Rule2024-29463
Review of the Secondary National Ambient Air Quality Standards for Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter
Primary source
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Published
December 27, 2024
Effective
January 27, 2025
Issuing agencies
Environmental Protection Agency
Abstract
Based on the Environmental Protection Agency's (EPA's) review of the air quality criteria for ecological effects and secondary national ambient air quality standards (NAAQS) for oxides of nitrogen (N oxides), oxides of sulfur (SO<INF>X</INF>), and particulate matter (PM), the EPA is revising the existing secondary sulfur dioxide (SO<INF>2</INF>) standard to an annual average, averaged over three consecutive years, with a level of 10 parts per billion (ppb). Additionally, the Agency is retaining the existing secondary standards for N oxides and PM, without revision. The EPA is also finalizing revisions to the data handling requirements for the secondary SO<INF>2</INF> NAAQS.
Full Text
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[Federal Register Volume 89, Number 248 (Friday, December 27, 2024)]
[Rules and Regulations]
[Pages 105692-105788]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2024-29463]
[[Page 105691]]
Vol. 89
Friday,
No. 248
December 27, 2024
Part II
Environmental Protection Agency
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40 CFR Part 50
Review of the Secondary National Ambient Air Quality Standards for
Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter; Final
Rule
Federal Register / Vol. 89 , No. 248 / Friday, December 27, 2024 /
Rules and Regulations
[[Page 105692]]
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Part 50
[EPA-HQ-OAR-2014-0128; FRL-5788-05-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: Final rule.
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SUMMARY: Based on the Environmental Protection Agency's (EPA's) review
of the air quality criteria for ecological effects and secondary
national ambient air quality standards (NAAQS) for oxides of nitrogen
(N oxides), oxides of sulfur (SO<INF>X</INF>), and particulate matter
(PM), the EPA is revising the existing secondary sulfur dioxide
(SO<INF>2</INF>) standard to an annual average, averaged over three
consecutive years, with a level of 10 parts per billion (ppb).
Additionally, the Agency is retaining the existing secondary standards
for N oxides and PM, without revision. The EPA is also finalizing
revisions to the data handling requirements for the secondary
SO<INF>2</INF> NAAQS.
DATES: This final rule is effective on January 27, 2025.
ADDRESSES: The EPA has established a docket for this action under
Docket ID No. EPA-HQ-OAR-2014-0128. All documents in the docket are
listed on the <a href="https://www.regulations.gov">https://www.regulations.gov</a> website. 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 will be publicly available only in hard copy form.
Publicly available docket materials are available electronically
through <a href="https://www.regulations.gov">https://www.regulations.gov</a>.
FOR FURTHER INFORMATION CONTACT: Ms. Ginger Tennant, Environmental
Protection Agency, Health and Environmental Impacts Division, Office of
Air Quality Planning and Standards (mail code C539-04), Research
Triangle Park, NC 27711; telephone number: (919) 541-4072; email
address: <a href="/cdn-cgi/l/email-protection#d5a1b0bbbbb4bba1fbb2bcbbb2b0a795b0a5b4fbb2baa3"><span class="__cf_email__" data-cfemail="b2c6d7dcdcd3dcc69cd5dbdcd5d7c0f2d7c2d39cd5ddc4">[email protected]</span></a>.
SUPPLEMENTARY INFORMATION:
Table of Contents
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 Decisions
A. Introduction
1. Background
a. Basis for Existing Secondary Standards
b. Prior Review of Deposition-Related Effects
c. General Approach for This Review
2. Overview of Air Quality and Deposition
a. Sources, Emissions and Atmospheric Processes Affecting
SO<INF>X</INF>, N Oxides and PM
b. Recent Trends in Emissions, Concentrations, and Deposition
c. Relationships Between Concentrations and Deposition
3. Overview of Welfare Effects Evidence
a. Nature of Effects
(1) Direct Effects of SO<INF>X</INF> and N Oxides in Ambient Air
(2) Acid Deposition-Related Ecological Effects
(3) Nitrogen Enrichment and Associated Ecological Effects
(4) Other Deposition-Related Effects
b. Public Welfare Implications
c. Exposure Conditions and Deposition-Related Metrics
(1) Acidification and Nitrogen Enrichment in Aquatic Ecosystems
(2) Deposition-Related Effects in Terrestrial Ecosystems
(3) Other Effects of N Oxides, SO<INF>X</INF> and PM in Ambient
Air
4. Overview of Exposure and Risk Assessment for Aquatic
Acidification
a. Key Design Aspects
b. Key Limitations and Uncertainties
c. Summary of Results
B. Conclusions
1. Basis for Proposed Decision
a. Policy-Relevant Evaluations in the Policy Assessment
(1) Effects Not Related to S and N Deposition
(2) Evidence of Ecosystem Effects of S and N Deposition
(3) Sulfur Deposition and SO<INF>X</INF>
(4) Nitrogen Deposition and N Oxides and PM
b. CASAC Advice
c. Administrator's Proposed Conclusions
2. Comments on the Proposed Decision
a. Sulfur Oxides
(1) Comments Regarding Adequacy of the Existing Standard
(2) Comments in Support of Proposed Adoption of a New Annual
Standard
(3) Comments in Disagreement With Proposed Adoption of a New
Annual Standard
(4) Comments Regarding Retaining the Existing Secondary Standard
b. Nitrogen Oxides and Particulate Matter
(1) Comments in Support of Proposed Decisions
(2) Comments in Disagreement With Proposed Decisions
3. Administrator's Conclusions
C. Decision on the Secondary Standards
III. Interpretation of the Secondary SO<INF>2</INF> NAAQS
A. Background
B. Interpretation of the Secondary SO<INF>2</INF> Standard
IV. Ambient Air Monitoring Network for SO<INF>2</INF>
A. Public Comments
B. Conclusion on the Monitoring Network
V. Clean Air Act Implementation Considerations for the Revised
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 Revised Secondary SO<INF>2</INF>
Standard
D. Transportation Conformity Program
E. General Conformity Program
VI. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 14094: Modernizing 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
K. Congressional Review Act (CRA)
L. Judicial Review
VII. References
Executive Summary
This document presents the Administrator's final decisions in the
current review of the secondary NAAQS for SO<INF>X</INF>, N oxides, and
PM. Specifically, this document summarizes the background and rationale
for the Administrator's final decisions to revise the secondary
SO<INF>2</INF> standard to an annual average, averaged over three
consecutive years, with a level of 10 ppb, and to retain the existing
standards for N oxides and PM. 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
[[Page 105693]]
effects of SO<INF>X</INF>, N oxides, and PM \1\ as described in the
Integrated Science Assessment (ISA) and conducted quantitative air
quality, deposition, and risk analyses. The Administrator's final
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 related analyses;
advice from the Clean Air Scientific Advisory Committee (CASAC); and
public comments on the proposed decision.
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\1\ Welfare effects of PM considered in the review of the PM
secondary standards completed in 2020, and reconsidered more
recently, include effects on visibility and climate and materials
damage (88 FR 5558, January 27, 2023).
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Sections 108 and 109 of the Clean Air Act (CAA) 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.'' As a result
of the current review, the Administrator concluded that the current 3-
hour secondary SO<INF>2</INF> standard is not requisite to protect the
public welfare from any known or anticipated adverse effects associated
with the presence of SO<INF>X</INF> in ambient air, and that it should
be revised to an annual average SO<INF>2</INF> standard, averaged over
three years, with a level of 10 ppb to provide the requisite protection
for the effects of SO<INF>X</INF>, including those related to
atmospheric deposition of sulfur (S) compounds in sensitive ecosystems.
The Administrator also decided 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 N oxides does not call into question the adequacy
of protection provided by the existing standard. Additionally, the
Administrator concludes that no change to the annual secondary
PM<INF>2.5</INF> standard is warranted and that the existing
PM<INF>2.5</INF> secondary standard should be retained without
revision.
This document additionally includes revisions related to
implementation of the proposed secondary SO<INF>2</INF> annual
standard. Specifically, the EPA is enacting revisions to the data
handling requirements in appendix T of part 50 to include
specifications needed for the new annual average standard. This
document also describes the SO<INF>2</INF> monitoring network and its
adequacy for surveillance for the revised annual standard. Lastly, the
document discusses implementation processes pertinent to implementation
of the 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.'' \2\
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\2\ 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.''
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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. 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.\3\
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\3\ This section of the Act requires the Administrator to
complete these reviews and make any revisions that may be
appropriate ``at five-year intervals.''
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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
[[Page 105694]]
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), 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. Rather, EPA has
established ambient air quality standards that specify air quality by
quantifying pollution in the ambient air to address effects of such
pollution, whether from ambient concentrations or deposition.
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 under the new source performance
standards in 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.\4\
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\4\ 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.
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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).\5\ 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 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).
