such as the Mojave Desert and California coastal sage scrub
ecosystems in southern California, makes these ecosystems sensitive to
N deposition (ISA, Appendix 6, section 6.3.8).
The current evidence includes relatively few studies of N
enrichment recovery in terrestrial ecosystems. Among N addition studies
assessing responses after cessation of additions, it has been observed
that soil nitrate and ammonium concentrations recovered to levels
observed in untreated controls within 1 to 3 years of the cessation of
additions, but soil processes such as N mineralization and litter
decomposition were slower to recover (ISA, Appendix 6, section 6.3.2;
Stevens, 2016). A range of recovery times have been reported for
mycorrhizal community composition and abundance from a few years in
some systems to as long as 28 or 48 years in others (ISA, Appendix 6,
section 6.3.2; Stevens, 2016; Emmett et al., 1998; Strengbom et al.,
2001). An N addition study in the midwestern U.S. observed that plant
physiological processes recovered in less than 2 years, although
grassland communities were slower to recover and still differed from
controls 20 years after the cessation of N additions (ISA, Appendix 6,
section 6.3.2; Isbell et al., 2013).
d. Other Deposition-Related Effects
Additional categories of effects for which the current evidence is
sufficient to infer causal relationships with deposition of S or N
compounds or PM include changes in mercury methylation processes in
freshwater ecosystems, changes in aquatic biota due to sulfide
phytotoxicity, and ecological effects from PM deposition (ISA, Table
IS-1). The current evidence, including that newly available in this
review, is sufficient to infer a causal relationship between S
deposition and the alteration of mercury methylation in surface water,
sediment, and soils in wetland and freshwater ecosystems. The process
of mercury methylation is influenced in part by surface water
SO<INF>4</INF><SUP>2-</SUP> concentrations, as well as the presence of
mercury. Accordingly, in waterbodies where mercury is present, S
deposition, particularly that associated with SO<INF>X,</INF> has a
role in production of methylmercury, which contributes to methylmercury
accumulation in fish (ISA, Appendix 12, section 12.8). Newly available
evidence has improved our scientific understanding of the types of
organisms involved in the methylation process, as well as the
environments in which they are found, and factors that influence the
process, such as oxygen content, temperature, pH, and carbon supply,
which themselves vary temporally, seasonally, and geographically (ISA,
Appendix 12, section 12.3). The currently available evidence is also
sufficient to infer a new causal relationship between S deposition and
changes in biota due to sulfide phytotoxicity, including alteration of
growth and productivity, species physiology, species richness,
community composition, and biodiversity in wetland and freshwater
ecosystems (ISA, section IS.9). Sulfur deposition can contribute to
sulfide and associated phytotoxicity in freshwater wetlands and lakes,
with the potential to contribute to effects on plant community
composition in freshwater wetlands (ISA, Appendix 12, section 12.2.3).
With regard to PM deposition, the currently available evidence is
sufficient to infer a likely causal relationship between deposition of
PM and a variety of effects on individual organisms and ecosystems
(ISA, Appendix 15, section 15.1). Particulate matter includes a
heterogeneous mixture of particles differing in origin, size, and
chemical composition. In addition to N and S and their transformation
products, other PM components, such as trace metals and organic
compounds, when deposited to ecosystems, may affect biota. Material
deposited onto leaf surfaces can alter leaf processes and PM components
deposited to soils and waterbodies may be taken up into biota, with the
potential for effects on biological and ecosystem processes. Studies
involving ambient air PM, however, have generally involved conditions
that would not be expected to meet the current secondary standards for
PM. Further, although in some limited cases, effects have been
attributed to particle size (e.g., soiling of leaves by large coarse
particles near industrial facilities or unpaved roads), ecological
effects of PM have been largely attributed more to its chemical
components, such as trace metals, which can be toxic in large amounts
(ISA, Appendix 15, sections 15.2 and 15.3.1). The evidence largely
comes from studies involving areas experiencing elevated concentrations
of PM, such as near industrial areas or historically polluted cities
(ISA, Appendix 15, section 15.4).
