# 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 *https://www.regulations.gov.* For comments submitted at *Regulations.gov* , follow the online instructions for submitting comments. Once submitted, comments cannot be edited or removed from *Regulations.gov* . 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. 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 *https://www.epa.gov/dockets/commenting-epa-dockets.* 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: *[email protected].*
**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 <sub>X</sub> ) 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 *https://www.epa.gov/sites/default/files/2016-04/documents/20151001healthfs.pdf* 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 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 *https://www.epa.gov/aqs* .
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 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]*https://www.epa.gov/newsreleases/administrator-zeldin-moves-forward-ensuring-us-states-are-not-punished-foreign-air* .
[^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 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 (§§ 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 <sub>2.5</sub> 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.
| AQS site ID | Site name | Monitoring agency | Fourth highest daily maximum 8-hour | 2021 | 2022 | 2023 | 2021-2023 DV |
| --- | --- | --- | --- | --- | --- | --- | --- |
| 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 | |
| 04-013-7020 | Senior Center | SRPMIC | 0.077 | N/A | N/A | 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 | |
**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 § 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:
• 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.
• Daytime temperatures often exceed 100 °F, while the Urban Heat Island effect causes overnight temperatures to remain elevated.
• Sustained valley/mountain breeze circulation pattern traps ozone in basin.
• The inversion layer traps ozone and ozone precursors from the previous day, enhancing ozone concentration due to strong vertical mixing after sunrise.
• Westerly regional and long-range transport patterns bring pollution and wildfire smoke from throughout western North America.
• Frequent long-range transport due to wind shifts during the summer monsoon season brings ozone and ozone precursors from Mexico into the region.
• Stratospheric intrusions contribute to downward transport of ozone-rich air mass from upper levels.
• High-resolution satellite data link summer ozone exceedances to consistently higher NO <sub>2</sub> levels (especially morning/early afternoon) and reveal profound, distinct impacts of wildfire smoke on NO <sub>2</sub> 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 <sub>X</sub> 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 <sub>2.5</sub> 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.
| Source apportionment modeling results | 2023 Ozone DV including | 2023 Ozone DV excluding | 2023 Ozone |
| --- | --- | --- | --- |
| Average of All Nonattainment Area Monitors | 75 | 60 | 15 |
| Average of All Nonattainment Area Monitors with Wildfire Days Excluded | 73 | 57 | 16 |
| Maximum Ozone Concentration Monitor | 80 | 66 | 14 |
| Maximum Ozone Concentration Monitor with Wildfire Days Excluded | 77 | 62 | 15 |
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”).
| Site | 2018-2020 DV | Natural | US | IA | Mix | DV without IA |
| --- | --- | --- | --- | --- | --- | --- |
| Phoenix, AZ-05 | 79 | 27.0 | 38.3 | 8.5 | 5.1 | 70 |
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 Mexico, biogenic emissions, wild and prescribed fires, lightning NO <sub>X</sub> , 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.
| | 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 |
**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.” *https://www.arl.noaa.gov/hysplit/.*
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 <sub>X</sub> and VOC are provided in tables 5 and 6 of this document respectively.
| Category | Maricopa NAA | 2017 | 2023 | Percent change | Mexico border states | 2017 | 2023 | Percent 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 | | |
| Category | Maricopa Nonattainment Area | 2017 | 2023 | Percent change | Mexico Border States | 2017 | 2023 | Percent 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 | | | | | | |
| 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.
**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.