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\5\ 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.
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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,\6\ 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 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).
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\6\ 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.
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2. SO<INF>X</INF>
The EPA first promulgated secondary NAAQS for SO<INF>X</INF> 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>,\7\ 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).
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\7\ 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).
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In 1979, the EPA announced initiation of a concurrent review of the
air quality criteria for SO<INF>X</INF> and PM and plans for
development of a combined AQCD for these pollutants (44 FR 56730,
October
[[Page 105695]]
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:
SO<INF>X</INF>, N oxides, 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 SO<INF>X</INF> 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>X</INF> (53 FR 14926, April 26, 1988).
The proposed decision reflected 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
CAA Amendments of 1990 (see section I.C.3.) with the decision not to
revise the secondary standard. The EPA concluded that revisions to the
standard to address acidic deposition and related SO<INF>X</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. 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; . . . (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 S
and 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 \8\ of the air
quality criteria
[[Page 105696]]
and secondary NAAQS for oxides of nitrogen and sulfur (70 FR 73236,
December 9, 2005). The review focused on the evaluation of the
protection provided by the standards 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
standards were developed to protect against, and (2) effects associated
with the deposition of N oxides and SO<INF>X</INF> to sensitive aquatic
and terrestrial ecosystems (77 FR 20218, April 3, 2012).
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\8\ Although the EPA has historically reviewed separately the
secondary standards for oxides of nitrogen and oxides of sulfur, the
EPA conducted a joint review of these standards in recognition of
the chemical interactions in the atmosphere and associated
contributions to acid deposition and related environmental effects.
The joint review was also responsive to a National Research Council
recommendation that the EPA consider pollutants in combination, as
appropriate, in considering the NAAQS (NRC, 2004).
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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 EPA released a final ISA
(referred to as 2008 ISA below) 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),\9\ 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).
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\9\ The REAs for NAAQS reviews may be presented in appendices to
the PA or in stand-alone documents (e.g., U.S. EPA 2020b, 2020c, and
PA for current review [U.S. EPA, 2024]).
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Drawing on the information in the 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). For the purpose
of protection against the direct effects on vegetation of exposure to
gaseous oxides of nitrogen and sulfur, the final 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 (details provided in section II.A.1.b. below). Based on
consideration 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).
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. Alternatively, the
Administrator proposed to revise the secondary standards by adopting
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 of
which were set in 2010, noting that these new primary standards, while
not set based on consideration of atmospheric deposition,\10\ were
likely to reduce oxides of nitrogen and sulfur emissions and associated
nitrogen and sulfur deposition in sensitive ecosystems (76 FR 46084,
August 1, 2011). After consideration of public comments, the EPA
decided to retain the existing standards (without revision) to address
the direct effects on vegetation of exposure to gaseous oxides of
nitrogen and sulfur. At that time, the EPA also described its decision
that it was not appropriate to set new secondary standards at that time
to address deposition-related effects associated with oxides of
nitrogen and sulfur (77 FR 20218, April 3, 2012).
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\10\ The 1-hour primary standards set in 2010 were a
NO<INF>2</INF> standard of 100 ppb, as the 98th percentile of 1-hour
daily maximum concentrations, averaged over three years, and a
SO<INF>2</INF> standard of 75 ppb, as the 99th percentile of daily
maximum 1-hour concentrations, averaged over three years (75 FR
6474, February 9, 2010; 75 FR 35520, June 22, 2010).
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The EPA's 2012 decision was challenged by the Center for Biological
Diversity and other environmental groups, who argued that the EPA,
having decided that the existing standards were not adequate to protect
against adverse public welfare effects such as damage to sensitive
ecosystems, was required to identify the requisite level of protection
for the public welfare and to issue 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 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 . . . '.'' \11\ 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.\12\
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\11\ Center for Biological Diversity, et al. v. EPA, 749 F.3d
1079, 1087 (2014).
\12\ Id. at 1088.
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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).\13\
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\13\ 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).
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In October 1979, the EPA announced the first review of the air
quality criteria and NAAQS for PM (44 FR 56730, October 2, 1979). A
combined AQCD for PM and SO<INF>X</INF> was released in 1982, after
CASAC and public review of drafts (U.S. EPA, 1982b). Soon after, the
OAQPS released a Staff Paper (U.S. EPA, 1982d), two drafts of which had
received public and CASAC review (Friedlander, 1982). In 1984, the EPA
proposed replacing the secondary standard with an annual TSP standard
with a level within the range of 70-90 [mu]g/m\3\, as an expected
annual arithmetic
[[Page 105697]]
mean (49 FR 10408, March 20, 1984). After consideration of public
comment and review by the CASAC and the public, the OAQPS released an
Addendum to the Staff Paper in 1986 (Lippman, 1986; U.S. EPA, 1986). In
1987, the EPA completed the review by adopting two new primary PM NAAQS
and setting the secondary standards identical to the primary standards
in all respects, all with a new indicator for PM (particles with a
nominal mass median diameter of 10 microns, PM<INF>10</INF>). The new
primary and secondary standards included (1) a 24-hour standard of 150
[mu]g/m\3\, in terms of one expected exceedance per year, on average
over three years and (2) an annual secondary standard of 50 [mu]g/m\3\,
as an 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 OAQPS
released a Staff Paper in November 1997, after CASAC and public review
of two drafts (U.S. EPA, 1996; Wolff, 1996). The EPA proposed revisions
to the PM standards in 1996 and promulgated final standards in 1997 (61
FR 65738; December 13, 1996; 62 FR 38652, July 18, 1997). With the 1997
decision, the EPA added new standards, using particles with a nominal
mean aerodynamic diameter less than or equal to 2.5 [mu]m
(PM<INF>2.5</INF>) as the indicator for fine particles. The new
secondary PM<INF>2.5</INF> standards were set equal to the primary
PM<INF>2.5</INF> standards, in all respects, as follows: (1) an annual
standard with a level of 15.0 [mu]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,\14\ and (2) a 24-hour standard
with a level of 65 [mu]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. The EPA also retained the primary and secondary annual
PM<INF>10</INF> standards, without revision, and revised the form of
the 24-hour primary and secondary PM<INF>10</INF> standards to be based
on the 99th percentile of 24-hour PM<INF>10</INF> concentrations at
each monitor in an area.
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\14\ 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).
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Following promulgation of the 1997 PM 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 (PM<INF>2.5</INF>) 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 (PM<INF>10</INF>) 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 the 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).
The EPA released the final AQCD in October 2004, after the CASAC and
public review of several drafts (U.S. EPA, 2004a, b). The OAQPS
released a Staff Paper in December 2005 (U.S. EPA, 2005). Also in
December 2005, the EPA proposed to revise the PM NAAQS and solicited
public comment on a broad range of options (71 FR 2620, January 17,
2006). In September 2006, after consideration of public comment, the
EPA revised the PM NAAQS, making revisions to the secondary standards
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 61144, 61203-61210, October
17, 2006). The EPA revised the level of the 24-hour PM<INF>2.5</INF>
standards to 35 [mu]g/m\3\, retained the level of the annual
PM<INF>2.5</INF> standards at 15.0 [mu]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. For PM<INF>10</INF>, the EPA
revoked the annual standards and retained the 24-hour standards, both
with a level of 150 [mu]g/m\3\.
Several parties filed petitions for review of the 2006 p.m. NAAQS
decision, with one raising the issue of the secondary PM<INF>2.5</INF>
standards being 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), remanding the
standards to the EPA stating the Agency had failed to adequately
explain how setting the secondary standards identical to the primary
standards provided the required public welfare protection, including
for visibility impairment (Id. at 528-32). The EPA responded to the
court's remands as part of the subsequent PM NAAQS review.
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). To
inform planning for the review, the EPA held science/policy issue
workshops later that year (72 FR 34003, June 20, 2007; 72 FR 34005,
June 20, 2007). Plans for the review and for welfare assessments were
developed in 2008 and 2009; the ISA was completed in 2009, an urban-
focused visibility assessment was completed in 2010 and the PA was
released in 2011 (U.S. EPA, 2008b; U.S. EPA, 2009b; U.S. EPA, 2009c;
U.S. EPA, 2010; U.S. EPA, 2011). In June 2012, the EPA proposed
revisions to the PM NAAQS and in December 2012 announced its final
decisions to revise the primary and secondary PM<INF>2.5</INF> annual
standards (77 FR 38890, June 29, 2012; 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.
[[Page 105698]]
D. Current Review
In August 2013, the EPA issued a call for information in the
Federal Register for information related to the current review of the
air quality criteria for SO<INF>X</INF> and N oxides 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 atmospheric transformations and
deposition involving the three pollutants (N oxides, SO<INF>X</INF> and
PM) and associated ecological effects (U.S. EPA, 2017). In so doing,
the EPA clarified that other effects of PM, including materials damage,
climate effects and visibility effects are beyond the scope of this
review (IRP, p. 1-2 and section 2.1).