2. Public Welfare Implications
The public welfare implications of the evidence regarding S and N
related welfare effects are dependent on the type and severity of the
effects, as well as the extent of the effect at a particular biological
or ecological level of organization or spatial scale. We discuss such
factors here in light of judgments and conclusions regarding effects on
the public welfare that have been made in NAAQS reviews.
As provided in section 109(b)(2) of the CAA, the secondary standard
is to ``specify a level of air quality the attainment and maintenance
of which in the judgment of the Administrator . . . is requisite to
protect the public welfare from any known or anticipated adverse
effects associated with the presence of
[[Page 26642]]
such air pollutant in the ambient air.'' The secondary standard is not
meant to protect against all known or anticipated welfare effects
related to oxides of N and S, and particulate matter, but rather those
that are judged to be adverse to the public welfare, and a bright-line
determination of adversity is not required in judging what is requisite
(78 FR 3212, January 15, 2013; 80 FR 65376, October 26, 2015; see also
73 FR 16496, March 27, 2008). Thus, the level of protection from known
or anticipated adverse effects to public welfare that is requisite for
the secondary standard is a public welfare policy judgment made by the
Administrator. The Administrator's judgment regarding the available
information and adequacy of protection provided by an existing standard
is generally informed by considerations in prior reviews and associated
conclusions.
The categories of effects identified in the CAA to be included
among welfare effects are quite diverse, and among these categories,
any single category includes many different types of effects that are
of broadly varying specificity and level of resolution. For example,
effects on vegetation and effects on animals are categories identified
in CAA section 302(h), and the ISA recognizes effects of N and S
deposition at the organism, population, community, and ecosystem level,
as summarized in section II.C.1 above (ISA, sections IS.5 to IS.9). As
noted in the last review of the secondary NAAQS for NO<INF>X</INF> and
SO<INF>X</INF>, while the CAA section 302(h) lists a number of welfare
effects, ``these effects do not define public welfare in and of
themselves'' (77 FR 20232, April 3, 2012).
The significance of each type of effect with regard to potential
effects on the public welfare depends on the type and severity of
effects, as well as the extent of such effects on the affected
environmental entity, and on the societal use of the affected entity
and the entity's significance to the public welfare. Such factors have
been considered in the context of judgments and conclusions made in
some prior reviews regarding public welfare effects. For example, in
the context of secondary NAAQS decisions for ozone (O<INF>3</INF>),
judgments regarding public welfare significance have given particular
attention to effects in areas with special Federal protections (such as
Class I areas), and lands set aside by States, Tribes and public
interest groups to provide similar benefits to the public welfare (73
FR 16496, March 27, 2008; 80 FR 65292, October 26, 2015).\34\ In the
2015 O<INF>3</INF> NAAQS review, the EPA recognized the ``clear public
interest in and value of maintaining these areas in a condition that
does not impair their intended use and the fact that many of these
lands contain O<INF>3</INF>-sensitive species'' (73 FR 16496, March 27,
2008).
---------------------------------------------------------------------------
\34\ For example, the fundamental purpose of parks in the
National Park System ``is to conserve the scenery, natural and
historic objects, and wildlife in the System units and to provide
for the enjoyment of the scenery, natural and historic objects, and
wildlife in such manner and by such means as will leave them
unimpaired for the enjoyment of future generations'' (54 U.S.C.
100101). Additionally, the Wilderness Act of 1964 defines designated
``wilderness areas'' in part as areas ``protected and managed so as
to preserve [their] natural conditions'' and requires that these
areas ``shall be administered for the use and enjoyment of the
American people in such manner as will leave them unimpaired for
future use and enjoyment as wilderness, and so as to provide for the
protection of these areas, [and] the preservation of their
wilderness character . . .'' (16 U.S.C. 1131 (a) and (c)). Other
lands that benefit the public welfare include national forests which
are managed for multiple uses including sustained yield management
in accordance with land management plans (see 16 U.S.C. 1600(1)-(3);
