I. Ozone Standards

The Phoenix-Mesa area, which includes a large portion of Maricopa County, as well as smaller portions of Pinal and Gila Counties, is currently designated as a nonattainment (NA) area for the 2015 8-hour ozone National Ambient Air Quality Standard (NAAQS). The ozone NAAQS is a health-based ambient concentration standard initially promulgated by the Clean Air Act (CAA) and used by the U.S. Environmental Protection Agency (EPA) to dictate air permitting requirements for ozone precursors, nitrogen oxides (NO_X) and volatile organic compounds (VOC), across the country. EPA classifies areas as nonattainment (NA) based on the area’s design value (i.e., the amount by which the NAAQS is exceeded in that area) into one of the following categories, listed in increasing order of severity: marginal, moderate, serious, severe, and extreme. As an area’s ozone NA classification becomes of higher severity, the air permitting and compliance requirements for NO_X and VOC become more stringent to bring the area into attainment of the NAAQS.

The Phoenix-Mesa area was initially designated in 2018 by the EPA as “marginal” NA for the 2015 ozone NAAQS, the lowest ozone NA designation. After failing to attain the NAAQS by the initial August 3, 2021 deadline, the Phoenix-Mesa area was redesignated to “moderate” NA classification in 2022, with a deadline to attain the NAAQS by August 3, 2024. Ambient monitor concentrations in the Phoenix-Mesa area showed that the 2015 ozone NAAQS was exceeded during the 2021-2023 period. Per CAA § 181(b)(2)(A), EPA has a statutory obligation to make an attainment determination within six (6) months of the deadline (i.e., February 3, 2025). However, an attainment determination has not been made as of September 2025. Changes in the EPA Administration, as well as the availability of 2022 – 2024 validated ambient monitor data, are thought to have a role in the timing of the attainment determination. Lee Zeldin, sworn in as EPA Administrator on January 29, 2025, has stated EPA’s commitment to supporting Arizona with addressing ozone nonattainment issues in a way that allows Phoenix to also continue its economic growth, and this was the focus of Administrator Zeldin’s visit to Arizona in March 2025. Most notably, EPA’s May 2025 decision to rescind Guidance on the Preparation of Clean Air Act Section (CAA) 179B Demonstrations for Nonattainment Areas Affected by International Transport of Emissions served as another milestone in EPA’s ozone nonattainment work. By rescinding this guidance, EPA allows states such as Arizona to demonstrate that foreign air pollution may be negatively contributing to ambient ozone concentrations, which in turn influences EPA’s attainment designations.

Due to the absence of an attainment determination by EPA so far into 2025, the Phoenix-Mesa NA area has retained its moderate designation status.

A Critical Regulatory Crossroads: The EPA Consent Decree

A significant development currently unfolding involves the EPA and a proposed consent decree that will directly impact the Phoenix-Mesa nonattainment area. On April 8, 2025, a complaint was filed by the Center for Biological Diversity and Center for Environmental Health against EPA due to the absence of an EPA attainment determination by the 2024 deadline for the Mariposa County, California, and Phoenix-Mesa, Arizona moderate ozone NA areas. As a result of the ongoing litigation, a notice of proposed consent decree was issued on August 28, 2025 which would resolve all claims in the case by establishing deadlines for the EPA to take final action on the abovementioned attainment determinations.

Key deadlines outlined in the proposed consent decree are identified below.

• Public comments on the proposed consent decree are due by September 29, 2025. Comments may be submitted, identified by Docket ID No. EPA-HQ-OGC-2025-0754, online.
• The EPA must sign the notice of final rule for the Phoenix-Mesa area attainment determination by February 3, 2026.
• Publication of this rule in the Federal Register would need to occur no later than 20 business days after signature, likely becoming effective 30 days after publication.

This consent decree is a pivotal moment, as the EPA’s attainment determination will dictate the future air quality regulatory landscape for the Phoenix-Mesa area.

How will the Attainment Determination Affect My Facility?

While the final decision is pending, it expected that EPA will determine that the area did not attain the 2015 NAAQS based on 2021 – 2023 ambient monitor data. Furthermore, EPA may elect to retain the Phoenix-Mesa area’s current NA classificationas “moderate” or redesignate the area to “serious”. If the moderate status is retained, the current nonattainment requirements will continue to apply. It is expected for EPA to elect this approach, given that Arizona is a border state and EPA has placed an emphasis on addressing the impacts international emissions have on U.S. ambient concentrations for attainment determinations. However, it is still possible that EPA could instead choose to redesignate the Phoenix-Mesa area to “serious” classification.

If the Phoenix-Mesa area is reclassified to serious NA, the Major Source Thresholds (MSTs) and Significant Emissions Rates (SERs) for NO_X and VOCs would be lowered within the NA area. For many facilities, in particular those located within the Maricopa County Air Quality Department (MCAQD) jurisdiction, this could mean that current emission levels, which might be considered minor under a moderate classification, could be reclassified as major sources. Lowered thresholds could also result in a larger number of facilities requiring the more complex Nonattainment New Source Review (NNSR) permitting process for new or expanding facilities. NNSR permitting requires additional components such as Lowest Achievable Emission Rate (LAER), Emission Offsets, and generally results in long permitting timeframes, which impact target construction schedules for projects. This could also make permitting harder and more costly for affected facilities, impacting future projects and expansions. A summary of these impacts is provided in Table 1.

TABLE 1. Serious Ozone Nonattainment Redesignation Impacts Summary

Arizona also continues to address the CAA requirements associated with the 2024 annual NAAQS for fine particulate matter (PM_2.5). The Arizona Department of Environmental Quality (ADEQ) issued a Notice of Proposed Rulemaking on September 5, 2025, which proposed to revise the annual PM_2.5 NAAQS in the Arizona Administrative Code (A.A.C.) to 9 μg/m³, reflect the changes in the federal rule which became effective on May 6, 2024. The comment period for these proposed changes began on September 5, 2025, and concludes on October 7, 2025, with a virtual public hearing scheduled for that final date.

However, EPA itself is currently engaged in litigation in the U.S. Court of Appeals for the District of Columbia Circuit to decide the future of the 2024 annual PM_2.5 NAAQS. Earlier in 2025, an abeyance period was granted to EPA in Kentucky v. EPA as part of the ongoing case developments. An August 13, 2025 motion included an EPA request for additional 45-day abeyance. According to the motion, EPA has decided to proceed with a new notice and comment rulemaking. Preparation of the proposed rule, including research, drafting, and review, is already underway.

• EPA expects the proposed NAAQS reconsideration rule to be signed in Fall of 2025;
• EPA is targeting a final rule to be signed by February 2026.

This creates a dynamic situation where ADEQ is proposing updates to align with current federal NAAQS, while those federal standards are simultaneously under review and subject to potential change. The uncertainty means that the federal PM_2.5 NAAQS could change again, potentially leading to a higher value than currently anticipated or reverting to a previous standard of 12 micrograms per cubic meter (µg/m³).

Clearing the Air: Your Toolkit for What’s Next

To conclude, the Phoenix-Mesa ozone NA Area and Arizona’s implementation of the PM_2.5 NAAQS are at a critical juncture. The upcoming ozone attainment determination, coupled with ongoing federal reconsideration of PM_2.5 NAAQS, will impact future air quality regulations in the region. Understanding these regulatory developments is essential for effective strategic planning and facility compliance.

For specific office support, reach out to Camille Ponce or Maheshwar Mane in Trinity’s Phoenix office.