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 public
meetings in May and August 2017 (82 FR 15701, March 30, 2017; 82 FR
35200, July 28, 2017; Diez Roux and Fernandez, 2017). A second external
review draft ISA was released in 2018 and reviewed by the CASAC at
public meetings in September 2018 and April 2020 (83 FR 2018; July 9,
2018; 85 FR 16093, March 30, 2020; Cox, Kendall, and Fernandez,
2020a).\15\ The EPA released the final ISA in October 2020 (85 FR
66327, October 19, 2020; U.S. EPA, 2020a).
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\15\ A change in CASAC membership contributed to an extended
time period between the two public meetings.
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In 2023, the draft PA, including the REA for aquatic acidification
as an appendix,\16\ was released for review by the CASAC and for public
comment (88 FR 34852, May 31, 2023). The CASAC conducted its review at
public meetings in June and September 2023 and conveyed its advice to
the Administrator on the standards and comments on the draft PA in late
September 2023 (88 FR 17572, March 23, 2023; 88 FR 45414, July 17,
2023; Sheppard, 2023). In January 2024, the EPA released the final PA
(89 FR 2223, January 12, 2024; U.S. EPA, 2024). In April 2024, the EPA
proposed to revise the secondary SO<INF>2</INF> standard and retain the
secondary standards for N oxides and PM (89 FR 26620, April 15, 2024).
During the subsequent public comment period, public comments were
received both orally during a virtual public hearing on May 8, 2024 (89
FR 26114, April 15, 2024) and in writing to the docket (as discussed in
section II.B.2. below).\17\ Significant comments received are addressed
in this preamble to this final action and in the accompanying Response
to Comments document, which can be found in the docket for this review.
The schedule for completion of this review has been governed by a
consent decree that requires the EPA to sign for publication a notice
of final rulemaking concerning review of the NAAQS for N oxides,
SO<INF>X</INF> and PM no later than December 10, 2024 (Center for
Biological Diversity v. Regan [No. 4:22-cv-02285-HSG (N.D. Cal.)]).
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\16\ The planning document for quantitative aquatic
acidification exposure/risk analyses was also made available for
public comment and consultation 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\ The public hearing transcript and any written testimony
provided are also in the docket.
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Materials upon which the decision in this review is based,
including the documents described above, are available to the public in
the docket for this review.\18\ The EPA is basing its decision in this
review on studies and related information included in the air quality
criteria, which have undergone CASAC and public review. The studies
assessed in the ISA and PA, and the integration of the scientific
evidence presented in them, have undergone extensive critical review by
the EPA, the CASAC, and the public. The rigor of that review makes
these studies, and their integrative assessment, the most reliable
source of scientific information on which to base decisions on the
NAAQS, decisions that all recognize to be of great import. Decisions on
the NAAQS can have profound impacts on public health and welfare, and
NAAQS decisions should be based on studies that have been rigorously
assessed in an integrated manner not only by the EPA but also by the
statutorily mandated independent scientific advisory committee, as well
as the public review that accompanies this process.
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\18\ The docket for this review, Docket ID No. EPA-HQ-OAR-2014-
0128, has incorporated the ISA docket (Docket ID No. EPA-HQ-ORD-
2013-0620) by reference. Both are publicly accessible at <a href="https://www.regulations.gov">https://www.regulations.gov</a>.
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Some commenters have referred to and discussed individual
scientific studies on the welfare effects of SO<INF>X</INF>, N oxides,
and PM that were not included in the ISA (``new'' studies) and that
have not gone through this comprehensive review process. In considering
and responding to comments for which such ``new'' studies were cited in
support, the EPA has provisionally considered the cited studies in the
context of the findings of the ISA (Weaver, 2024). The EPA's
provisional consideration of these studies did not and could not
provide the kind of in-depth critical review described above, but
rather was focused on determining whether they warranted reopening the
review of the air quality criteria to enable the EPA, the CASAC and the
public to consider them further as part of this review. This approach,
and the decision to rely on studies and related information included in
the air quality criteria, which have undergone CASAC and public review,
is consistent with the EPA's practice in prior NAAQS reviews and its
interpretation of the requirements of the CAA. Since the 1970
amendments, the EPA has taken the view that NAAQS decisions are to be
based on scientific studies and related information that have been
assessed as a part of the pertinent air quality criteria, and the EPA
has consistently followed this approach. This longstanding
interpretation was strengthened by new legislative requirements enacted
in 1977, which added section 109(d)(2) of the Act concerning CASAC
review of air quality criteria. See 71 FR 61144, 61148 (October 17,
2006, final decision on review of NAAQS for particulate matter) for a
detailed discussion of this issue and the EPA's past practice.
As discussed in the EPA's 1993 decision not to revise the ozone
(O<INF>3</INF>) NAAQS, ``new'' studies may sometimes be of such
significance that it is appropriate to delay a decision in a NAAQS
review and to supplement the pertinent air quality criteria so the
studies can be taken into account (58 FR at 13013-13014, March 9,
1993). In the present case, the EPA's consideration of ``new'' studies
concludes that, taken in context, the ``new'' information and findings
do not materially change any of the broad scientific conclusions made
in the air quality criteria regarding the health and welfare effects of
the subject pollutants in ambient air. For this reason, reopening the
air quality criteria review is not warranted. Accordingly, the EPA is
basing the final decisions in this review on the studies and related
information included in the air quality
[[Page 105699]]
criteria that have undergone rigorous review by the EPA, the CASAC, and
the public. The EPA will consider these ``new'' studies for inclusion
in the air quality criteria for the next review, which will provide the
opportunity to fully assess these studies through a more rigorous
review process involving the EPA, the CASAC, and the public.
II. Rationale for Decisions
This section presents the rationale for the Administrator's
decisions in the review of the secondary NAAQS 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 reviews of the secondary standards
for N oxides, SO<INF>X</INF> and PM. This information on ecological
effects associated with SO<INF>X</INF>, N oxides and PM and pertaining
to their presence in ambient air, which includes studies generally
published between January 2008 and May 2017 (and considered in the
ISA), is integrated with the information and conclusions from previous
assessments and presented in the ISA (ISA, section IS.1.2).\19\ 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; (3) public comments
received during the development of these documents; and (4) public
comments received on the proposed decisions.
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\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 2008 ISA) and May 2017. Studies published after
the literature cutoff date for this ISA were also considered in the
ISA 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).
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Before presenting the rationale for the Administrator's final
decisions and their foundations, section II.A.1. provides an
introduction that also summarizes the basis for the existing standards
(section II.A.1.a.), provides background on the prior review of
deposition-related effects of N oxides and SO<INF>X</INF> (section
II.A.1.b.), and summarizes the general approach in this review (section
II.A.1.c.). Section II.A.2. provides an overview of the air quality
information and analyses relating S and N deposition to concentrations
of SO<INF>X</INF>, N oxides and PM. Section II.A.3. provides an
overview of the currently available ecological effects evidence as
summarized in the ISA, focusing on consideration of key policy-relevant
aspects, and section II.A.4. provides an overview of the exposure and
risk information for this review, drawing on the quantitative analyses
of aquatic acidification risk, presented in the PA. Section II.B.1.
provides a summary of the Administrator's proposed decisions (section
II.B.1.c.), which drew on both evidence-based and exposure/risk-based
considerations from the PA (section II.B.1.a.) and advice from the
CASAC (section II.B.1.b.). Section II.B.2. discusses comments received
on the proposed decision, and section II.B.3. presents the
Administrator's conclusions and associated rationale. The final
decisions are summarized in section II.C.
A. Introduction
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-related 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 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 the public welfare. A public welfare policy decision draws
upon scientific information and analyses about welfare effects,
exposures 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 may be 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 decisions on
secondary standards, the Administrator seeks to establish standards
that are neither more nor less stringent than necessary for this
purpose. In evaluating the public welfare protection afforded by the
standards, the four basic elements of the NAAQS (indicator, averaging
time, level, and form) are considered collectively.\20\
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\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.
