Proposed Rule2025-20357
Determination of Attainment by the Attainment Date but for International Emissions for the 2015 Ozone National Ambient Air Quality Standards; Phoenix-Mesa Nonattainment Area, Arizona
Primary source
Metadata and text below are from the Federal Register, a public-domain U.S. government work. Always verify the official published version before relying on it for any legal matter.
Published
November 19, 2025
Issuing agencies
Environmental Protection Agency
Abstract
The Environmental Protection Agency (EPA or "Agency") is proposing to determine that the Phoenix-Mesa nonattainment area ("Phoenix-Mesa area") would have attained the 2015 ozone national ambient air quality standards (NAAQS) by the August 3, 2024 "Moderate" area attainment date, but for emissions emanating from outside the United States. If we finalize this proposed action, the Phoenix-Mesa area would no longer be subject to the Clean Air Act (CAA) requirements pertaining to reclassification upon failure to attain and therefore would remain classified as a Moderate nonattainment area for the 2015 ozone NAAQS. This action, when finalized, will fulfill the EPA's statutory obligation to determine whether the Phoenix-Mesa area attained the NAAQS by the attainment date.
Full Text
<html>
<head>
<title>Federal Register, Volume 90 Issue 221 (Wednesday, November 19, 2025)</title>
</head>
<body><pre>
[Federal Register Volume 90, Number 221 (Wednesday, November 19, 2025)]
[Proposed Rules]
[Pages 52019-52030]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2025-20357]
=======================================================================
-----------------------------------------------------------------------
ENVIRONMENTAL PROTECTION AGENCY
40 CFR Part 52
[EPA-R09-OAR-2025-2833; FRL-13057-01-R9]
Determination of Attainment by the Attainment Date but for
International Emissions for the 2015 Ozone National Ambient Air Quality
Standards; Phoenix-Mesa Nonattainment Area, Arizona
AGENCY: Environmental Protection Agency (EPA).
ACTION: Proposed rule.
-----------------------------------------------------------------------
SUMMARY: The Environmental Protection Agency (EPA or ``Agency'') is
proposing to determine that the Phoenix-Mesa nonattainment area
(``Phoenix-Mesa area'') would have attained the 2015 ozone national
ambient air quality standards (NAAQS) by the August 3, 2024
``Moderate'' area attainment date, but for emissions emanating from
outside the United States. If we finalize this proposed action, the
Phoenix-Mesa area would no longer be subject to the Clean Air Act (CAA)
requirements pertaining to reclassification upon failure to attain and
therefore would remain classified as a Moderate nonattainment area for
the 2015 ozone NAAQS. This action, when finalized, will fulfill the
EPA's statutory obligation to determine whether the Phoenix-Mesa area
attained the NAAQS by the attainment date.
DATES: Comments must be received on or before December 19, 2025.
ADDRESSES: Submit your comments, identified by Docket ID No. EPA-R09-
OAR-2025-2833 at <a href="https://www.regulations.gov">https://www.regulations.gov</a>. For comments submitted at
<a href="http://Regulations.gov">Regulations.gov</a>, follow the online instructions for submitting
comments. Once submitted, comments cannot be edited or removed from
<a href="http://Regulations.gov">Regulations.gov</a>. The EPA may publish any comment received to its public
docket. Do not submit electronically any information you consider to be
Confidential Business Information (CBI) or other information whose
disclosure is restricted by statute. Multimedia submissions (audio,
video, etc.) must be accompanied by a written comment.
[[Page 52020]]
The written comment is considered the official comment and should
include discussion of all points you wish to make. The EPA will
generally not consider comments or comment contents located outside of
the primary submission (i.e., on the web, cloud, or other file sharing
system). For additional submission methods, please contact the person
identified in the FOR FURTHER INFORMATION CONTACT section. For the full
EPA public comment policy, information about CBI or multimedia
submissions, and general guidance on making effective comments, please
visit <a href="https://www.epa.gov/dockets/commenting-epa-dockets">https://www.epa.gov/dockets/commenting-epa-dockets</a>. If you need
assistance in a language other than English or if you are a person with
disabilities who needs a reasonable accommodation at no cost to you,
please contact the person identified in the FOR FURTHER INFORMATION
CONTACT section.
FOR FURTHER INFORMATION CONTACT: Karina O'Connor, EPA Region IX, 75
Hawthorne St., San Francisco, CA 94105; telephone number: (415) 725-
8713, email address: <a href="/cdn-cgi/l/email-protection#29464a464747465b0742485b404748694c5948074e465f"><span class="__cf_email__" data-cfemail="39565a565757564b1752584b505758795c4958175e564f">[email protected]</span></a>.
SUPPLEMENTARY INFORMATION: Throughout this document ``we,'' ``us,'' or
``our'' refer to the EPA.
Table of Contents
I. Background
A. 2015 Ozone National Ambient Air Quality Standard and Area
Designations
B. Clean Air Act Requirements for Moderate Ozone Nonattainment
Areas
C. Requirement for Determination of Attainment of the 2015 Ozone
NAAQS
D. International Transport and Clean Air Act Section 179B
E. Exceptional Events and Other Data Modification
II. Phoenix-Mesa Ozone Determination of Attainment but for
International Emissions
A. Phoenix-Mesa Ozone Nonattainment Area
B. Ozone Monitoring Sites in Phoenix-Mesa
C. Summary of the State's Submission
D. EPA Review of the State's Submission
III. Proposed Action
IV. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
B. Executive Order 14192: Unleashing Prosperity Through
Deregulation
C. Paperwork Reduction Act (PRA)
D. Regulatory Flexibility Act (RFA)
E. Unfunded Mandates Reform Act (UMRA)
F. Executive Order 13132: Federalism
G. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
H. Executive Order 13045: Protection of Children From
Environmental Health and Safety Risks
I. Executive Order 13211: Actions That Significantly Affect
Energy Supply, Distribution, or Use
J. National Technology Transfer Advancement Act (NTTAA)
I. Background
A. 2015 Ozone National Ambient Air Quality Standard and Area
Designations
Ground-level ozone pollution is formed from the reaction of
volatile organic compounds (VOC) and oxides of nitrogen
(NO<INF>X</INF>) in the presence of sunlight. These two pollutants,
referred to as ozone precursors, are emitted by many types of sources,
including on- and non-road motor vehicles and engines, power plants and
industrial facilities, and smaller area sources such as lawn and garden
equipment and paints. Scientific evidence indicates that adverse public
health effects occur following exposure to ground-level ozone
pollution. Exposure to ozone can harm the respiratory system (the upper
airways and lungs), can aggravate asthma and other lung diseases, and
is linked to premature death from respiratory causes. People most at
risk from breathing air containing ozone include people with asthma,
children, older adults, and people who are active outdoors, especially
outdoor workers.\1\
---------------------------------------------------------------------------
\1\ EPA Fact Sheet--Ozone and Health, available at <a href="https://www.epa.gov/sites/default/files/2016-04/documents/20151001healthfs.pdf">https://www.epa.gov/sites/default/files/2016-04/documents/20151001healthfs.pdf</a> and in the docket for this action.
---------------------------------------------------------------------------
Under CAA section 109, the EPA promulgates NAAQS (or ``standards'')
for pervasive air pollutants, such as ozone. The EPA has previously
promulgated NAAQS for ozone in 1979, 1997, and 2008.\2\ On October 26,
2015, the EPA revised the NAAQS for ozone to establish new 8-hour
standards.\3\ In that action, the EPA promulgated identical revised
primary and secondary ozone standards designed to protect public health
and welfare that specified an 8-hour ozone level of 0.070 parts per
million (ppm).\4\ Specifically, the standards require that the 3-year
average of the annual fourth highest daily maximum 8-hour average ozone
concentration (i.e., the design value) may not exceed 0.070 ppm.\5\
When the design value (DV) does not exceed 0.070 ppm at each ambient
air quality monitoring site within the area, the area is deemed to be
attaining the ozone NAAQS.\6\
---------------------------------------------------------------------------
\2\ 44 FR 8202 (February 8, 1979), 62 FR 38856 (July 18, 1997),
and 73 FR 16436 (March 27, 2008).
\3\ 80 FR 65292.
\4\ Because the 2015 primary and secondary NAAQS for ozone are
identical, for convenience, the EPA refers to them in the singular
as ``the 2015 ozone NAAQS'' or as ``the standard.''
\5\ A design value is a statistic used to compare data collected
at an ambient air quality monitoring site to the applicable NAAQS to
determine compliance with the standard. The design value for the
2015 ozone NAAQS is the 3-year average of the annual fourth highest
daily maximum 8-hour average ozone concentration. The design value
is calculated for each air quality monitor in an area and the area's
design value is the highest design value among the individual
monitoring sites in the area.
\6\ The data handling convention in 40 CFR part 50, appendix U
dictates that concentrations shall be reported in ``ppm'' to the
third decimal place, with additional digits to the right being
truncated. Thus, a computed 3-year average ozone concentration of
0.071 ppm is greater than 0.070 ppm and would exceed the standard,
but a design value of 0.0709 is truncated to 0.070 and attains the
2015 ozone NAAQS.