16 U.S.C. 1601(d)(1)).
---------------------------------------------------------------------------
Judgments regarding effects on the public welfare can depend on the
intended use for, or service (and value) of, the affected vegetation,
ecological receptors, ecosystems and resources and the significance of
that use to the public welfare (73 FR 16496, March 27, 2008: 80 FR
65377, October 26, 2015). Uses or services provided by areas that have
been afforded special protection can flow in part or entirely from the
vegetation that grows there or other natural resources. Ecosystem
services range from those directly related to the natural functioning
of the ecosystem to ecosystem uses for human recreation or profit, such
as through the production of lumber or fuel (Constanza et al., 2017;
ISA, section IS.13). The spatial, temporal, and social dimensions of
public welfare impacts are also influenced by the type of service
affected. For example, a national park can provide direct recreational
services to the thousands of visitors that come each year, but also
provide an indirect value to the millions who may not visit but receive
satisfaction from knowing it exists and is preserved for the future (80
FR 65377, October 26, 2015).
In the last review of the secondary NAAQS for NO<INF>X</INF> and
SO<INF>X</INF>, ecosystem services were discussed as a method of
assessing the magnitude and significance to the public of resources
affected by ambient air concentrations of oxides of nitrogen and sulfur
and associated deposition in sensitive ecosystems (77 FR 20232, April
3, 2012). That review recognized that although there is no specific
definition of adversity to public welfare, one paradigm might involve
ascribing public welfare significance to disruptions in ecosystem
structure and function. The concept of considering the extent to which
a pollutant effect will contribute to such disruptions has been used
broadly by the EPA in considering effects. An evaluation of adversity
to public welfare might also consider the likelihood, type, magnitude,
and spatial scale of the effect, as well as the potential for recovery
and any uncertainties relating to these considerations (77 FR 20218,
April 3, 2012).
The types of effects on aquatic and terrestrial ecosystems
discussed in section II.C.1 above differ with regard to aspects
important to judging their public welfare significance. For example, in
the case of effects on timber harvest, such judgments may consider
aspects such as the heavy management of silviculture in the U.S., while
judgments for other categories of effects may generally relate to
considerations regarding natural areas, including specifically those
areas that are not managed for harvest. For example, effects on tree
growth and survival have the potential to be significant to the public
welfare through impacts in Class I and other areas given special
protection in their natural/existing state, although they differ in how
they might be significant.
In this context, it may be important to consider that S and N
deposition-related effects, such as changes in growth and survival of
plant and animal species, could, depending on severity, extent, and
other factors, lead to effects on a larger scale including changes in
overall productivity and altered community composition (ISA, section
IS.2.2.1 and Appendices 5, 6, 8, 9, and 10). Further, effects on
individual species could contribute to impacts on community composition
through effects on growth and reproductive success of sensitive species
in the community, with varying impacts to the system through many
factors including changes to competitive interactions (ISA, section
IS.5.2 and Appendix 6, section 6.3.2).
In acid-impacted surface waters, acidification primarily affects
the diversity and abundance of fish and other aquatic life, and the
ecosystem services derived from these organisms. (2011 PA, section
4.4.5). In addition to other types of services, fresh surface waters
support several cultural services, such as aesthetic, recreational, and
educational services. The type of service that is likely to be most
widely and significantly affected by aquatic
[[Page 26643]]
acidification is recreational fishing. Multiple studies have documented
the economic benefits of recreational fishing. Freshwater rivers and
lakes of the northeastern United States, surface waters that have been
most affected by acidification, are not a major source of commercially
raised or caught fish; they are, however, a source of food for some
recreational and subsistence fishers and for other consumers (2009 REA,
section 4.2.1.3). It is not known if and how consumption patterns of
these fishers may have been affected by the historical impacts of
surface water acidification in the affected systems. Non-use services,
which include existence (protection and preservation with no
expectation of direct use) and bequest values, are arguably a
significant source of benefits from reduced acidification (Banzhaf et
al., 2006). Since the 2012 review, additional approaches and methods
have been applied to estimate the potential effects of aquatic
acidification on uses and services of affected aquatic ecosystems; with
regard to economic impacts, however, ``for many regions and specific
services, poorly characterized dose-response between deposition,
ecological effect, and services are the greatest challenge in
developing specific data on the economic benefits of emission
reductions'' (ISA, Appendix 14, p. 14-23).