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Generally, 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
[[Page 105700]]
appropriate inferences to be drawn as to risks to public welfare, and
that the choice of 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 associated
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. Background
Ecological effects of N oxides, SO<INF>X</INF> and PM include those
related to direct contact of the airborne pollutants with plants and
those related to atmospheric deposition of N- and S-containing
compounds into sensitive ecosystems. As summarized in section II.A.1.a.
below, it is the former category of effects (from direct contact) that
were considered in establishing the existing standards, with those
effects as the basis for the secondary standards for N oxides and
SO<INF>X</INF>. In the last review of those standards, deposition-
related effects were also considered. However, as summarized in section
II.A.1.b. below, the extent of the uncertainties associated with the
complex methodology investigated for defining a deposition-based
standard in that review were found to be so significant that the
Administrator concluded that the limitations and uncertainties in the
available information were too great to support establishment of a new
standard using this methodology that could be concluded to provide the
requisite protection for such effects under the Act (77 FR 20218, April
3, 2012). As described in the proposal for the current action, and
generally summarized in section II.A.1.c. below, in the current review
we have taken a different approach to considering standards that might
be expected to provide the appropriate level of protection from
deposition-related effects.
a. Basis for Existing Secondary Standards
The existing 3-hour secondary SO<INF>2</INF> standard, with its
level of 0.5 ppm, and the annual secondary NO<INF>2</INF> standard,
with its level of 0.053 ppm 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 standards is to provide protection to the
public welfare related to direct effects on vegetation (U.S. DHEW,
1969a; U.S. EPA, 1971). There are three secondary PM standards--
established in 1997 (annual PM<INF>2.5</INF> standard) and 2006 (24-
hour PM<INF>2.5</INF> and PM<INF>10</INF> standards)--variously based
on consideration of materials damage, visibility impacts, climate
effects and ecological effects.\21\
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\21\ As noted in section I.D. above, the 2020 review of the PM
secondary NAAQS and its reconsideration focused on visibility
effects, materials damage and climate effects, while the ecological
effects of PM are being addressed in this combined review (89 FR
16205, March 6, 2024).
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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, as well as reductions
in productivity, species richness, and diversity (U.S. DHEW, 1969a;
U.S. EPA, 1982c; U.S. EPA, 2008a). 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 are the basis for the existing secondary
standard for SO<INF>X</INF>.
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 effects
of direct contact of airborne N oxides with 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
effects considered in that review of the secondary PM standards include
effects on visibility, materials damage, and climate effects, as well
as ecological effects; 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 in the 2013
review included 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).
b. 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 preceding secondary standards reviews for those
pollutants (77 FR 20236, April 3, 2012). Considering the extensive
evidence available in the 2012 review, the Agency concluded that the
most significant risks of adverse effects of N oxides and
SO<INF>X</INF> to the public welfare were those related to deposition
of N and S compounds in 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 atmospheric deposition of N and S compounds in
sensitive aquatic and terrestrial ecosystems, described in the 2009 REA
and 2011 PA (77 FR 20218, April 3, 2012).
The 2009 REA assessed atmospheric deposition of N and S compounds
and the risks it posed of two categories of ecosystem effects:
acidification and nutrient enrichment in both terrestrial
[[Page 105701]]
and aquatic ecosystems (U.S. EPA, 2009a). In so doing, however, the
2009 REA and 2011 PA recognized that the different types of effects
varied in the strength of the evidence and of the information
characterizing quantitative linkages between pollutants in ambient air
and ecosystem responses, and in associated potential public welfare
implications. The support in the evidence for quantitative assessment
of aquatic acidification-related effects was strongest and the least
uncertain.
With regard to nutrient enrichment-related effects, despite the
extensive evidence of deleterious effects of excessive ecosystem
loading of nitrogen, the identification of options to provide
protection from deposition-related effects was limited by several
factors. These included the influence in terrestrial ecosystems of
other air pollutants such as O<INF>3</INF>, and limiting factors such
as moisture and other nutrients, and their potential to confound the
characterization of the effects of changes in any one stressor, such as
N deposition, in those systems (2011 PA, section 6.3.2). Forest
management practices were also recognized to have the ability to
significantly affect nitrogen cycling within a given forest ecosystem
(2008 ISA section 3.3.2.1 and Annex C, section C.6.3). In aquatic
systems, appreciable contributions of non-atmospheric sources to
nutrient loading in most large waterbodies, and limitations in data and
tools, contributed uncertainties to characterizations of incremental
adverse impacts of atmospheric N deposition (2011 PA, section 6.3.2).
With regard to terrestrial acidification effects, data limitations
contributed uncertainty to identification of appropriate indicator
reference levels, and the potential for other stressors to confound
relationships between deposition and terrestrial acidification effects
was recognized with regard to empirical case studies described in the
2008 ISA.
Based on the strong support in the evidence for the relationship
between atmospheric deposition of acidifying N and S compounds and loss
of acid neutralizing capacity (ANC) in sensitive ecosystems, with
associated aquatic acidification effects, the REA analyses for this
endpoint (aquatic acidification) received greatest emphasis in the
review relative to other deposition-related effects. This emphasis on
aquatic acidification-related effects of N oxides and SO<INF>X</INF>
also reflected the advice from the CASAC. Accordingly, the 2011 PA
focused on aquatic acidification effects 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 ANC levels)
for consideration in establishing such a standard (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 ``transference ratios.'' The latter is the term assigned to factors
applied to deposition values (estimated to achieve the minimum
specified ANC) to back-calculate or estimate the highest ambient air
concentrations of SO<INF>X</INF> and N oxides that would meet the AAI-
based standard level (2011 PA, Chapter 7).\22\ 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.
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\22\ 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 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.
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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). As the EPA said:
``[T]he 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-
[[Page 105702]]
related effects associated with N oxides and SO<INF>X</INF>, it was not
appropriate under section 109 of the CAA (given the uncertainties
summarized immediately above) 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). This decision was upheld upon judicial review.
c. 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 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 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. The EPA's assessments are
primarily documented in the ISA and PA, both of which received CASAC
review and public comment, as summarized in section I.D. above.
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 contact effects of the airborne
pollutants and also the 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 contact 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.A.3. 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, summarized briefly in the sections below (and in detail in
the PA), 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 atmospheric deposition and
ecological 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 intervals of 15 to 30 years (1984-2001
and 1967-1997). These studies reported that although atmospheric
deposition in the Northeast declined across those intervals, soil
acidity increased (ISA, Appendix4, 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, including changes in emissions,
ambient air concentrations and associated deposition.
The two-pronged approach applied in the PA for deposition-related
effects includes the consideration of deposition levels that may be
associated with ecological effects of potential concern and
consideration of relationships between ambient air concentrations and
levels of deposition. In considering the ecological effects evidence,
the focus is on effects for which the evidence is most robust with
regard to established quantitative relationships between deposition and
ecosystem effects. Such quantitative information for terrestrial
ecosystems is derived primarily from analysis of the evidence presented
in the ISA. For aquatic ecosystems, the primary focus has been 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.A.4. below (PA,
section 5.1 and Appendix 5A). Regarding the second prong of the
approach, we employed several different types of analyses 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.A.2. below). 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, informed 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 and inform the Administrator's
final decisions in this review, as discussed in section II.B.3. below.
In summary, the approach to evaluating the standards with regard to
protection from ecological effects related to ecosystem deposition of N
and S compounds in this review involves multiple components: (1) review
of the scientific evidence to identify the ecological effects
associated with the three pollutants, those related
[[Page 105703]]
both 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;
and (3) analysis of relationships between ambient air concentrations of
the pollutants and deposition of N and S compounds to understand
aspects of these relationships that can inform judgments on ambient air
standards that protect against air concentrations associated with
direct effects and against deposition associated with deposition-
related effects that are judged adverse to the public welfare. As
discussed in the PA and the proposal, however, relating ambient air
concentrations of N oxides and PM 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 secondary 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 also related 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 decision, as summarized
in section II.B.3. below.
2. Overview of 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 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 pollutant and precursor emissions (section
II.A.2.a.) to ambient air concentrations (section II.A.2.b.) and to
eventual deposition (section II.A.2.c.) varies by pollutant and is
influenced by a series of atmospheric processes and chemical
transformations that occur at multiple spatial and temporal scales
(ISA, Appendix 2; 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>. Although emissions of N
oxides have appreciably declined, NH<INF>3</INF> emissions have risen.
Together, these co-occurring trends have reduced the influence of N
oxides on total N deposition (PA, sections 6.2.1, 6.4.2 and 7.2.3.3).
Geographic variability and temporal changes in the percentage of PM
composed of N- (and S-) containing compounds, are other factors
affecting decisions in this review.
a. Sources, Emissions and Atmospheric Processes Affecting
SO<INF>X</INF>, N Oxides and PM
Sulfur dioxide is generally present at higher concentrations in the
ambient air than the other gaseous and highly reactive SO<INF>X</INF>
(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 electricity generating units (48% of total),
industrial processes (27%), and other stationary source fuel combustion
(9%).