---------------------------------------------------------------------------
Section 107(d) of the CAA provides that when the EPA promulgates a
new or revised NAAQS, the Agency must designate areas of the country as
nonattainment, attainment, or unclassifiable based on whether an area
is not meeting (or is contributing to air quality in a nearby area that
is not meeting) the NAAQS, meeting the NAAQS, or cannot be classified
as meeting or not meeting the NAAQS, respectively. Subpart 2 of part D
of title I of the CAA governs the classification, state planning, and
emissions control requirements for any areas designated as
nonattainment for a revised primary ozone NAAQS. In particular, CAA
section 181(a)(1) also requires the EPA to classify each ozone
nonattainment area at the time of designation, based on the extent of
the ozone problem in the area (based on the area's DV). Classifications
for ozone nonattainment areas range from ``Marginal'' to ``Extreme.''
CAA section 182 provides the specific attainment planning and
additional requirements that apply to each ozone nonattainment area
based on its classification. CAA section 182, as interpreted in the
EPA's implementing regulations at 40 CFR 51.1308 through 51.1317, also
establishes the timeframes by which air agencies must submit and
implement SIP revisions to satisfy the applicable attainment planning
elements, and the timeframes by which nonattainment areas must attain
the 2015 ozone NAAQS.
Effective on August 3, 2018, the EPA designated 51 areas throughout
the country, including Phoenix-Mesa, Arizona, nonattainment for the
2015 ozone NAAQS.\7\ In a separate action, the EPA assigned
classification thresholds and attainment dates based on the
[[Page 52021]]
severity of each nonattainment area's ozone problem, determined by the
area's DVs and classified the Phoenix-Mesa area as Marginal.\8\ The EPA
established the attainment date for ``Marginal,'' ``Moderate,'' and
``Serious'' nonattainment areas as three years, six years, and nine
years, respectively, from the effective date of the final designations.
Thus, the attainment date for Marginal nonattainment areas for the 2015
ozone NAAQS was August 3, 2021, the attainment date for Moderate areas
was August 3, 2024, and the attainment date for Serious areas is August
3, 2027. On October 7, 2022, the EPA determined that 22 areas,
including the Phoenix-Mesa area addressed in this action, did not
attain the standards by the Marginal attainment date, and these areas
were reclassified as Moderate by operation of law.\9\
---------------------------------------------------------------------------
\7\ 83 FR 25776 (June 4, 2018), effective August 3, 2018. The
EPA later designated the San Antonio area as a 2015 ozone NAAQS
nonattainment area effective September 24, 2018. 83 FR 35136 (July
25, 2018).
\8\ 83 FR 10376 (March 9, 2018), effective May 8, 2018.
\9\ 87 FR 60897.
---------------------------------------------------------------------------
B. Clean Air Act Requirements for Moderate Ozone Nonattainment Areas
The list of applicable requirements for ozone nonattainment areas
classified as Moderate includes a baseline emissions inventory, source
emission statement rules, nonattainment new source review program
requirements, an attainment demonstration, a reasonably available
control measures (including reasonably available control technology)
demonstration, a reasonable further progress (RFP) demonstration, and
contingency measures for failure to attain or achieve RFP.
Attainment contingency measures are triggered upon the EPA's
determination that an area failed to attain a given NAAQS by its
applicable attainment date. For ozone nonattainment areas, such a
finding would be made pursuant to CAA section 181(b)(2), as described
in section I.C of this document. However, CAA section 179B(b) provides
that where a state demonstrates to the EPA that the area would have
attained the ozone NAAQS by the applicable attainment date but for
emissions emanating from outside the U.S., the area is not subject to
the reclassification provisions in section 181(b)(2) and will not be
reclassified to a higher nonattainment level. Therefore, following the
EPA's approval of a demonstration under section 179B(b), attainment
contingency measures will not be triggered. Given these considerations,
the EPA interprets the CAA not to require the state to have EPA-
approved contingency measures for failure to attain for the NAAQS at
issue in an area with an approved section 179B(b) demonstration.\10\
---------------------------------------------------------------------------
\10\ See 84 FR 58641, 58660 (November 1, 2019) (proposing the
same interpretation with respect to Imperial County for the 2008
ozone NAAQS); 85 FR 11817 (February 27, 2020) (finalizing the same).
---------------------------------------------------------------------------
RFP contingency measures under 172(c)(9) are generally triggered
upon the EPA's determination that an area failed to achieve RFP.
However, in the case of Moderate ozone nonattainment areas, the EPA's
long-standing interpretation is that RFP contingency measures can be
triggered only by a finding that the area has failed to attain the
NAAQS by the attainment date under 181(b)(2).\11\ This interpretation
is based on the RFP requirements of the CAA, the purpose of which is to
``ensur[e] attainment of the applicable [NAAQS] by the applicable
date.'' \12\ Consistent with this purpose, under CAA section 182(g),
ozone nonattainment areas classified ``Serious'' or higher are required
to meet RFP emissions reduction ``milestones'' and to demonstrate
compliance with those milestones, except when the milestone coincides
with the attainment date and the standard has been attained.\13\ This
specific statutory exemption from milestone compliance demonstration
submittals for areas that attained by the attainment date indicates
that Congress intended that a finding that an area attained the
standard--the finding made in a determination of attainment by the
attainment date--would serve as a demonstration that RFP requirements
for the area have been met. In other words, if a Serious or above area
has attained the NAAQS by the attainment date, the RFP milestones have
been sufficiently achieved. Accordingly, such a finding would also
indicate that RFP contingency measures could not be triggered and are
therefore no longer necessary.
---------------------------------------------------------------------------
\11\ See 57 FR 13498, 13511 (contrasting Moderate areas, for
which contingency measures would be triggered ``when the area fails
to attain the standard by the attainment date'' with Serious and
above areas, for which contingency measures would also be triggered
``if the area fails to meet the rate-of-progress requirements for
any milestone other than one falling on an attainment year''). See
also Memorandum from G.T. Helms, Chief Ozone/Carbon Monoxide
Programs Branch, to Air Branch Chief, Regions I-X (``The test for
moderate areas will be whether they attained the standard because
the attainment date for moderate areas coincides with the milestone
demonstration date. Failure to attain will cause an area to be
required to implement its contingency measures . . .'').
\12\ CAA section 171(1).
\13\ CAA section 182(g)(2).
---------------------------------------------------------------------------
In the case of Moderate areas, there are no RFP milestone
compliance demonstration requirements.\14\ Therefore, the EPA has
previously concluded that ``a determination of attainment by the
attainment date for a Moderate area serves as demonstration that RFP
requirements for the area have been met and that RFP contingency
measures are no longer needed.'' \15\
---------------------------------------------------------------------------
\14\ CAA section 182(g)(1)(exempting areas classified as
Moderate from milestone requirements).
\15\ 84 FR 52838, 52847 (October 3, 2019). See also 85 FR 33571
(June 2, 2020) (finalizing this interpretation).
---------------------------------------------------------------------------
C. Requirement for Determination of Attainment of the 2015 Ozone NAAQS
Section 181(b)(2)(A) of the CAA requires that within six months
following the applicable attainment date, the EPA shall determine
whether an ozone nonattainment area attained the ozone standard based
on the area's design value as of that date. If the EPA determines that
an area failed to attain, CAA section 181(b)(2)(A) requires the area to
be reclassified by operation of law to the higher of: (1) the next
higher classification for the area, or (2) the classification
applicable to the area's design value as of the determination of
failure to attain.\16\ Section 181(b)(2)(B) of the CAA requires the EPA
to publish the determination of failure to attain and accompanying
reclassification in the Federal Register no later than six months after
the attainment date, which was February 3, 2025 for the Phoenix-Mesa
area.
---------------------------------------------------------------------------
\16\ If the EPA were to determine that the Phoenix-Mesa
nonattainment area failed to attain by the attainment date, it would
be classified to the next highest classification of Serious. The
reclassified area would then be subject to the Serious area
requirement to attain the 2015 ozone NAAQS as expeditiously as
practicable, but not later than August 3, 2027.
---------------------------------------------------------------------------
The EPA's proposed determination that the Phoenix-Mesa area would
have attained the 2015 ozone NAAQS but for international emissions is
based in part upon data that have been collected and quality-assured by
the Arizona Department of Environmental Quality (ADEQ), the Maricopa
County Air Quality Department (MCAQD), the Pinal County Air Quality
Control District (PCAQCD), and the Salt River Pima Maricopa Indian
Community (SRPMIC) in accordance with 40 CFR part 58 and recorded in
the EPA's Air Quality System (AQS) database.\17\
---------------------------------------------------------------------------
\17\ The EPA maintains the AQS, a database that contains ambient
air pollution data collected by the EPA, state, local, and tribal
air pollution control agencies. The AQS also contains meteorological
data, descriptive information about each monitoring station
(including its geographic location and its operator) and data
quality assurance/quality control information. The AQS data are used
to (1) assess air quality, (2) assist in attainment/non-attainment
designations, (3) evaluate SIPs for non-attainment areas, (4)
perform modeling for permit review analysis, and (5) prepare reports
for Congress as mandated by the CAA. Access is through the website
at <a href="https://www.epa.gov/aqs">https://www.epa.gov/aqs</a>.
