The Environmental Protection Agency published a proposed rule in the Federal Register on October 23, 2025, to approve Arizona's state implementation plan revisions addressing interstate transport requirements for the 2012 fine particulate matter national ambient air quality standards. This action stems from submissions by the Arizona Department of Environmental Quality in 2015 and 2022, which demonstrate that emissions from Arizona sources do not significantly contribute to nonattainment or interfere with maintenance of these standards in other states. The proposal highlights the Clean Air Act's good neighbor provision, which mandates states to prevent their emissions from adversely affecting downwind air quality. By approving these revisions, the EPA affirms Arizona's compliance with key infrastructure requirements under section 110(a)(2)(D)(i)(I) of the Act, potentially setting a precedent for how western states handle similar obligations amid unique geographical and climatological challenges.
Statutory and Regulatory Background
The Clean Air Act requires states to submit implementation plans within three years of a new or revised national ambient air quality standard to ensure adequate provisions for various elements, including interstate transport. For the 2012 PM2.5 standards, established on January 15, 2013, at 12.0 micrograms per cubic meter for the primary annual standard, this includes prohibiting emissions that significantly contribute to nonattainment (prong 1) or interfere with maintenance (prong 2) in other states.
The EPA has developed a four-step framework to evaluate these obligations, drawing from the Cross-State Air Pollution Rule promulgated in 2011 and subsequent guidance. Step one identifies downwind receptors projected to struggle with attainment or maintenance. Step two assesses upwind state contributions, using a one percent threshold of the NAAQS to determine linkages. Steps three and four involve identifying and implementing emission reductions if necessary. The 2016 PM2.5 Transport Memo provided projections for 2017 and 2025, identifying problem receptors primarily in California, Idaho, and Pennsylvania. This framework ensures independent evaluation of each prong, as emphasized in the 2008 court decision in North Carolina v. EPA.
Arizona's SIP Submissions
Arizona addressed these requirements through two submissions. The first, dated December 11, 2015, from the Arizona Department of Environmental Quality, covered infrastructure elements for the 2012 PM2.5 standards. A supplemental submission on February 10, 2022, further detailed compliance with section 110(a)(2), including interstate transport. Both underwent public comment periods with no adverse feedback, meeting procedural standards under the Clean Air Act and federal regulations.
These documents analyze emissions of direct PM2.5 and precursors like nitrogen oxides and sulfur dioxide, incorporating monitoring data, modeling, and factors such as terrain and wind patterns. Arizona concluded that its contributions to out-of-state receptors fall below the one percent threshold, negating the need for additional controls.
EPA's Evaluation and Analysis
The EPA's review, detailed in a technical support document, applies the four-step framework to Arizona's submissions. For step one, the agency references the 2016 memo's identification of receptors, focusing on those in California due to proximity. Step two evaluates Arizona's linkages, considering emissions inventories, photochemical modeling, and topographical barriers like mountain ranges that limit transport.
Modeling shows Arizona's maximum contribution to any downwind receptor is well below one percent of the 2012 PM2.5 NAAQS. For instance, projections for 2025 indicate negligible impacts on California's San Joaquin Valley and South Coast areas. The EPA also weighs climatological data, noting prevailing winds and arid conditions that disperse pollutants locally rather than transporting them interstate. This weight-of-evidence approach aligns with precedents from the Cross-State Air Pollution Rule, where similar thresholds determined state obligations.
Different perspectives emerge in this context. Environmental groups might argue for stricter thresholds to account for cumulative impacts, while industry stakeholders could view the one percent metric as appropriately balanced. State regulators in downwind areas, such as California, may monitor these approvals closely to ensure collective compliance. The EPA's proposal does not endorse any viewpoint but notes that Arizona's existing controls, including vehicle emission standards and dust regulations, sufficiently address potential transport without further measures.
Implications for Air Quality Management
This proposed approval underscores broader political and legal dynamics in federal air quality regulation. Interstate transport has been a flashpoint in litigation, as seen in challenges to the Cross-State Air Pollution Rule, which was largely upheld but remanded on certain quantifications. Arizona's case reflects western states' unique challenges, including vast distances and natural barriers, contrasting with denser eastern pollution patterns.
Short-term implications include strengthened SIPs that could facilitate Arizona's attainment demonstrations. Long-term, it may influence how the EPA evaluates transport for future NAAQS revisions, potentially incorporating advanced modeling like source apportionment. Perspectives vary: advocates for federalism praise state-led solutions, while critics highlight the need for uniform national standards to prevent a patchwork of protections.
Key takeaways from this development include confirmation of Arizona's minimal interstate impact and the effectiveness of the EPA's analytical framework. Potential next steps involve public comments due by November 24, 2025, followed by a final rule. Ongoing debates may center on evolving climate patterns affecting transport or updates to the PM2.5 standards themselves, challenging states to adapt plans accordingly. This action contributes to a national effort ensuring clean air across state lines without imposing undue burdens.