Once emitted to the atmosphere, SO<INF>2</INF> can either remain as
SO<INF>2</INF> in the gas phase and be transported and/or be dry
deposited, or it can be oxidized to form sulfate particles
(SO<INF>4</INF><SUP>2-</SUP>), with modeling studies suggesting that
oxidation accounts for more than half of SO<INF>2</INF> removal
nationally (PA, section 2.1.1). The rate of SO<INF>2</INF> oxidation
accelerates with greater availability of oxidants, which are generally
depleted near source stacks. Consequently, oxidization to
SO<INF>4</INF><SUP>2-</SUP> generally occurs in cleaner air downwind of
SO<INF>X</INF> sources (2008 ISA, section 2.6.3.1). 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> and have
an atmospheric lifetime ranging from 2 to 10 days (PA, section 2.1.1).
The areas of highest SO<INF>2</INF> and SO<INF>4</INF><SUP>2-</SUP>
deposition are generally near or downwind of SO<INF>X</INF> emissions
sources, with most S deposition occurring in the eastern U.S. (PA,
section 2.5.3). Geographic variation in precipitation also influences
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, and 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 distributing more regionally.
The term N oxides refers to all forms of oxidized nitrogen
compounds, including 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 2020 NEI estimates, with highway vehicles
(26% of total), stationary fuel combustion including electric
generating units (25%), and non-road mobile sources (19%) identified as
the largest contributors to total emissions (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>,
which may occur in either the fine or coarse particle size range, such
that not all particulate NO<INF>3</INF><SUP>-</SUP> is a component of
PM<INF>2.5</INF>. In general, gas phase N oxides 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>, which
has a similar atmospheric lifetime as particulate
SO<INF>4</INF><SUP>2-</SUP> and is generally removed by precipitation
in wet deposition.
In addition to N oxides, there is another category of nitrogen
pollutants, referred to as reduced nitrogen, which also contributes to
nitrogen deposition. The most common form of reduced N emitted into the
air is NH<INF>3</INF> gas (PA, sections 2.1.3 and 2.2.3), which is not
a criteria pollutant. The main 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, with a fraction of what is emitted being
converted to particle form, as ammonium (NH<INF>4</INF>\+\), which can
be transported away from sources and is most efficiently removed by
precipitation (PA, section 2.1.3).
Particulate matter is both emitted to the atmosphere and formed in
the atmosphere from precursor chemical gases, such as N Oxides,
SO<INF>X</INF> and NH<INF>3</INF>. Accordingly, PM<INF>2.5</INF>
contributing to S and N deposition generally results from chemicals
formed in the atmosphere after being emitted (e.g., particulate
SO<INF>4</INF><SUP>2-</SUP>, particulate NO<INF>3</INF><SUP>-</SUP>,
NH<INF>4</INF><SUP>+</SUP>). The majority of PM<INF>2.5</INF> mass in
recent periods (e.g., 2019-2021) is composed of materials that do not
contribute to S and N deposition (PA, section 2.4.3 and 6.4.2). For
example, at PM<INF>2.5</INF> monitoring sites across the U.S.,
SO<INF>4</INF><SUP>2-</SUP> generally comprises no more than about a
third of PM<INF>2.5</INF> mass (in eastern sites), with much lower
percentages at monitoring sites in much of the West and South (PA
Figure 2-30 and section 2.4.3). Similarly, nitrogen-containing species
are also a minority of PM<INF>2.5</INF> mass, representing less than
about 30% and down to about 5% or lower in some areas of South (PA,
sections 2.4.3 and 6.4.2).
b. 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-
[[Page 105704]]
term trend (PA, section 2.2). Total SO<INF>2</INF> emissions nationwide
declined by 87% between 2002 and 2022, including reductions of 91% in
emissions from electricity generating units and 96% in emissions from
mobile sources. Total anthropogenic NO<INF>X</INF> emissions also
trended downward from 2002 to 2022 by 70% nationwide, driven in part by
large reductions in emissions from highway vehicles (84%) and
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 increased by
over 20 percent nationwide between 2002 and 2022 (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. These trends in NO<INF>X</INF> and NH<INF>3</INF>
emissions have had ramifications for N deposition patterns across the
U.S., as described further below.
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 the past. This is true for both 3-hour and 1-
hour average concentrations. With regard to 3-hour SO<INF>2</INF>
concentrations, 2021 design values for the existing 3-hour standard at
all State and Local Air Monitoring Stations (SLAMS) with valid design
values (n= 333) \23\ are less than the level of the existing secondary
standard (500 ppb) \24\ and more than 75 percent of the sites have
design values below 20 ppb (PA, section 2.4.2). This reflects a
downward trend since 2000, with the median design value declining from
about 50 ppb to less than 10 ppb in 2021 (PA, Figure 2-27).
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\23\ 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>).
\24\ 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.
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Similarly, design values for the primary SO<INF>2</INF> standard
(annual 99th percentile of daily maximum 1-hour average concentrations,
averaged over 3 years) have also declined. In the mid-1990s, the median
value of all sites with valid 1-hour design values often exceeded 75
ppb (PA, Figure 2-26). Since then, the entire distribution of design
values (including source-oriented sites) has continued to decline such
that the median design value for the 1-hour primary standard across the
network of sites is now between 5 and 10 ppb (PA, Figure 2-26). Annual
average SO<INF>2</INF> concentrations have also declined over this
period. Additionally, both peak and mean SO<INF>2</INF> concentrations
are higher at source-oriented sites than monitoring locations that are
not source-oriented.\25\
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\25\ 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,
such as in Hawaii (PA, section 2.4.2).
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Regarding NO<INF>2</INF>, design values for the secondary standard
(annual averages) at all 399 sites with valid design values in 2021 are
below the 53 ppb level of the existing standard,\26\ and 98% of sites
have design values below 20 ppb. In 2021, the maximum design value was
30 ppb,\27\ and the median was 7 ppb, reflecting a downward trend since
2000 when the median annual design value was 15 ppb.
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\26\ Sites in the contiguous U.S. have met the existing
NO<INF>2</INF> secondary standard since around 1991 (PA, Figure 2-
22).
\27\ 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).
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Likewise, the median of the annual average PM<INF>2.5</INF>
concentrations also decreased substantially from 2000 to 2021, from
12.8 [mu]g/m\3\ to 8 [mu]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 [mu]g/m\3\ in 2000 to 21
[mu]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).
The changes in emissions and associated concentrations since 2000
have also contributed to appreciable changes in N and S deposition
nationwide (PA, sections 2.5.3 and 6.2.1). For S compounds, the
dramatic reduction in SO<INF>X</INF> emissions (87% nationwide)
resulted in concordant reductions in S deposition, 68% on average
across U.S. (PA, section 6.2.1). This decline is observed across the
contiguous U.S. (CONUS), with the largest reductions in regions
downwind of large sources such as electricity generating units. For N
deposition, the impact of the appreciable reduction in N oxides
emissions has been offset by deposition arising from increasing
emissions of reduced forms of nitrogen over the same timeframe.
c. 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.
These relationships were evaluated over a range of conditions (e.g.,
pollutant, region, time period), and with consideration of deposition
both near sources and at distance (allowing for pollutant transport and
associated transformation) using five different approaches (PA, Chapter
6 and Appendix 6A).
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 to examine the correlation between
observed decreases in emissions and concentration and observed changes
in deposition over the past two decades (PA, section 6.2.1). The
deposition estimates used in these analyses (termed TDep) \28\ 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). For the second
approach, we assessed how ambient air concentrations and associated
deposition levels are related within the CMAQ \29\ both across the U.S.
and then at certain Class I areas \30\ (PA, section
[[Page 105705]]
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 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 the hybrid
set of TDep estimates (PA, section 6.2.3).
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\28\ 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
contiguous U.S. (CONUS), which are referred to as the TDep datasets
(technical updates available from NADP, 2021; ISA, Appendix 2,
section 2.6). TDep datasets do not currently exist for areas outside
of the CONUS.
\29\ 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.
\30\ 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).\31\ Once those potential zones of
influence were established, we evaluated the relationships between air
quality metrics for the three pollutants \32\ at sites within those
zones (sites of influence) and 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 concentration
among the upwind monitoring sites identified for each downwind
ecoregion. 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 were weighted higher (PA,
section 6.2.4.1).
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\31\ Upwind sites of influence were identified for all 84
ecoregions (level III categorization) in the contiguous U.S.
Identification of monitoring sites linked to each downwind ecoregion
was based on HYSPLIT modeling for a 120-hour period and focusing on
monitoring site locations estimated to contribute at least 0.5% of
hits to the downwind ecoregion in the trajectory modeling (PA,
Appendix 6, section 6A.2).
\32\ 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.