---------------------------------------------------------------------------
[[Page 52022]]
The level of the 2015 ozone NAAQS is 0.070 parts per million
(ppm).\18\ Under the EPA regulations at 40 CFR part 50, appendix U, the
2015 ozone NAAQS is attained at a site when the 3-year average of the
annual fourth highest daily maximum 8-hour average ambient ozone
concentration (i.e., DV) does not exceed 0.070 ppm. When the DV does
not exceed 0.070 ppm at each ambient air quality monitoring site within
the area, the area is deemed to be attaining the ozone NAAQS. Each
area's DV is determined by the highest DV among monitors with valid
DVs.\19\ The data handling convention in appendix U dictates that
concentrations shall be reported in ``ppm'' to the third decimal place,
with additional digits to the right being truncated. Thus, a computed
3-year average ozone concentration of 0.071 ppm is greater than 0.070
ppm and would exceed the standards, but a computed 3-year average ozone
concentration of 0.0709 ppm is truncated to 0.070 ppm and attains the
2015 ozone NAAQS.
---------------------------------------------------------------------------
\18\ See 40 CFR 50.19.
\19\ According to appendix U to 40 CFR part 50, ambient
monitoring sites with a DV of 0.070 ppm or less must meet minimum
data completeness requirements in order to be considered valid.
These requirements are met for a 3-year period at a site if daily
maximum 8-hour average ozone concentrations are available for at
least 90% of the days within the ozone monitoring season, on
average, for the 3-year period, with a minimum of at least 75% of
the days within the ozone monitoring season in any one year. Ozone
monitoring seasons are defined for each State in appendix D to 40
CFR part 58. DVs greater than 0.070 ppm are considered to be valid
regardless of the data completeness.
---------------------------------------------------------------------------
For the Phoenix-Mesa area, the Moderate attainment date was August
3, 2024. Because the DV is based on the three most recent, complete
calendar years of data, attainment must occur no later than December 31
of the year prior to the attainment date (i.e., December 31, 2023, in
the case of Moderate nonattainment areas for the 2015 ozone NAAQS).
Therefore, the EPA's determinations for this area are based in part
upon the complete, quality-assured, and certified ozone monitoring data
from calendar years 2021, 2022, and 2023. The DV for this period is
0.080 ppm, indicating that the Phoenix area did not attain the 2015
ozone NAAQS by its August 3, 2024 attainment date.\20\
---------------------------------------------------------------------------
\20\ 2023 Design Value_20241106_AMP480_2237472.
---------------------------------------------------------------------------
D. International Transport and Clean Air Act Section 179B
CAA section 179B(b) provides that where a state demonstrates to the
Administrator's satisfaction that an ozone nonattainment area would
have attained the NAAQS by the applicable attainment date but for
emissions emanating from outside the United States (U.S.), that area
shall not be subject to the mandatory reclassification provision of CAA
section 181(b)(2).\21\ In the event an air agency does not demonstrate
to the Administrator's satisfaction that it would have attained the
NAAQS but for international emissions, it will be reclassified to the
next higher classification.
---------------------------------------------------------------------------
\21\ Note that the statute cites 42 U.S.C. 7511(a)(2), but that
provision establishes ozone attainment deadlines for severe areas
under the 1-hour standard. The EPA has long interpreted the citation
in CAA section 179B(b) to be a scrivener's error that was supposed
to refer to 42 U.S.C. 7511(b)(2), which refers to consequences for
failure to attain by the attainment date.
---------------------------------------------------------------------------
Anthropogenic emissions sources outside of the U.S. can affect to
varying degrees the air quality of nonattainment areas in the U.S. In a
nonattainment area affected by international emissions, an air agency
may elect under CAA section 179B to develop and submit to the EPA a
demonstration intended to show that a nonattainment area would attain,
or would have attained, the relevant NAAQS by the applicable statutory
attainment date ``but for'' emissions emanating from outside the
U.S.\22\ Under CAA section 179B, the EPA evaluates such demonstrations,
and if it agrees with the air agency's demonstration, the EPA considers
the impacts of international emissions in taking specific regulatory
actions.
---------------------------------------------------------------------------
\22\ All references to CAA section 179B are to 42 U.S.C. 7509a.
International border areas, as added Pub. L. 101-549, title VIII,
section 818, 104 Stat. 2697 (November 15, 1990).
---------------------------------------------------------------------------
CAA section 179B provides the EPA with authority to consider
impacts from international emissions in two contexts: (1) a
``prospective'' state demonstration submitted as part of an attainment
plan, which the EPA considers when determining whether the SIP
adequately demonstrates that a nonattainment area will attain the NAAQS
by its future attainment date (CAA section 179B(a)); or (2) a
``retrospective'' state demonstration, which the EPA considers after
the attainment date in determining whether a nonattainment area
attained the NAAQS by the attainment date (CAA section 179B(b)-(d)).
First, CAA section 179B(a) provides that, ``[n]otwithstanding any
other provision of law, an implementation plan or plan revision
required under this chapter shall be approved by the Administrator if:
(1) such plan or revision meets all the requirements applicable to it .
. . other than a requirement that such plan or revision demonstrate
attainment and maintenance of the relevant national ambient air quality
standards by the attainment date specified under the applicable
provision of this chapter, or in a regulation promulgated under such
provision; and (2) the submitting state establishes to the satisfaction
of the Administrator that the implementation plan of such state would
be adequate to attain and maintain the relevant national ambient air
quality standards by the attainment date . . . but for emissions
emanating from outside of the United States,'' (emphasis added). The
EPA refers to CAA section 179B(a) demonstrations as ``prospective''
demonstrations because they are intended to assess future air quality,
taking into consideration the impact of international emissions.
Second, CAA section 179B(b) provides that, for ozone nonattainment
areas, ``[n]otwithstanding any other provision of law, any State that
establishes to the satisfaction of the Administrator that . . . such
State would have attained the national ambient air quality standard . .
. by the applicable attainment date but for emissions emanating from
outside of the United States,'' (emphasis added) shall not be subject
to reclassification to a higher classification category by operation of
law, as otherwise required in CAA section 181(b)(2).\23\ The EPA refers
to demonstrations developed under CAA section 179B(b) as
``retrospective'' demonstrations because they involve analyses of past
air quality (e.g., air quality data from the years evaluated for
determining whether an area attained by the attainment date). Thus, an
EPA-approved retrospective demonstration provides relief from
reclassification that would have resulted from the EPA determining that
the area failed to attain the NAAQS by the relevant attainment date.
---------------------------------------------------------------------------
\23\ The EPA's longstanding view is that CAA section 179B(b)
contains an erroneous reference to section 181(a)(2), and that
Congress actually intended to refer here to section 181(b)(2), which
addresses reclassification requirements for ozone nonattainment
areas. See ``State Implementation Plans; General Preamble for the
Implementation of Title I of the Clean Air Act Amendments of 1990,''
57 FR 13498, 13569, footnote 41 (April 16, 1992).
---------------------------------------------------------------------------
Irrespective of whether developing and submitting a prospective or
retrospective CAA section 179B demonstration, states still must meet
all nonattainment area requirements
[[Page 52023]]
applicable for the relevant NAAQS and area classification. The 2015
Ozone NAAQS Implementation Rule did not include regulatory requirements
specific to CAA section 179B but did provide guidance on certain
points. In the preamble to the rule, the EPA confirmed that: (1) only
areas classified Moderate and higher must show that they have
implemented RACM/RACT; (2) CAA section 179B demonstrations are not
geographically limited to nonattainment areas adjoining an
international border; and, (3) a state demonstration prepared under CAA
section 179B can consider emissions emanating from sources in North
America (i.e., Canada or Mexico) or sources on other continents.\24\ In
the preamble to that rule, the EPA encouraged air agencies to consult
with the appropriate EPA regional office in developing CAA section 179B
demonstrations.
---------------------------------------------------------------------------
\24\ 83 FR 62998, 63009.
---------------------------------------------------------------------------
The EPA issued more detailed guidance regarding CAA section 179B on
December 18, 2020, which included recommendations to assist state,
local, and Tribal air agencies that intend to develop a CAA section
179B demonstration (``179B Guidance'').\25\ On April 7, 2025, the
Administrator rescinded the 179B Guidance and announced that the ``EPA
intends to work with state and local air agencies to develop the
evidence necessary to grant regulatory relief under CAA Section
[179B].'' \26\ Accordingly, in this action, the EPA is applying
interpretations and policies that differ in certain respects from those
set forth in the rescinded 179B Guidance and previous actions under
179B(b), consistent with the discretion provided to the EPA under the
Act and relevant case law.\27\
---------------------------------------------------------------------------
\25\ ``Guidance on the Preparation of Clean Air Act Section 179B
Demonstrations for Nonattainment Areas Affected by International
Transport of Emissions'' issued on December 18, 2020. The EPA also
issued a notice of availability in the Federal Register on January
7, 2021 (86 FR 1105).
\26\ <a href="https://www.epa.gov/newsreleases/administrator-zeldin-moves-forward-ensuring-us-states-are-not-punished-foreign-air">https://www.epa.gov/newsreleases/administrator-zeldin-moves-forward-ensuring-us-states-are-not-punished-foreign-air</a>.