Nitrogen loading in aquatic ecosystems, particularly large
estuarine and coastal water bodies, has and continues to pose risks to
the services provided by those ecosystems, with clear implications to
the public welfare (2011 PA, section 4.4.2; ISA, Appendix 14, section
14.3.2). For example, the large estuaries of the eastern U.S. are an
important source of fish and shellfish production, capable of
supporting large stocks of resident commercial species and serving as
breeding grounds and interim habitat for several migratory species
(2009 REA, section 5.2.1.3). These estuaries also provide an important
and substantial variety of cultural ecosystem services, including
water-based recreational and aesthetic services. And as noted for fresh
waters above, these systems have non-use benefits to the public (2011
PA, section 4.4.5). Studies reviewed in the ISA have explored both
enumeration of the number of ecosystem services that may be affected by
N loading, and the pathways by which this may occur, as well as
approaches to valuation of such impacts. A finding of one such analysis
was that ``better quantitative relationships need to be established
between N and the effects on ecosystems at smaller scales, including a
better understanding of how N shortages can affect certain
populations'' (ISA, Appendix 14, sections 14.5 and 14.6). The relative
contribution of atmospheric deposition to total N loading varies widely
among estuaries, however, and has declined in more recent years (ISA,
Appendix 10, section 10.10.1).
A complication to consideration of public welfare implications that
is specific to N deposition in terrestrial systems is its potential to
increase growth and yield of plants, that may be agricultural and
forest crops. Such increased growth and yield may be judged and valued
differently than changes in growth of other species. As noted in
section II.C.1 above, enrichment in natural ecosystems can, by
increasing growth of N limited plant species, change competitive
advantages of species in a community, with associated impacts on the
composition of the ecosystem's plant community. The public welfare
implications of such effects may vary depending on their severity,
prevalence, or magnitude, such as with only those rising to a
particular severity (e.g., with associated significant impact on key
ecosystem functions or other services), magnitude or prevalence
considered of public welfare significance. Impacts on some of these
characteristics (e.g., forest or forest community composition) may be
considered of greater public welfare significance when occurring in
Class I or other protected areas, due to the value that the public
places on such areas. In considering such services in past reviews for
secondary standards for other pollutants (e.g., O<INF>3</INF>), the
Agency has given particular attention to effects in natural ecosystems,
indicating that a protective standard, based on consideration of
effects in natural ecosystems in areas afforded special protection,
would also ``provide a level of protection for other vegetation that is
used by the public and potentially affected by O<INF>3</INF> including
timber, produce grown for consumption and horticultural plants used for
landscaping'' (80 FR 65403, October 26, 2015).
Although more sensitive effects are described with increasingly
greater frequency in the evidence base of effects related to ecosystem
deposition of N and S compounds, the available information does not yet
provide a framework that can specifically tie various magnitudes or
prevalences of changes in a biological or ecological indicator (e.g.,
lichen abundance or community composition \35\) to broader effects on
the public welfare. The ISA finds that while there is an improved
understanding from information available in this review of the number
of pathways by which N and S deposition may affect ecosystem services,
most of these relationships remain to be quantified (ISA, Appendix 14,
section 14.6).\36\ This gap creates uncertainties when considering the
public welfare implications of some biological or geochemical responses
to ecosystem acidification or N enrichment, and accordingly complicates
judgments on the potential for public welfare significance. That
notwithstanding, while shifts in species abundance or composition of
various ecological communities may not be easily judged with regard to
public welfare significance, at some level, such changes, especially if
occurring broadly in specially protected areas, where the public can be
expected to place high value, might reasonably be concluded to impact
the public welfare. An additional complexity in the current review with
regard to assessment of effects associated with existing deposition
rates is that the current, much-improved air quality and associated
reduced deposition is within the context of a longer history that
included appreciably greater deposition in the middle of the last
century, the environmental impacts of which may remain, affecting
ecosystem responses.