---------------------------------------------------------------------------
The full set of quantitative results of the characterization of air
quality and deposition relationships is discussed more thoroughly in
Chapter 6 and Appendix 6A of the PA. 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
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. As with any assessment, there are
uncertainties and limitations, as discussed in the PA (PA, sections 6.3
and 6.4). For example, the SLAMS analyses are site-based comparisons
that do not account for deposition associated with the transport of
pollutants emitted some distance upwind. Similarly, the other analyses
have their own limitations ranging from model uncertainty to
limitations in geographical scope. In combination, these analyses
supported the PA conclusion of a strong association between
SO<INF>2</INF> and S deposition. The results and associated information
for N oxides and PM, however, indicate more variable relationships,
both between NO<INF>2</INF> concentrations and N deposition, and
between 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 with S deposition estimates
in the TDep dataset at the local scale (correlation coefficient of
0.70),\33\ especially in the earlier periods of the record and across
the eastern U.S. (PA, section 6.2.3). This association is also seen in
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).
---------------------------------------------------------------------------
\33\ The correlation coefficients reported here, from the PA,
are based on Spearman's rank correlation coefficient. These
nonparametric coefficients are generally used with data that are not
normally distributed 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 at
relatively remote sites, generally distant from emissions sources. For
example, an evaluation of the associations of total S TDep estimates
with SO<INF>4</INF><SUP>2-</SUP> concentrations and of wet S deposition
with the sum of SO<INF>2</INF> + SO<INF>4</INF><SUP>2-</SUP> at 27
sites in 27 Class I areas concluded that the correlations for S
deposition with particulate SO<INF>4</INF><SUP>2-</SUP> and total S
(i.e., SO<INF>2</INF> + SO<INF>4</INF><SUP>2-</SUP>) were lower than
what was exhibited for S deposition and SO<INF>2</INF> concentrations
at the SLAMS (PA, section 6.2.2). The analyses also found poor
correlation (correlation coefficient of 0.33) between total S
deposition estimates (TDep) and PM<INF>2.5</INF> mass at IMPROVE sites
in the 27 Class I areas (PA, sections 2.3.3 and 6.2.2.3). While this
set of analyses is based on data at a relatively limited number of
sites (e.g., compared to the SLAMS network), the results do not
indicate advantages to PM<INF>2.5</INF> mass, particulate
SO<INF>4</INF><SUP>2-</SUP>, or total S (SO<INF>4</INF><SUP>2-</SUP>
plus SO<INF>2</INF>) over SO<INF>2</INF> (alone) as an indicator for a
secondary NAAQS to address S deposition-related effects.
Both NO<INF>2</INF> and certain components of PM<INF>2.5</INF>
(NO<INF>3</INF><SUP>-</SUP> and NH<INF>4</INF><SUP>+</SUP>) 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 to relating ambient air concentrations of
NO<INF>2</INF> and PM<INF>2.5</INF> mass to N deposition. First, not
all N deposition is caused by these pollutants (PA, Chapter 2 and
section 6.1.1). Ammonia, which is not a criteria pollutant, also
contributes 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>\+\) and these comprise less than about 30% of
PM<INF>2.5</INF> mass across the U.S., below 5% in some regions (PA,
Figure 6-56). As a result of these two factors, the associations
between NO<INF>2</INF> concentrations and N deposition, and between
PM<INF>2.5</INF> concentrations and N deposition are less robust than
what is observed for SO<INF>2</INF> and S deposition. The multi-faceted
approach to evaluating these relationships confirmed this expectation.
For example, there are
[[Page 105706]]
weaker associations of N deposition with NO<INF>2</INF> observations at
SLAMS across the U.S. than what is observed in the similar S deposition
and SO<INF>2</INF> analysis (PA, section 6.4.2). There is little
correlation for N deposition with NO<INF>2</INF> concentrations, 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 at the identified upwind sites 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 for NO<INF>2</INF>
concentrations with N deposition was confirmed by 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 with trends in total or reduced
atmospheric N deposition. Since 2010, NO<INF>2</INF> concentrations
have continued to drop while N deposition nationally has remained
steady (PA, section 6.2.1). As for S deposition and S compound metrics,
the PA also investigated relationships between N deposition and air
quality metrics other than the current indicator species
(NO<INF>2</INF>) in the 27 Class I areas where collocated data were
available. Recognizing that such information was not available in
other, less remote areas of the U.S., including areas where
contributing emissions are highest or at the regulatory SLAMS monitors,
no clear advantages of these other parameters (e.g., nitric acid,
particulate NO<INF>3</INF><SUP>-</SUP>, and NH<INF>4</INF>\+\) over
NO<INF>2</INF> or PM<INF>2.5</INF> mass were indicated. Across all
analyses, the evidence indicates NO<INF>2</INF> to be a weak indicator
of total atmospheric N deposition, especially in areas where
NH<INF>3</INF> is prevalent and where PM<INF>2.5</INF> mass is
dominated by species other than NO<INF>3</INF><SUP>-</SUP> or
NH<INF>4</INF><SUP>+</SUP> (PA, section 6.4.2).
3. Overview of Welfare Effects Evidence
More than 3,000 welfare effects studies, including approximately
2,000 studies newly available since the last review, have been
considered in the ISA.<SUP>34 35</SUP> While expanding the evidence for
some effect categories, the studies on acid deposition, an important
category of effects in the last review, are largely consistent with the
evidence that was previously available. The subsections below briefly
summarize the nature of welfare effects of S oxides, N oxides and PM
(section II.A.3.a.), the potential public welfare implications of these
effects (section II.A.3.b.), and exposure concentrations and
deposition-related metrics (section II.A.3.c.).
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\34\ 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.
\35\ The study count and citations are available on the project
page for the ISA on the Health & Environmental Research Online
(HERO) website (<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>).
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a. Nature of Effects
The welfare effects evidence base evaluated in the current review
includes decades of extensive research on the ecological effects of N
oxides, SO<INF>X</INF> and PM. The sections below provide an overview
of the nature of the direct effects of gas-phase exposure to oxides of
nitrogen and sulfur (section II.A.3.a.(1)), acid deposition-related
ecological effects (section II.A.3.a.(2)), N enrichment and associated
effects (section II.A.3.a.(3)), and other effects (section
II.A.3.a.(4)).
(1) Direct Effects of SO<INF>X</INF> and N Oxides in Ambient Air
A well-established body of scientific evidence 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. The
scientific evidence available for these effects in 1971 is the basis
for the current secondary NAAQS for SO<INF>X</INF> and N oxides.
The current scientific evidence continues to be sufficient to infer
a causal relationship between gas-phase SO<INF>2</INF> and injury to
vegetation (ISA, Appendix 3, section 3.6.1). High concentrations have
been associated with damage to plant foliage (ISA, Appendix 3, section
3.2). In addition to foliar injury, which is usually a rapid response,
and which can vary significantly among species and growth conditions
(which affect stomatal conductance), SO<INF>2</INF> exposures have also
been documented to reduce plant photosynthesis and growth. 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). Multiple factors, including reduced
deposition, buffering and other environmental variables, may play a
role in such species recovery. (ISA, Appendix 3, section 3.2, Schaberg
et al., 2014). Some of this evidence seems to suggest a somewhat faster
recovery than might be expected from deposition-related soil
acidification alone, 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). 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).
The current scientific evidence also continues to be sufficient to
infer a causal relationship between gas-phase NO, NO<INF>2</INF> and
PAN and injury to vegetation (ISA, Appendix 3, section 3.6.2). 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).
With regard to HNO<INF>3</INF>, the evidence is sufficient to infer
a causal relationship between exposure to HNO<INF>3</INF> and changes
to vegetation (ISA, Appendix 3, section 3.6.3). The evidence suggests 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).
(2) Acid Deposition-Related Ecological Effects
The connection between SO<INF>X</INF> and N oxide emissions to
ambient air,
[[Page 105707]]
atmospheric deposition of S and/or N compounds, and the acidification
of acid-sensitive soils and surface waters is well documented by many
decades of evidence, particularly in the eastern U.S. (ISA, section
IS.5; Appendix 8, section 8.1). Sulfur oxides and N oxides in ambient
air undergo reactions to form 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 sulfuric acid
[H<INF>2</INF>SO<INF>4</INF>]), NH<INF>4</INF>\+\, and by chemical and
biological processes. When NO<INF>3</INF><SUP>-</SUP>, or
SO<INF>4</INF>2<SUP>-</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<SUP>2+</SUP>], sodium
[Na<SUP>+</SUP>], or potassium [K<SUP>+</SUP>]), rather than hydrogen
ions (H<SUP>+</SUP>), 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<SUP>+</SUP> 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).
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
aluminum (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).
(a) 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 among 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 and the ramifications for biological functioning of
freshwater ecosystems (ISA, Appendix 8, section 8.1). The more recently
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 concentration (ISA, Appendix
8, Table 8-9).