\27\ See, e.g., FCC v. Fox Television Stations, Inc., 556 U.S.
502, 515 (2009) (referencing Motor Vehicle Mfrs. Ass'n of United
States, Inc. v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29 (1983))
(an agency is free to change a prior policy and ``need not
demonstrate . . . that the reasons for the new policy are better
than the reasons for the old one; it suffices that the new policy is
permissible under the statute, that there are good reasons for it,
and that the agency believes it to be better.'')
---------------------------------------------------------------------------
The CAA does not specify what technical analyses would be
sufficient to demonstrate ``to the satisfaction of the Administrator''
that a ``State would have attained the [ozone NAAQS] by the applicable
attainment date, but for'' international emissions. Moreover, the best
reading of the phrase ``to the satisfaction of the Administrator'' is
that it provides inherent flexibility to the EPA to determine what
analyses are sufficient for this purpose.\28\ As described in previous
EPA 179B(b) determinations, ``[g]iven the extensive number of technical
factors and meteorological conditions that can affect international
transport of air pollution, and the lack of specific guidance in the
Act, the EPA evaluates CAA section 179B demonstrations based on the
weight of evidence of all information and analyses provided by the air
agency.'' \29\ We have further explained that, ``[t]he EPA considers
and qualitatively weighs all evidence based on its relevance to CAA
section 179B and the nature of international contributions as described
in the demonstration's conceptual model. Every demonstration should
include fact-specific analyses tailored to the nonattainment area in
question.\30\ The EPA is retaining this overall weight-of-evidence
approach to evaluating 179B(b) demonstrations, which we find to be
consistent with the discretion granted to the agency to under section
179B(b). However, we no longer consider specific characteristics as
necessarily suggesting the need for a more detailed demonstration with
additional evidence.\31\ Similarly, we are no longer applying our
previous policy that, ``[w]hen a CAA section 179B demonstration shows
that international contributions are larger than domestic
contributions, the weight of evidence will be more compelling than if
the demonstration shows domestic contributions exceeding international
contributions.'' \32\
---------------------------------------------------------------------------
\28\ Loper Bright Enterprises v. Raimondo, 609 U.S. 369, 395
(2024) (``[Other statutes] empower an agency . . . to regulate
subject to the limits imposed by a term or phrase that `leaves the
agencies with flexibility,' . . . such as `appropriate' or
`reasonable'.'').
\29\ 87 FR 50030, 50033 (August 15, 2022); see also 87 FR 21842,
21852 (April 13, 2022).
\30\ Id.
\31\ In previous actions on 179B demonstrations, the EPA has
stated the following characteristics would suggest the need for a
more detailed demonstration with additional evidence: (1) affected
monitors are not located near an international border; (2) specific
international sources and/or their contributing emissions are not
identified or are difficult to identify; (3) exceedances on
internationally influenced days are in the range of typical
exceedances attributable to local sources; and (4) exceedances
occurred in association with other processes and sources of
pollutants, or on days where meteorological conditions were
conducive to local pollutant formation (e.g., for ozone, clear skies
and elevated temperatures). See 87 FR 60897, 60906 (October 7,
2022); 87 FR 50030, 50033 (August 15, 2022).
\32\ 179B Guidance, p. 7. See also 87 FR 60897, 60906.
---------------------------------------------------------------------------
In evaluating previous CAA section 179B demonstrations, the EPA has
also considered what measures an air agency has implemented to control
local emissions. Specifically, the EPA has stated, ``(f)or the EPA to
concur with a state's CAA section 179B retrospective demonstration, the
weight of evidence should show the area could not attain with on-the-
books measures and potential reductions associated with controls
required for that particular NAAQS and classification that are to be
implemented by the attainment date.'' \33\ The EPA has also noted that,
``[b]ecause CAA section 179B does not relieve an air agency of its
planning or control obligations, the air agency should show that it has
implemented all required emissions controls at the local level as part
of its demonstration.'' \34\ The EPA is now determining that its prior
interpretation of CAA section 179B(b) with regard to planning and
control obligations was not the best reading of the statute. CAA
section 179B(b) does not expressly require that a state meet all CAA
requirements for an area's classification before the EPA can approve a
retrospective demonstration. Accordingly, the EPA is now proposing to
change our policy with respect to analysis of potential controls as
part of a 179B(b) demonstration. Under the proposed new interpretation,
states will no longer be expected to show that they could not attain
with on-the-books measures and potential reductions associated with
controls required to be implemented by the attainment date in order to
qualify for approval of a 179B(b) determination. For example, areas
classified Moderate and higher would no longer need to show that they
could not attain by implementing RACM/RACT. However, approval of a
179B(b) demonstration does not relieve a state of its obligation to
adopt and submit the required SIP elements for its existing
classification, with the exception of contingency measures as described
in section III. of this document.
---------------------------------------------------------------------------
\33\ 87 FR 21842, 21852 (April 13, 2022). See also 87 FR 50030,
50034 (August 15, 2022).
\34\ Id. See also 83 FR 62998, 63010 (``The EPA is not
finalizing our proposed requirement that all demonstrations under
CAA section 179B(b) must include a showing that the air agency
adopted all RACM, including RACT. . . . For purposes of CAA section
179B demonstrations for the 2015 ozone NAAQS, we are maintaining the
approach used for prior ozone standards that only areas classified
Moderate and higher must show that they have implemented RACM/
RACT.'')
---------------------------------------------------------------------------
E. Exceptional Events and Other Data Modification
Congress has recognized that it may not be appropriate for the EPA
to use
[[Page 52024]]
certain monitoring data collected by the ambient air quality monitoring
network and maintained in the AQS in certain regulatory determinations.
Thus, in 2005, Congress provided the statutory authority for the
exclusion of data influenced by ``exceptional events'' meeting specific
criteria by adding section 319(b) to the CAA. To implement this 2005
CAA amendment, the EPA promulgated the 2007 Exceptional Events
Rule.\35\ The 2007 Exceptional Events Rule created a regulatory process
codified at 40 CFR parts 50 and 51 (Sec. Sec. 50.1, 50.14 and 51.930).
These regulatory sections, which superseded the EPA's previous guidance
on handling data influenced by events, contain definitions, procedural
requirements, requirements for air agency demonstrations, criteria for
the EPA's approval of the exclusion of event-affected air quality data
from the data set used for regulatory decisions, and requirements for
air agencies to take appropriate and reasonable actions to protect
public health from exceedances or violations of the NAAQS. In 2016, the
EPA promulgated a comprehensive revision to the 2007 Exceptional Events
Rule.\36\ Under the Exceptional Events Rule, if a state demonstrates to
the EPA's satisfaction that emissions from a wildfire caused a specific
air pollution concentration in excess of the NAAQS at a particular air
quality monitoring location and otherwise satisfies the requirements of
40 CFR 50.14, the EPA must exclude that data from use in determinations
of exceedances and violations.\37\
---------------------------------------------------------------------------
\35\ 72 FR 13560 (March 22, 2007).
\36\ 81 FR 68216 (October 3, 2016). We refer herein to the 2016
revision as the ``Exceptional Events Rule.''
\37\ 40 CFR 50.14(b)(4).
---------------------------------------------------------------------------
In 2019, the EPA issued guidance clarifying what types of
regulatory determinations fall under the Exceptional Events Rule and
identifying ``other determinations, actions, and analyses that are not
covered by the scope of the Exceptional Events Rule, but for which the
exclusion, selection, or adjustment of monitoring data may be
appropriate and allowable'' under the CAA and applicable rules and
guidance.\38\ Among other things, the Clarification Memo on Data
Modification cites the EPA's Guideline on Air Quality Models
(``Guideline''),\39\ which allows for modification of the ambient data
record for purposes of certain modeling analyses. In particular, the
Guideline allows for ``removal of data from specific days or hours when
a monitor is being impacted by activities that are not typical or not
expected to occur again in the future (e.g., construction, roadway
repairs, forest fires, or unusual agricultural activities).'' \40\
These types of events are commonly known as ``atypical events.'' The
Clarification Memo on Data Modification also cites modeling guidance
issued by the EPA in 2018,\41\ which, in turn, discusses atypical
events in the context of (1) establishing a base design value,\42\ and
(2) developing relative response factors (RRFs).\43\
---------------------------------------------------------------------------
\38\ Memorandum from Richard Wayland, Director Air Quality
Assessment Division, ``Additional Methods, Determinations, and
Analyses to Modify Air Quality Data Beyond Exceptional Events,''
dated April 4, 2019 (``Clarification Memo on Data Modification'').
\39\ 40 CFR part 51, appendix W.
\40\ 40 CFR part 51, appendix W, section 8.3.2.c.ii. See also
id. sections 8.3.2.d and 8.3.3.d.
\41\ EPA, ``Modeling Guidance for Demonstrating Attainment of
Air Quality Goals for Ozone, PM<INF>2.5</INF> and Regional Haze,''
dated November 2018 (``Modeling Guidance'').
\42\ Id. at 102-103.
\43\ Id. at 105-106.