---------------------------------------------------------------------------
\35\ As recognized in section II.C.1.c above, lichen communities
have important roles in ecosystem function, such as in hydrologic
cycling, nutrient cycling, and as sources of food and habitat for
other species (ISA, appendix 6).
\36\ While ``there is evidence that N and S emissions/deposition
have a range of effects on U.S. ecosystem services and their social
value'' and ``there are some economic studies that demonstrate such
effects in broad terms,'' ``it remains methodologically difficult to
derive economic costs and benefits associated with specific
regulatory decisions/standards'' (ISA, appendix 14, pp. 14-23 to 14-
24).
---------------------------------------------------------------------------
In summary, several considerations are recognized as important to
judgments on the public welfare significance of the array of welfare
effects at different exposure conditions. These include uncertainties
and limitations that must be taken into account regarding the magnitude
of key effects that might be concluded to be adverse to ecosystem
health and associated services. Additionally, there are numerous
locations vulnerable to public welfare impacts from S or N deposition-
related effects on terrestrial and aquatic ecosystems and their
associated services. Other important considerations include the
exposure circumstances that may elicit effects and
[[Page 26644]]
the potential for the significance of the effects to vary in specific
situations due to differences in sensitivity of the exposed species,
the severity and associated significance of the observed or predicted
effect, the role that the species plays in the ecosystem, the intended
use of the affected species and its associated ecosystem and services,
the presence of other co-occurring predisposing or mitigating factors,
and associated uncertainties and limitations.
3. Exposure Conditions and Deposition-Related Metrics
The ecological effects identified in section II.C.1 above vary
widely with regard to the extent and level of detail of the available
information that describes the exposure circumstances that may elicit
them. The information for direct effects of SO<INF>X</INF>, N oxides
and PM in ambient air is somewhat more straight-forward to consider as
it is generally in terms of concentrations in air. For deposition-
related effects, the information may be about S and N compounds in soil
or water or may be for metrics intended to represent atmospheric
deposition of those compounds. For the latter, as recognized in section
II.A.3 above, we face the challenge of relating that information to
patterns of ambient air concentrations.
With regard to the more complex consideration of deposition-related
effects such as ecosystem acidification and N enrichment, there is also
wide variation in the extent and level of detail of the evidence
available to describe the ecosystem characteristics (e.g., physical,
chemical, and geological characteristics, as well as atmospheric
deposition history) that influence the degree to which deposition of N
and S associated with the oxides of S and N and PM in ambient air
elicit ecological effects. One reason for this relates to the
contribution of many decades of uncontrolled atmospheric deposition
before the establishment of NAAQS for PM, oxides of S and oxides of N
(in 1971), followed by the subsequent decades of continued deposition
as standards were implemented and updated. The impacts of this
deposition history remain in soils of many parts of the U.S. today
(e.g., in the Northeast and portions of the Appalachian Mountains in
both hardwood and coniferous forests, as well as areas in and near the
Los Angeles Basin), with recent signs of recovery in some areas (ISA,
Appendix 4, section 4.6.1; 2008 ISA, section 3.2.1.1). This backdrop
and associated site-specific characteristics are among the challenges
faced in identifying deposition targets that might be expected to
provide protection going forward against the array of effects for which
we have evidence of occurrence in sensitive ecosystems as a result of
the deposition of the past.
Critical loads (CLs) are frequently used in studies that
investigate associations between various chemical, biological,
ecological and ecosystem characteristics and a variety of N or S
deposition-related metrics. The term critical load, which in general
terms refers to an amount (or a rate of addition) of a pollutant to an
ecosystem that is estimated to be at (or just below) that which would
resu
[…truncated; see source link]Indexed from Federal Register on April 15, 2024.
This is legal information, not legal advice. Laws vary by jurisdiction and change frequently. Always verify current law with official sources and consult a licensed attorney in your jurisdiction for advice on your specific situation.