The effects of waterbody acidification on fish species are
especially well documented, with many species (e.g., brown and brook
trout and Atlantic salmon) experiencing adverse effects from
acidification and the earliest lifestages being most sensitive (ISA,
Appendix 8, section 8.3). Many effects of acidic surface waters on
fish, particularly effects on gill function or structure, relate to low
pH or the combination of low pH and elevated dissolved aluminum (ISA,
Appendix 7, section 7.1.2.5 and Appendix 8, sections 8.3.6.1 and
8.6.4). 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 aluminum, 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 microequivalents per liter ([mu]eq/L) in waters of the
Northeast with low to moderate levels of dissolved organic carbon (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 to 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 (summarized in section
II.A.4. below). Water quality models are generally better at estimating
ANC than at estimating other indicators of acidification-related risk,
such as pH.
[[Page 105708]]
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
(ISA, Appendix 7, section 7.1.2.6). In waters with high concentrations
of naturally occurring organic acids, however, ANC may not be a good
indicator of risk to biota as those acids can reduce bioavailability of
aluminum, thus buffering the effects usually associated with low pH and
high total aluminum concentrations (Waller et al., 2012; ISA, Appendix
8, section 8.3.6.4).
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). Headwater streams tend to be
more sensitive to such episodes due to their smaller watersheds and, in
the East, due to their underlying geology (ISA, Appendix 8, section
8.5.1).
National survey data available in the last review, and dating back
to the early 1980s through 2004, 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 1984-1987 survey of
waterbodies in the Adirondacks found 27% of streams to have ANC values
below zero, with a minimum value of -134 [mu]eq/L (Sullivan et al.,
2006). Values of ANC below 20 [mu]eq/L in Shenandoah stream sites have
been reported as having a greater risk of episodic acidification and
associated reduced populations of sensitive species, such as the native
brook trout, compared to sites with higher ANC (Bulger et al., 1999;
Bulger et al., 2000). 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).
(b) Terrestrial Ecosystems
Longstanding evidence, supported and strengthened by evidence newly
available in this review, describes the changes in soil biogeochemical
processes caused by acidifying deposition of N and S to terrestrial
systems that are linked to changes in terrestrial biota, with
associated impacts on ecosystem characteristics (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 aluminum from soil
to drainage water, and depletion of 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). 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 aluminum toxicity (related to
increased availability of inorganic aluminum 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 physiological effects of acidifying deposition on terrestrial
biota can also result in changes in species composition whereby
sensitive species, such as red spruce and sugar maple, 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<SUP>2+</SUP> addition or gradient experiments)
has been associated with greater growth and seedling colonization by
sugar maple, while American beech is 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). Soil acid-
base chemistry has also been found to be a predictor of understory
species composition (ISA, Appendix 5, section 5.2.2.1), and limited
evidence has indicated an influence of soil acid-base chemistry on
diversity and composition of soil bacteria, fungi, and nematodes (ISA,
Appendix 5, section 5.2.4.1). In addition to Ca<SUP>2+</SUP> addition
experiments, observational gradient studies have also evaluated
relationships between soil chemistry indicators of acidification (e.g.,
soil pH, base cation to aluminum (Bc:Al) ratio, base saturation, and
aluminum) 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). The 2020 ISA
also 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).
[[Page 105709]]
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
following section]) 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, p. 7-45 and section 7.1.5).
(3) Nitrogen Enrichment and Associated Ecological Effects
Ecosystems in the U.S. vary in their sensitivity to N enrichment,
with organisms in their natural environments 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 can contribute to N
enrichment. In addition to atmospheric deposition, other sources of N
compounds can play relatively greater or lesser roles in ecosystem N
loading, depending on location. For example, many waterbodies receive
appreciable amounts of N from agricultural runoff and municipal or
industrial wastewater discharges. For many aquatic ecosystems, sources
of N other than atmospheric deposition, including fertilizer and waste
treatment, contribute more to ecosystem N than atmospheric deposition
(ISA Appendix 7, sections 7.1 and 7.2). Additionally, the impacts of
historic N deposition in both aquatic and terrestrial ecosystems pose
complications to discerning the potential effects of more recent
deposition rates.
(a) Aquatic and Wetland Ecosystems
Nitrogen additions to freshwater, estuarine and near-coastal
ecosystems, including N from atmospheric deposition, 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 the alteration of
biogeochemistry in freshwater, estuarine and near-coastal marine
systems (ISA, Appendix 7, sections 7.1 and 7.2). Consistent with
findings in the last review, the current body of evidence is also
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).
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).
[[Page 105710]]
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 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 that nutrient enrichment
may play a role 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).
The impact of N additions on wetlands, and whether the wetlands may
serve as a source, sink, or transformer of atmospherically deposited N
varies with 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).
(b) 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
[[Page 105711]]
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). Newly available
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 related to soil pH
(ISA, Appendix 6, section 6.3.3.2).
Recent evidence includes associations of variation in lichen
community 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 differences in
atmospheric N pollution in forests of the West Coast, Rocky Mountains,
and 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 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 for plant and microbial community
composition effects of increased N availability, indicating that
competition for resources, such as water in arid and semi-arid
environments, may exacerbate the effects of N addition on diversity
(ISA, Appendix 6, section 6.2.6). The newly available studies in arid
and semiarid ecosystems, particularly in southern California 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). 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).
(4) 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 other than N
and S 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 (ISA, Table ES-1). 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
[[Page 105712]]
biodiversity in wetland and freshwater ecosystems (ISA, section IS.9).
With regard to PM deposition other than N and S 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).
b. Public Welfare Implications
In evaluating the public welfare implications of the evidence
regarding S and N related welfare effects, we must consider the type,
severity, and geographic extent of the effects. In this section, we
discuss such factors 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 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
there are many different types of effects that vary broadly with regard
to 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.A.3.a. 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).
How important ecological impacts are to the public welfare depends
on the type, severity and extent of the effects, as well as the
societal use of the resource and the significance of the resource to
the public welfare. Such factors can also be 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 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).\36\ 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).
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\36\ 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)).
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Judgments regarding effects on the public welfare can depend on the
intended use, including conservation, 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 as well as other natural
features and 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 (Costanza 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
[[Page 105713]]
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.A.3.1. 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. 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 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. Additionally, 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 some areas in recent
years (ISA, Appendix 10, section 10.10.1).
A complication to considering the public welfare implications
specific to N deposition in terrestrial systems is the potential for N
to increase growth and yield of plants that, depending on the type of
plant and its use by human populations (e.g., food for livestock or
human populations, trees for lumber), could be judged beneficial to the
public. Such increased growth and yield may be judged and valued
differently than changes in growth of other species. As noted in
section II.A.3.a. 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, and magnitude. Impacts on some ecosystem 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 the welfare effects evidence base describes 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)
\37\ 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).\38\ This
[[Page 105714]]
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.
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\37\ As recognized in section II.A.3.a.(3)(b) 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).
\38\ 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).
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In summary, several considerations are important to judgments on
the public welfare significance of given welfare effects under
different exposures. 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 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.
c. Exposure Conditions and Deposition-Related Metrics
The ecological effects identified in section II.A.3.a. above vary
widely in their extent and the resolution of the available information
that describes the exposure circumstances under which they occur. 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 presented 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.1.c. 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 influences the degree to which deposition of N
and S associated with the oxides of S and N and PM in ambient air may
be linked to 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 from the range of effects for which we
have evidence 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 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 result in an
ecological effect of interest, has multiple interpretations and
applications (ISA, p. IS-14). The dynamic nature of ecosystem pollutant
processing and the broad array of factors that influence it adds
complications to critical load identification and interpretation. Time
is an important dimension, which is sometimes unstated (e.g., in
empirical or observational analyses) and is sometimes explicit (e.g.,
in steady-state or dynamic modeling analyses) (ISA, section IS.2.2.4).
Further, this variety in meanings stems in part from differing
judgments and associated identifications regarding the ecological
effect (both type and level of severity) on which the critical load
focuses and judgment of its significance or meaning.
Studies, based on which CLs are often identified, vary widely with
regard to the specific ecosystem characteristics being evaluated, as
well as the benchmarks selected for judging them. The specific details
of these various judgments influence the strengths and limitations, and
associated uncertainty, of using critical load information from such
studies for different applications. The summary that follows is
intended to reach beyond individual critical loads developed over a
variety of studies and ecosystems and consider the underlying study
findings about key aspects of the environmental conditions and
ecological characteristics studied. A more quantitative variation of
this is the methodology developed for the aquatic acidification REA in
this review, presented in the PA and summarized in section II.A.4.
below. In those analyses, the concept of a critical load is employed
with steady-state modeling that relates deposition to waterbody acid
neutralizing capacity.