---------------------------------------------------------------------------
II. Phoenix-Mesa Ozone Determination of Attainment but for
International Emissions
A. Phoenix-Mesa Ozone Nonattainment Area
The Phoenix area is located in south-central Arizona along the
channel and flood plain of the Salt River and includes the majority of
Maricopa County and smaller portions of Gila County and Pinal County.
In 2023 the Phoenix-Mesa-Chandler metro population was 5.1 million. The
elevation of downtown Phoenix is about 350 meters above mean sea level
(MSL) and it is surrounded by mountains as high as 1,400 meters above
MSL. Given its location in the Sonoran Desert, the area experiences a
continental summer climate with hot, dry summers dominated by
persistent high-pressure systems.
B. Ozone Monitoring Sites in Phoenix-Mesa
There are currently 25 ozone monitoring sites in the Phoenix-Mesa
nonattainment area.
Table 1 of this document shows the annual fourth highest daily
maximum 8-hour average ozone concentration and 2021-2023 DV for each
monitor in the Phoenix-Mesa area. All monitors have violating 2021-2023
DVs, except for the Buckeye monitor, which is located in an upwind
rural area, and the Humboldt Mountain monitor, which is located in a
downwind rural area.
Table 1--2021-2023 Fourth Highest Daily Maximum 8-Hour Average Ozone Concentrations and Design Values at All Monitors in the Phoenix-Mesa, AZ Area
--------------------------------------------------------------------------------------------------------------------------------------------------------
Fourth highest daily maximum 8-hour
average ozone concentration (ppm) 2021-2023 DV
AQS site ID Site name Monitoring agency --------------------------------------- (ppm)
2021 2022 2023
--------------------------------------------------------------------------------------------------------------------------------------------------------
04-007-0010........................ Tonto National Monument..... ADEQ.......................... 0.075 0.074 0.076 0.075
04-013-0019........................ West Phoenix................ MCAQD......................... 0.078 0.076 0.077 0.077
04-013-1003........................ Mesa........................ MCAQD......................... 0.083 0.078 0.080 0.080
04-013-1004........................ North Phoenix............... MCAQD......................... 0.081 0.077 0.077 0.078
04-013-1010........................ Falcon Field................ MCAQD......................... 0.080 0.078 0.080 0.079
04-013-2001........................ Glendale.................... MCAQD......................... 0.079 0.081 0.079 0.079
04-013-2005........................ Pinnacle Peak............... MCAQD......................... 0.079 0.077 0.076 0.077
04-013-3002........................ Central Phoenix............. MCAQD......................... 0.081 0.072 0.078 0.077
04-013-3003........................ South Scottsdale............ MCAQD......................... 0.081 0.068 0.074 0.074
04-013-4003........................ South Phoenix............... MCAQD......................... 0.074 0.069 0.076 0.073
04-013-4004........................ West Chandler............... MCAQD......................... 0.073 0.071 0.069 0.071
04-013-4005........................ Tempe....................... MCAQD......................... 0.079 0.073 0.080 0.077
04-013-4008........................ Cave Creek.................. MCAQD......................... 0.074 0.071 0.076 0.073
04-013-4010........................ Dysart...................... MCAQD......................... 0.077 0.075 0.074 0.075
04-013-4011........................ Buckeye..................... MCAQD......................... 0.071 0.071 0.067 0.069
[[Page 52025]]
04-013-7020........................ Senior Center............... SRPMIC........................ 0.077 \a\ N/A \a\ N/A \b\ Invalid
04-013-7021........................ Red Mountain................ SRPMIC........................ 0.078 0.075 0.079 0.077
04-013-7022........................ Lehi........................ SRPMIC........................ 0.076 0.079 0.082 0.079
04-013-7024........................ High School................. SRPMIC........................ 0.074 0.077 0.076 0.075
04-013-9508........................ Humboldt Mountain........... MCAQD......................... 0.066 0.072 0.073 0.070
04-013-9702........................ Blue Point.................. MCAQD......................... 0.072 0.077 0.080 0.076
04-013-9704........................ Fountain Hills.............. MCAQD......................... 0.076 0.076 0.070 0.074
04-013-9997........................ JLG Supersite............... ADEQ.......................... 0.082 0.076 0.083 0.080
04-021-3001........................ Apache Junction............. PCAQCD........................ 0.074 0.072 0.072 0.072
04-021-8001........................ Queen Valley................ ADEQ.......................... 0.073 0.077 0.075 0.075
--------------------------------------------------------------------------------------------------------------------------------------------------------
Sources: 2023 Design Value_20241106_AMP480_2237472; Modeling Protocol table 2-1.
\a\ The required annual 75 percent completeness criterion was not met, therefore the annual fourth highest daily maximum values were not provided.
\b\ The design value for the Senior Center site is invalid due to construction and was therefore not operational beginning August 18, 2022 and all of
2023.
C. Summary of the State's Submission
On September 24, 2025, the Maricopa Association of Governments
(MAG) submitted to the EPA for review the ``MAG 2025 Clean Air Act
Section 179B(b) Retrospective Demonstration of the Impact of
International Emissions on Ozone Concentrations in the Maricopa
Nonattainment Area'' (``Demonstration'').\44\ The Demonstration
includes two reports developed by consultants for MAG and submitted as
appendices to the Demonstration.\45\ Using several lines of evidence,
MAG evaluated the extent to which ambient ozone levels in the Phoenix-
Mesa area have been affected by international emissions. This
evaluation includes a conceptual model of ozone formation in the
Phoenix-Mesa area including a discussion of the meteorological and
topographic conditions that influence ozone formation; modeling to
quantify international contribution; information about precursor
emissions and ozone air quality trends; and an analysis of the
transport patterns influencing the area. In addition, MAG provided
information to support the exclusion of days that it found to be
possibly influenced by wildfires, as discussed in section II.C.1. of
this document.
---------------------------------------------------------------------------
\44\ Letter dated September 24, 2025, from Ed Zuercher,
Executive Director, MAG, to Lee Zeldin, Administrator, EPA.
\45\ Ramboll Americas Engineering Solutions, Inc. (``Ramboll''),
``Technical Analyses in Support of the MAG 2025 Clean Air Act Sec.
179B(b) Retrospective Demonstration for the Maricopa Ozone
Nonattainment Area,'' dated September 2025 and submitted as appendix
A of the Demonstration (``Ramboll Report'') and Sonoma Technology,
``Evaluation of Atypical Exceedance Days of the 2015 Ozone Standard
in the Maricopa Ozone Nonattainment Area for Wildfire Impacts (2021-
2023)'', dated September 2025 and submitted as appendix C of the
Demonstration (``Atypical Events Evaluation'').
---------------------------------------------------------------------------
1. Exclusion of Wildfire-Influenced Days
MAG discussed the handling of potential wildfire impacts in section
1.5 of the Demonstration. As an initial step, MAG, ADEQ, MAQD, and
PCAQD identified 76 exceedance days as potentially influenced by
wildfire in 2021-2023. MAG then selected a subset of 31 days from the
original 76 days to exclude from the base DV \46\ used in modeling. MAG
explained that it did not submit these days as exceptional events
because, ``[t]he action requested in this demonstration (adjustment of
base modeling year design value) does not readily fall under the type
of actions that would trigger using the Exceptional Events Rule,'' \47\
but rather more closely aligns with data modification appropriate for
developing a representative base design value for use in modeling
analyses, as discussed in the Clarification Memo on Data Modification,
Modeling Guideline, and Modeling Guidance.
---------------------------------------------------------------------------
\46\ MAG used the 2023 DV for each monitoring site as the base
design value or anchor point for determining whether, when estimated
modeled international anthropogenic contributions are removed, the
monitor would have had an attaining design value.
\47\ Demonstration, p. 13.
---------------------------------------------------------------------------
The Atypical Events Evaluation, which was included as appendix C to
the Demonstration, provided information to support the exclusion of
these 31 days at exceeding monitors in the Phoenix-Mesa area as
atypical events based on evidence that ``wildfire-generated smoke, and
ozone precursors from the smoke, have impacted ozone concentrations''
in the nonattainment area.\48\ The EPA's evaluation of these analyses
is provided in the EPA's ``Technical Support Document for Review of
Atypical Events on 2015 8-Hour Ozone Phoenix-Mesa, AZ Nonattainment
Area for the 179B(b) Demonstration'' (``Atypical Events TSD'') and
summarized in section II.D. of this document.
---------------------------------------------------------------------------
\48\ Atypical Events Evaluation, p. 4.
---------------------------------------------------------------------------
2. Conceptual Model
MAG provided a conceptual model describing ozone formation in the
Phoenix-Mesa area in section 2 and appendix B of the Demonstration.
Overall, MAG cited the following as key ozone formation factors for the
area:
<bullet> Multiple days of intense solar radiation, high
temperatures and stagnant conditions, combined with sustained natural
and anthropogenic emissions, lead to a photochemical buildup of ozone
pollution.
<bullet> Daytime temperatures often exceed 100 [deg]F, while the
Urban Heat Island effect causes overnight temperatures to remain
elevated.
<bullet> Sustained valley/mountain breeze circulation pattern traps
ozone in basin.