While recognizing the inherent connections between watersheds and
waterbodies, such as lakes and streams, the organization of this
section recognizes the more established state of the information,
tools, and data for aquatic ecosystems for characterizing relationships
between atmospheric deposition and acidification and/or nutrient
enrichment effects under air quality associated with the current
standards (PA, Chapter 5).\39\ Further, we
[[Page 105715]]
recognize the generally greater role of atmospheric deposition in
waterbodies impacted by aquatic acidification compared to its role in
eutrophication-related impacts of surface waters, particularly rivers
and estuaries in and downstream of populated watersheds, to which
direct discharges have also long contributed, as recognized in section
II.A.3.a(3) above (ISA, Appendix 13, section 13.1.3.1; ISA, Appendix 7,
section 7.1.1.1; 2008 ISA, section 3.2). Therefore, with regard to
deposition-related effects, we focus first on the quantitative
information for aquatic ecosystem effects in sections II.A.3.c.(1)
below. Section II.A.3.c.(2) discusses the available evidence regarding
relationships between deposition-related exposures and the occurrence
and severity of effects on trees and understory communities in
terrestrial ecosystems. Section II.A.3.c.(3) discusses the currently
available information related to consideration of exposure
concentrations associated with other welfare effects of nitrogen and
sulfur oxides and PM in ambient air.
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\39\ With regard to other deposition-related effects of S
compounds, quantitative tools or approaches for relating S
deposition to ecosystem impacts are not currently well developed. As
summarized in section II.A.3.a.(4) above, these effects, in wetland
and freshwater ecosystems, include the alteration of Hg methylation
in surface water, sediment, and soils; and changes in biota due to
sulfide phytotoxicity including alteration of growth and
productivity, species physiology, species richness, community
composition, and biodiversity. No studies are in the available
evidence regarding the estimation of critical loads for
SO<INF>X</INF> deposition related to these non-acidifying effects of
S deposition into these ecosystems (ISA, Appendix 12, section 12.6).
---------------------------------------------------------------------------
(1) Acidification and Nitrogen Enrichment in Aquatic Ecosystems
Prior to the peak in S deposition levels that occurred in the 1970s
and early 1980s, when deposition likely exceeded 30 kg S/ha-yr in some
areas (PA, Appendix B, Figure 5B-9), surface water
SO<INF>4</INF><SUP>2-</SUP> concentrations were increasing in response
to the extremely high S deposition of the preceding years.
Subsequently, and especially more recently, surface water
SO<INF>4</INF><SUP>2-</SUP> concentrations have generally decreased,
particularly in the Northeast (Robinson et al., 2008; ISA, section
7.1.5.1.4). Some studies of long-term projections in some waterbodies
(e.g., in the Blue Ridge Mountains region in Virginia), however,
continue to indicate little or slow reduction in acidic ions, even as
emissions have declined. This is an example of the competing role of
changes in S adsorption on soils and the release of historically
deposited S from soils into surface water,\40\ which some modeling has
suggested will delay chemical recovery in those water bodies (ISA,
Appendix 7, sections 7.1.2.2 and 7.1.5.1).
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\40\ Some modeling studies in some areas have indicated the
potential for a lagged response even as emissions and deposition
decline; this lag reflects a reduction in soil absorption of
SO<INF>4</INF><SUP>-2</SUP> and leaching of previously accumulated S
from watersheds (ISA, Appendix 7, section 7.1.2.2).
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In the 2012 review of the oxides of N and S, quantitative analyses
relating deposition in recent times (e.g., since 2000) to ecosystem
acidification, and particularly aquatic acidification, were generally
considered to be less uncertain, and the ability of those analyses to
inform NAAQS policy judgments more robust, than analyses related to
deposition and ecosystem nutrient enrichment or eutrophication (2011
PA). While quantitative assessment approaches for aquatic
eutrophication as a result of total N loading are also well
established, and the evidence base regarding atmospheric deposition and
nutrient enrichment has expanded since the 2012 review, the
significance of non-air N loading to rivers, estuaries and coastal
waters (as recognized in section II.A.3.a. above) continues to
complicate the assessment of nutrient enrichment-related risks
specifically related to atmospheric N deposition. Accordingly, the REA
analyses developed in this review focus on aquatic acidification. The
REA and its findings regarding deposition rates associated with
different levels of aquatic acidification risk are summarized in
section II.A.4. below. Thus, the paragraphs below focus on available
quantitative information regarding atmospheric deposition and N
enrichment in aquatic ecosystems.\41\ The overview provided here draws
on the summary in the PA of the evidence as characterized in the ISA
with regard to deposition level estimates that studies have related to
various degrees of different effects with associated differences in
potential for or clarity in public welfare significance (PA, section
5.2).
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\41\ Separate quantitative analyses have not been performed in
this review for N enrichment-related effects in these waterbodies in
recognition of a number of factors, including modeling and
assessment complexities, and site- or waterbody-specific data
requirements, as well as, in some cases, issues of apportionment of
atmospheric sources separate from other influential sources.
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The eutrophication of wetlands and other aquatic systems is
primarily associated with nitrogen inputs, whether from deposition or
other sources. Atmospheric deposition is the main source of new N
inputs to some freshwater wetlands and fresh waterbodies, such as
headwater streams and high-elevation lakes, while other N inputs, such
as agricultural runoff and wastewater effluent, can be significant
contributors to waterbodies in agricultural and populated areas (ISA,
Appendix 9, section 9.1 and Appendix 11, section 11.3.1). Rates of
total N deposition associated with eutrophication-related effects in
aquatic systems ranges from a few kilograms per hectare per year (kg/
ha-yr) for differences in diatom community composition in high
elevation lakes to over 500 kg N/ha-yr for some effects in saltwater
wetlands. While the evidence for these effects contributes to ISA
causal determinations, it is often very location-specific and less
informative for other uses, such as in quantitative assessments
relating deposition to waterbody response across broad geographic
areas.
In estuaries and coastal systems, the well-established
relationships between N loading and algal blooms and associated water
quality impacts have been the focus of numerous water quality modeling
projects that have quantified eutrophication processes across a wide
variety of U.S. ecosystems. These projects, which have generally
involved quantification of N loading and association with various water
quality indicators, have informed management decision-making in
multiple estuaries, including Chesapeake Bay, Narraganset Bay, Tampa
Bay, Neuse River Estuary and Waquoit Bay (ISA, Appendix 7, section
7.2). The indicators of nutrient enrichment employed include
chlorophyll a, dissolved oxygen, and reduced abundance of submerged
aquatic vegetation, among others (ISA, section IS.7.3 and Appendix 10,
section 10.6).
The decision-making in these projects generally focuses on
identification of total N loading targets for purposes of attaining
water quality standards, informed by modeling work that includes
apportionment of sources, which vary by system. We note that the
assignment of targets to different source types (e.g., groundwater,
surface water runoff, and atmospheric deposition) in different
waterbodies and watersheds varies for both practical and policy
reasons. Further, during the multi-decade time period across which
these activities have occurred, atmospheric deposition of N in coastal
areas has declined. In general, however, atmospheric deposition targets
for N for the large systems summarized above have been approximately 10
kg/ha-yr.
The establishment of target N loads to surface waterbodies is in
many areas related to implementation of the total maximum daily load
(TMDL) requirements of section 303(d) of the Clean Water Act.\42\
Nutrient load allocation and reduction activities in some large
estuaries predate
[[Page 105716]]
development of CWA 303(d) TMDLs. The multiple Chesapeake Bay Agreements
signed by the U.S. EPA, District of Columbia, and states of Virginia,
Maryland, and Pennsylvania first established the voluntary government
partnership that directs and manages bay cleanup efforts and
subsequently included commitments for reduction of N and phosphorus
loading to the bay. Efforts prior to 2000 focused largely on point-
source discharges, with slower progress for nonpoint-source reductions
via strategies such as adoption of better agricultural practices,
reduction of atmospheric N deposition, enhancement of wetlands and
other nutrient sinks, and control of urban sprawl (2008 ISA, section
3.3.8.3). Studies since 2000 estimate atmospheric deposition as a major
N source in the overall N budget for the Chesapeake Bay \43\ (ISA,
section 7.2.1; Howarth, 2008; Boyer et al., 2002). The TMDL established
for the Chesapeake Bay in 2010, under requirements of section 303(d) of
the Clean Water Act, included a loading allocation for atmospheric
deposition of N directly to tidal waters, which was projected to be
achieved by 2020 based on air quality progress under existing CAA
regulations and programs (U.S. EPA, 2010).\44\
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\42\ Under the CWA, section 303(d), every two years, states and
other jurisdictions are required to list impaired waterbodies not
meeting water quality standards. For waterbodies on the list, a TMDL
must be developed that identifies the maximum amount of pollutant a
waterbody can receive and still meet water quality standards, e.g.,
standards for dissolved oxygen and chlorophyll a (which are
indicators of eutrophication).
\43\ For example, a 2011 analysis estimated atmospheric
deposition to the Chesapeake Bay watershed to account for
approximately 25% of total N inputs to the estuary (ISA, Appendix 7,
section 7.2.1).
\44\
[…truncated; see source link]Indexed from Federal Register on December 27, 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.