<bullet> The inversion layer traps ozone and ozone precursors from
the previous day, enhancing ozone concentration due to strong vertical
mixing after sunrise.
<bullet> Westerly regional and long-range transport patterns bring
pollution and wildfire smoke from throughout western North America.
<bullet> Frequent long-range transport due to wind shifts during
the summer monsoon season brings ozone and ozone precursors from Mexico
into the region.
[[Page 52026]]
<bullet> Stratospheric intrusions contribute to downward transport
of ozone-rich air mass from upper levels.
<bullet> High-resolution satellite data link summer ozone
exceedances to consistently higher NO<INF>2</INF> levels (especially
morning/early afternoon) and reveal profound, distinct impacts of
wildfire smoke on NO<INF>2</INF> and formaldehyde (HCHO).\49\
---------------------------------------------------------------------------
\49\ Demonstration, p. 17.
---------------------------------------------------------------------------
As part of its conceptual model, MAG also provided information
regarding ozone and precursor emissions monitoring, ozone exceedances
during the 2021-2023 design value period, meteorological evidence of
transport from Mexico, and a conceptual model of international
transport of emissions. Specifically, MAG discussed two international
transport patterns: a lower-level atmospheric flow pattern that brings
humid air northward from Mexico into the Sonoran Desert and Great Basin
states during July and August, and a mid- to upper-level atmospheric
pattern linking Asia to the western U.S., which is more significant
during the late spring through early summer.
3. Photochemical Modeling
In the Demonstration, MAG included results of modeling performed by
MAG and its consultant Ramboll, using both the Comprehensive Air
Quality Model with extensions (CAMx) and the Community Multiscale Air
Quality (CMAQ) model to simulate ozone air quality over southwestern
North America during the 2023 summer ozone season.
a. Source Apportionment Modeling
The primary line of evidence in the Demonstration is CAMx modeling
with Ozone Source Apportionment Technology (OSAT) performed by Ramboll
using MAG's 2023 modeling platform to quantify local, regional, and
international contributions to ozone within the nonattainment area.
This modeling is described in section 3 and appendix B of the
Demonstration and section 3.2 of the Ramboll Report. The MAG modeling
platform consists of a set of three nested grids: a southwestern North
America domain at 36 km horizontal resolution, an Arizona domain at 12
km resolution, and a domain for Maricopa NAA at 4 km resolution. Each
domain resolves the atmosphere in 23 layers spanning from the surface
up to the tropopause. MAG modeled the 2023 summer ozone season (April-
September) using the Weather Research and Forecasting (WRF) model to
develop three-dimensional meteorological fields for all grids and the
Sparse Matrix Operator Kernel Emissions (SMOKE) processing system to
develop anthropogenic, biogenic, and fire emission inventories within
the domain. MAG developed three-dimensional initial/boundary conditions
(IC/BC) for the master domain from Ramboll's application of the GEOS-
Chem \50\ global chemistry model for the 2023 year. The GEOS-Chem runs
included: (1) a full representation of ozone, particulate matter,
precursors, and products from the entire globe, and (2) a scenario in
which all international anthropogenic (IA) emissions were removed
(known as a ``zero-out'' run). OSAT uses time- and space-resolved BC
concentration differences between the two global model scenarios to
track the global IA contributions to total simulated ozone at NAA
monitors.
---------------------------------------------------------------------------
\50\ GEOS-Chem is a global 3-D model of atmospheric chemistry
driven by meteorological input from the Goddard Earth Observing
System (GEOS) of the NASA Global Modeling and Assimilation Office.
---------------------------------------------------------------------------
Section 3.2.1 of the Ramboll Report describes the modeling
configuration. OSAT tracked three emissions sector groups
(anthropogenic, natural, and fire sources) and two source apportionment
IC/BC groups (one representing global IA ozone and precursors and the
other representing global natural and US emissions). Three geographic
regions were defined in OSAT at the resolution of each modeling grid:
the Maricopa NAA, the rest of the U.S. including 200 km territorial
waters, and Mexico. The sum of tracers for ozone formed under
NO<INF>X</INF> sensitive conditions and ozone formed under VOC
sensitive conditions associated with anthropogenic Mexico emissions and
IC/BC IA, represent the total international contribution from
international emissions for the purposes of this 179B(b) demonstration.
Section 3.2 of the Demonstration and section 3.2.2 of the Ramboll
Report summarize the 2023 OSAT results for Phoenix-Mesa area monitoring
sites, as extracted from the 4-km grid and post-processed to daily
maximum 8-hour average (MDA8) concentrations. To quantify IA impacts on
2021-2023 DVs, Ramboll used a modified version of the EPA's recommended
``modeled attainment test.'' \51\ In a typical attainment
demonstration, the relative (or fractional) change in concentration
between the modeled base year and a modeled future year provides a
relative response factor (RRF). Each monitoring site's RRF is then
multiplied by its monitored base year design value to provide an
estimated future design value for comparison to the NAAQS. In this
instance, instead of using a modeled base year and a modeled future
year to develop the RRFs, MAG and Ramboll used the post-processed CAMx
OSAT results, in the form of hourly cumulative ozone concentrations
excluding ozone tracers for the total IA (BC IA + Mexico IA) to develop
RRFs representing the fractional change in ozone concentration when IA
sources are excluded.\52\ In addition, Ramboll calculated adjusted RRFs
with wildfire-impacted days removed from the top-ten modeled days.
Ramboll then used SMAT-CE to apply these unadjusted and adjusted RRFs
to the unadjusted and adjusted 2021-2023 DVs respectively to provide an
estimated design values without IA sources for comparison to the
NAAQS.\53\ The results are summarized in table 3-1 of the Demonstration
and table 2 of this document.
---------------------------------------------------------------------------
\51\ See the EPA's ``Modeling Guidance for Demonstrating
Attainment of Air Quality Goals for Ozone, PM<INF>2.5</INF> and
Regional Haze,'' dated November 2018 (``Modeling Guidance''),
Chapter 4.
\52\ Demonstration appendix B, p. 146.
\53\ Unadjusted 2021-2023 DVs refers to MDA8 ozone DVs for all
official sites operating in the Maricopa NAA in AQS; adjusted DVs
refers to 2021-2023 MDA8 ozone DVs from MAG reflecting the removal
of 31 atypical wildfire days as discussed in section II.C.1. of this
document.
[[Page 52027]]
Table 2--Summary Results of Ozone Source Apportionment Modeling
----------------------------------------------------------------------------------------------------------------
2023 Ozone DV 2023 Ozone DV 2023 Ozone
including excluding impact of
Source apportionment modeling results international international international
emissions (ppb) emissions (ppb) emissions (ppb)
----------------------------------------------------------------------------------------------------------------
Average of All Nonattainment Area Monitors............. 75 60 15
Average of All Nonattainment Area Monitors with 73 57 16
Wildfire Days Excluded................................
Maximum Ozone Concentration Monitor.................... 80 66 14
Maximum Ozone Concentration Monitor with Wildfire Days 77 62 15
Excluded..............................................
----------------------------------------------------------------------------------------------------------------
Source: Demonstration, table 3-1.
Ramboll also provided site-averaged stacked area plots of simulated
MDA8 ozone contributions from BC IA and Mexico IA sectors to illustrate
the change in contributions from different international source regions
over time. Ramboll noted that, IA contributions via BC (Asia/
Intercontinental) dominate in April through June, and September, when
stronger westerly winds are predominant, whereas in July through
August, during southerly monsoonal flow, BC IA decreases significantly
while Mexican contributions increase.
MAG provided an evaluation of photochemical modeling performance in
section 3.3 and appendix B of the Demonstration. In addition, MAG
provided additional detail regarding the source apportionment modeling
results in section 7.2 of appendix B, including monitor-specific
results and month-by-month comparisons of international and local
anthropogenic source contributions. The monthly contribution results
indicate that during most of the ozone season (April, May, June, and
September) the international anthropogenic contribution is greater than
the local (non-attainment area) anthropogenic contribution, while the
local anthropogenic contribution is greater than the international
anthropogenic contribution in July and August.
b. Zero-Out Modeling
In section 3.4 and appendix B, sections 7.1and 7.4 of the
Demonstration, MAG described the results of ``zero-out'' CAMx and CMAQ
modeling simulations performed for comparison to the CAMx source
apportionment modeling. Specifically, MAG performed CAMx zero-out
simulations using the GEOS-Chem zero-out of international anthropogenic
emissions scenario as the IC/BC and removing Mexico manmade emissions
within the CAMx modeling domains. The zero-out simulations were
compared to base year modeling to develop RRFs and applied them to the
ozone DVs. This produced an estimated 2023 ozone DV in the absence of
IA sources of 70 ppb, based on a starting ozone design value of 80 ppb
(i.e., potentially wildfire-impacted exceedance days not excluded), and
65 ppb with potentially wildfire-impacted exceedance days excluded. The
analogous CMAQ zero-out modeling produced 2023 ozone DV of 70 ppb,
based on a starting ozone design value of 80 ppb, and 64 ppb with
potentially wildfire-impacted exceedance days excluded. MAG concluded
that all simulations presented in the Demonstration estimated an
attaining DV when IA sources were excluded.
4. Additional Analyses
In addition to the photochemical modeling described in section
II.C.3. of this document, MAG and Ramboll provided the results of three
secondary analyses used as supporting evidence: (1) EPA ozone
photochemical modeling for manmade international emissions; (2) air
parcel transport pattern analysis; and (3) ozone precursor emissions
trends in the nonattainment area and in Mexico.
a. EPA Modeling of International Contribution
Section 4.1 of the Demonstration and sections 3.3 and 3.4 of the
Ramboll Report summarize the results of photochemical modeling
performed by the EPA in 2021 and 2023. In the 2021 modeling,\54\ the
EPA ran CMAQ for the base year of 2016 on a national grid with 12 km
grid spacing and 35 vertical layers, then averaged total ozone and
sector concentrations across the 10 highest modeled days at each grid
cell coinciding with the peak monitor in each nonattainment area. The
resulting modeled mean sector concentrations were scaled to represent
their relative contribution to the 2018-2020 DV. Table 3 of this
document lists the relative contributions by source category applied to
the 2018-2020 DV value at the peak DV site in Phoenix.
---------------------------------------------------------------------------
\54\ Memorandum from Barron Henderson and Heather Simon (EPA,
OAQPS), Subject: ``Modeled U.S. and International Contributions for
2015 Ozone NAAQS Nonattainment Areas,'' dated December 10, 2021
(``Henderson Simon Memo'').
Table 3--Summary of EPA Source Attribution Modeling Results for Phoenix-Mesa
--------------------------------------------------------------------------------------------------------------------------------------------------------
Site 2018-2020 DV Natural US IA Mix DV without IA
--------------------------------------------------------------------------------------------------------------------------------------------------------
Phoenix, AZ-05................................. 79 27.0 38.3 8.5 5.1 70
--------------------------------------------------------------------------------------------------------------------------------------------------------
Source: Ramboll Report, table 3-3. DVs are truncated to whole numbers to reflect the form of the ozone NAAQS.
With respect to the Phoenix-Mesa area, Ramboll concluded that
``[a]ccording to EPA's modeling, the IA contribution was estimated at
8.5 ppb, but source apportionment modeling would estimate a larger
contribution by accounting for some fraction of the 5.1 ppb mixed
anthropogenic component (for a total ozone contribution range of 8.5-
13.6 ppb) and that ``[r]emoving just the IA sector from the 79 ppb DV
results in 70.5 ppb; i.e., the Maricopa NAA would have attained the
standard in 2020 but for the IA contribution.'' \55\
---------------------------------------------------------------------------
\55\ Ramboll Report, p. 32.
---------------------------------------------------------------------------
Ramboll also discussed the results of nationwide CAMx source
apportionment modeling performed by the EPA to estimate contributions
to 2023 ozone DVs from each state, Tribal lands, offshore marine vessel
and oil/gas development, portions of Canada and
[[Page 52028]]
Mexico, biogenic emissions, wild and prescribed fires, lightning
NO<INF>X</INF>, and BC.\56\ The EPA used the source apportionment
results to apportion the projected 2021-2023 DV at each monitoring
location. Ramboll summarized the results of the EPA's modeling,
averaged over the highest five sites in the nonattainment area for the
following categories: Arizona, California (the largest upwind
contributing state), Canada and Mexico, fires, and BC. Because none of
the projected 2023 DVs exceed 70 ppb, Ramboll also scaled those values
to the actual peak monitored 2021-2023 DV of 80 ppb. The results are
shown in table 4 of this document.
---------------------------------------------------------------------------
\56\ EPA Office of Air Quality Planning and Standards, ``Air
Quality Modeling Technical Support Document, 2015 Ozone NAAQS SIP
Disapproval Final Action'' (January 31, 2023). In this instance, BC
represents the entirety of global pollutants, both natural and
anthropogenic, other than from Canada and Mexico.
Table 4--Model-Projected and Actual 2023 Ozone DV and Source Contributions Averaged Over the Highest 5 Sites in the Maricopa NAA
--------------------------------------------------------------------------------------------------------------------------------------------------------
2023 DV AZ CA C&M Fires BC
--------------------------------------------------------------------------------------------------------------------------------------------------------
Projected DV............................................ 70 14.4 1.9 2.9 1.7 43.3
Actual DV............................................... 80 16.3 2.1 3.3 1.9 49.1
--------------------------------------------------------------------------------------------------------------------------------------------------------
Source: Ramboll Report, table 3-4.
a. Air Parcel Transport Analysis
Section 5.1 of the Ramboll Report provides the results of a HYSPLIT
\57\ back trajectory analysis of the medium-range (three-day) origins
of air parcels that arrived in the NAA on ozone exceedance days during
2021-2023. Ramboll ran the HYSPLIT model for each exceedance day and
grouped the results by transport patterns (i.e., northerly, easterly/
southeasterly, southerly, and westerly). Out of all 160 exceedance
days, 26 of the days (16 percent) were consistent with the northerly
transport pattern; 44 days (28 percent) were consistent with the
easterly pattern; 51 days (32 percent) were consistent with the
southerly pattern; and 39 days (24 percent) were consistent with the
westerly pattern.
---------------------------------------------------------------------------
\57\ The HYSPLIT model, which is provided by the National
Oceanic and Atmospheric Administration, ``is a complete system for
computing simple air parcel trajectories, as well as complex
transport, dispersion, chemical transformation, and deposition
simulations.'' <a href="https://www.arl.noaa.gov/hysplit/">https://www.arl.noaa.gov/hysplit/</a>.
---------------------------------------------------------------------------
Ramboll also recalculated the Maricopa NAA 2021-2023 DVs after
excluding days with potential transport from Mexico (i.e., all days
with easterly, southerly, or westerly patterns) and presented these
results in section 5.2 of the Ramboll Report, concluding that the
average reduction in DV due to exclusion of wildfires and Mexican
transport is 8.4 ppb.\58\
---------------------------------------------------------------------------
\58\ Ramboll Report, p. 32.
---------------------------------------------------------------------------
a. Precursor and Ozone Trends
Section 4.0 of the Ramboll Report provides a comparison of ozone
precursor emissions inventories and 2017-2023 trends for the Maricopa
NAA (developed by MAG and Maricopa County) and the three Mexican border
states of Baja California, Sonora, and Chihuahua (developed by the
Eastern Research Group, Inc.). Summaries of the results of these
analyses for NO<INF>X</INF> and VOC are provided in tables 5 and 6 of
this document respectively.
Table 5--2017 and 2023 NOX Emissions, Maricopa NAA US and Mexico Border States
----------------------------------------------------------------------------------------------------------------
Maricopa NAA Mexico border states
-----------------------------------------------------------------------------
Category Percent Percent
2017 (TPD) 2023 (TPD) change 2017 (TPD) 2023 (TPD) change
----------------------------------------------------------------------------------------------------------------
Point............................. 9.35 7.49 -20 136.77 158.63 16
Airports.......................... 8.43 9.79 16 3.11 3.33 7
Area.............................. 11.51 9.25 -20 29.27 29.35 0.3
Nonroad........................... 37.22 22.80 -39 53.12 50.81 -4
On-Road........................... 80.74 39.59 -51 749.72 788.63 5
Emissions Reduction Credits (ERC). ........... 0.01 ........... ........... ........... ...........
Mobile ERC........................ ........... 2.74 ........... ........... ........... ...........
-----------------------------------------------------------------------------
Total......................... 147.26 91.66 -38 971.99 1,030.75 6
----------------------------------------------------------------------------------------------------------------
Per capita (lbs/day).............. 0.062 0.036 -43 0.185 0.194 5
----------------------------------------------------------------------------------------------------------------
Table 6--2017 and 2023 VOC Emissions, Maricopa NAA US and Mexico Border States
----------------------------------------------------------------------------------------------------------------
Maricopa Nonattainment Area Mexico Border States
-----------------------------------------------------------------------------
Category Percent Percent
2017 (TPD) 2023 (TPD) change 2017 (TPD) 2023 (TPD) change
----------------------------------------------------------------------------------------------------------------
Point............................. 3.01 1.99 -34 26.88 31.04 15
Airports.......................... 4.21 4.90 16 0.66 0.70 6
Area.............................. 91.59 97.32 6 518.70 531.35 2
Nonroad........................... 20.23 18.40 -9 3.29 3.05 -7
On-Road........................... 48.82 37.16 -24 374.00 376.69 1
ERC............................... ........... 0.03 ........... ........... ........... ...........
[[Page 52029]]
Mobile ERC........................ ........... 1.37 ........... ........... ........... ...........
-----------------------------------------------------------------------------
Total......................... 167.86 161.18 -4 923.53 942.83 2
----------------------------------------------------------------------------------------------------------------
Per capita (lbs/day).............. 0.071 0.063 -11 0.175 0.177 1
----------------------------------------------------------------------------------------------------------------
D. EPA Review of the State's Submission
As part of meeting our duty to determine whether the Phoenix-Mesa
area attained the 2015 ozone NAAQS by the applicable attainment date,
the EPA evaluated air quality monitoring data submitted by ADEQ, MCAQD,
PCAQCD, and SRPMIC to determine the attainment status of the Phoenix-
Mesa nonattainment area as of its Moderate area attainment date. As
indicated in table 1 of this document, the 2021-2023 DV is 0.080 ppm,
indicating that the Phoenix-Mesa area did not attain the 2015 ozone
NAAQS by its August 3, 2024 attainment date.
The EPA has also evaluated MAG's Demonstration. Our evaluation is
included in the ``EPA Evaluation of the Clean Air Act Section 179B(b)
Demonstration for the Phoenix-Mesa 2015 Ozone NAAQS Nonattainment
Area--Modeling and Impact of International Emissions Technical Support
Document'' (``179B(b) TSD'') and the Atypical Events TSD provided in
the docket for this rulemaking and our findings are summarized here.
Overall, MAG's retrospective 179B(b) demonstration includes multiple
lines of evidence focusing on 2021, 2022, 2023, which are the key years
for evaluating attainment for a 2015 ozone Moderate nonattainment area.
MAG provided a conceptual model showing that the area's ozone
production is a complex mix of local emissions, inter-state and
international transport, and other meteorological factors. The primary
line of evidence provided by MAG was photochemical source apportionment
modeling, which indicated an international anthropogenic contribution
of 14 ppb to the 2021-2023 DV of 80 ppb in the Phoenix-Mesa area which,
when subtracted, would be an attaining value.
Other photochemical modeling, including zero-out modeling performed
by the EPA and MAG, indicated an international anthropogenic
contribution of 8.5-10.0 ppb to the Phoenix-Mesa area DV. Variation
among modeling approaches means that there can be a range in credible
estimates. Information regarding potential wildfire influences on 31
exceedance days in 2021-2023, while not conclusive, adds additional
supporting evidence. MAG also provided emissions estimates that ozone
precursor emissions in the NAA are decreasing, while emissions in
Mexico are increasing. Air parcel transport provided additional
qualitative evidence of influence of emissions from Mexico on the
Phoenix-Mesa area.
Based on the weight of evidence provided in these analyses
collectively, the EPA proposes to find that MAG has established to the
satisfaction of the Administrator that the Phoenix-Mesa area would have
attained the 2015 ozone NAAQS by the applicable attainment date, but
for emissions emanating from outside of the United States.
Specifically, as described in detail in the 179B(b) TSD, we find that,
collectively the weight of evidence supports the conclusion that the
2023 ozone design values at all monitoring sites in the Phoenix area
would have been at or below 0.070 ppm (70 ppb) but for the influence of
international emissions. Accordingly, the EPA is proposing to approve
the Demonstration and determine that the Phoenix-Mesa nonattainment
area (``Phoenix-Mesa area'') would have attained the 2015 ozone
national ambient air quality standards (NAAQS) by the August 3, 2024
``Moderate'' area attainment date, but for emissions emanating from
outside the United States. If finalized, this action will fulfill the
EPA's statutory obligation under CAA section 181(b)(2) to determine
whether the Phoenix area attained the 2015 ozone NAAQS as of the
attainment date of August 3, 2024.
III. Proposed Action
For the reasons discussed in this document and the associated
technical support documents, the EPA proposes to find that MAG has
established to the satisfaction of the Administrator that the Phoenix
area would have attained the 2015 ozone NAAQS by the Moderate area
attainment date of August 3, 2024, but for emissions emanating from
outside the United States. If finalized, per CAA section 179B(b),
Arizona will no longer be subject to the provisions of CAA section
181(b)(2), and the EPA's obligation under those provisions with respect
to the Phoenix-Mesa Moderate 2015 ozone nonattainment area will
therefore be fulfilled. The area will remain designated nonattainment
and thus the State must continue to comply with applicable requirements
for a Moderate ozone nonattainment area, except with respect to
contingency measures, as described below.
In addition, the EPA also proposes to find that, if we finalize our
179B(b) determination as proposed, the contingency measure requirements
of CAA section 172(c)(9) would no longer apply to the Phoenix-Mesa
Moderate 2015 ozone nonattainment area. Specifically, as explained in
section I.B. of this document, attainment contingency measures and RFP
contingency measures can only be triggered for a Moderate ozone
nonattainment area by a finding of failure to attain under CAA section
181(b)(2). As previously noted, if the EPA finalizes our determination
under 179B(b), Arizona will no longer be subject to the provisions of
181(b)(2). Therefore, neither contingency measures for failing to
attain nor RFP contingency measures could ever be triggered for this
area. According, the EPA proposes to find that, if we finalize the
179B(b) determination for the Phoenix-Mesa Moderate 2015 ozone
nonattainment area, the requirement for the state to submit attainment
and RFP contingency measures will no longer apply to this area.
The EPA is soliciting public comments on the issues discussed in
this document. We will accept comments from the public on this proposal
until December 19, 2025 and will consider comments before taking final
action.
[[Page 52030]]
IV. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review and Executive
Order 13563: Improving Regulation and Regulatory Review
This action is not a significant regulatory action and was
therefore not submitted to the Office of Management and Budget (OMB)
for review.
B. Executive Order 14192: Unleashing Prosperity Through Deregulation
Executive Order 14192 does not apply because it is not a
significant regulatory action and is therefore exempted from review
under Executive Order 12866.
C. Paperwork Reduction Act (PRA)
This rule does not impose any new information collection burden
under the PRA not already approved by the Office of Management and
Budget. This action proposes to find that the Phoenix-Mesa Moderate
ozone nonattainment area would have attained the 2015 ozone NAAQS by
the applicable attainment date, but for emissions emanating from
outside the United States. Thus, the proposed action does not establish
any new information collection burden that has not already been
identified and approved in the EPA's information collection
request.\59\
---------------------------------------------------------------------------
\59\ On January 31, 2025, the EPA submitted a request for an
extension without change of a previously approved information
collection request (ICR) titled ``Implementation of the 8-Hour
National Ambient Air Quality Standards for Ozone (Renewal).'' See
OMB Control Number 2060-0695 and ICR Reference No: 202501-2060-004
for EPA ICR Number 2347.05. The ICR renewal is pending OMB final
approval.
---------------------------------------------------------------------------
D. Regulatory Flexibility Act (RFA)
I certify that this action will not have a significant economic
impact on a substantial number of small entities under the RFA. This
action will not impose any requirements on small entities. The proposed
determination that the Phoenix-Mesa nonattainment area would have
attained the 2015 ozone NAAQS but for international emissions does not
in and of itself create any new requirements beyond what is mandated by
the CAA. Instead, this rulemaking only makes factual determinations and
does not directly regulate any entities.
E. Unfunded Mandates Reform Act (UMRA)
This action does not contain any unfunded mandate as described in
UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely affect
small governments. This action imposes no enforceable duty on any
state, local or Tribal governments or the private sector.
F. Executive Order 13132: Federalism
This action does not have federalism implications. It will not have
substantial direct effects on the states, on the relationship between
the national government and the states and Tribes, or on the
distribution of power and responsibilities among the various levels of
government. The division of responsibility between the Federal
Government and the states for the purposes of implementing the NAAQS is
established under the CAA.
G. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments
This action has Tribal implications. However, it will neither
impose substantial direct compliance costs on federally recognized
Tribal governments, nor preempt Tribal law.
The EPA has identified Tribal areas within the nonattainment area
covered by this proposed rule that would be potentially affected by
this rulemaking. Specifically, the Fort McDowell Yavapai Nation, Gila
River Indian Community of the Gila River Indian Reservation, Salt River
Pima-Maricopa Indian Community of the Salt River Reservation, and
Tohono O'odham Nation are located within the boundaries of the Phoenix-
Mesa nonattainment area.
The EPA has communicated with the potentially affected Tribes
located within the boundaries of the nonattainment area addressed in
this proposal.
H. Executive Order 13045: Protection of Children From Environmental
Health and Safety Risks
The EPA interprets Executive Order 13045 as applying only to those
regulatory actions that concern environmental health or safety risks
that the EPA has reason to believe may disproportionately affect
children, per the definition of ``covered regulatory action'' in
section 2-202 of the Executive Order. This action is not subject to
Executive Order 13045 because it does not concern an environmental
health risk or safety risk.
I. Executive Order 13211: Actions That Significantly Affect Energy
Supply, Distribution, or Use
This action is not subject to Executive Order 13211, because it is
not a significant regulatory action under Executive Order 12866.
J. National Technology Transfer Advancement Act (NTTAA)
This rulemaking does not involve technical standards.
List of Subjects in 40 CFR Part 52
Environmental protection, Administrative practice and procedure,
Air pollution control, Designations and classifications, Incorporation
by reference, Intergovernmental relations, Nitrogen oxides, Ozone,
Reporting and recordkeeping requirements, Volatile organic compounds.
Dated: November 13, 2025.
Michael Martucci,
Acting Regional Administrator, Region IX.
[FR Doc. 2025-20357 Filed 11-18-25; 8:45 am]
BILLING CODE 6560-50-P
</pre><script data-cfasync="false" src="/cdn-cgi/scripts/5c5dd728/cloudflare-static/email-decode.min.js"></script></body>
</html>Indexed from Federal Register on November 19, 2025.
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.