The Environmental Protection Agency issued a proposed rule 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 within the state do not significantly contribute to air quality issues in neighboring states. Under the Clean Air Act's good neighbor provision, states must ensure their emissions do not adversely affect downwind areas. The EPA's proposal highlights Arizona's compliance with these obligations, potentially setting a model for other western states facing similar evaluations. This development is significant as it aligns with ongoing efforts to manage cross-border air pollution amid evolving national standards.
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 address infrastructure elements, including interstate transport. Section 110(a)(2)(D)(i)(I) - known as the good neighbor provision - mandates that these plans prohibit emissions that significantly contribute to nonattainment (prong 1) or interfere with maintenance (prong 2) of the standards in other states. For the 2012 PM2.5 standards, which lowered the primary annual limit to 12.0 micrograms per cubic meter, the EPA has applied a four-step framework derived from prior rulemakings like the 2011 Cross-State Air Pollution Rule.
This framework involves identifying downwind receptors with projected attainment issues, determining upwind state contributions above a one percent threshold of the standard, assessing necessary emission reductions, and implementing enforceable measures. The EPA's 2013 guidance on infrastructure plans and its 2016 memorandum specific to the 2012 PM2.5 standards further inform this process. Legal precedents, such as the D.C. Circuit's ruling in North Carolina v. EPA (2008), emphasize giving independent weight to both prongs of the good neighbor provision.
Arizona's SIP Submissions
Arizona submitted its initial plan revision on December 11, 2015, followed by a supplement on February 10, 2022. These documents collectively address the interstate transport requirements for the 2012 PM2.5 standards. The Arizona Department of Environmental Quality conducted public comment periods for both submissions, with no comments received, meeting procedural requirements under the Clean Air Act and federal regulations.
In its analysis, Arizona evaluated emissions of direct PM2.5 and precursors like nitrogen oxides and sulfur dioxide. The state considered geographical factors, such as its arid terrain and distance from identified downwind receptors in California, Idaho, and Pennsylvania. Modeling and monitoring data indicated that Arizona's contributions fall below the one percent threshold, meaning no linkage to nonattainment or maintenance problems elsewhere.
EPA's Evaluation and Analysis
The EPA's review applies the four-step framework to Arizona's submissions. For step one, the agency identified receptors with projected issues, including 17 sites in California's San Joaquin Valley and South Coast areas, one in Shoshone County, Idaho, and one in Allegheny County, Pennsylvania, based on 2017 and 2025 design value projections from the 2016 memorandum.
In step two, the EPA assessed Arizona's contributions using air quality modeling, emissions inventories, and topographical data. The technical support document details that Arizona's emissions do not exceed the one percent threshold at any receptor. Factors like prevailing winds, which often direct emissions away from sensitive areas, and the state's existing controls on sources such as power plants and vehicles support this finding. As a result, the EPA concludes no further evaluation under steps three and four is needed, as Arizona is not linked to downwind problems.
This evaluation draws on verifiable data, including ambient monitoring and projections consistent with prior EPA actions. For instance, the one percent threshold aligns with the Cross-State Air Pollution Rule, upheld in part by the Supreme Court in EPA v. EME Homer City Generation (2014).
Perspectives and Implications
Stakeholders offer varied views on interstate transport rules. Environmental groups often advocate for stricter thresholds to protect downwind states, citing cumulative impacts from multiple upwind sources. Industry representatives, however, emphasize the economic burdens of additional controls without clear linkages, as seen in challenges to the Cross-State Air Pollution Rule.
Short-term implications include streamlined compliance for Arizona, potentially reducing regulatory uncertainty for emitters. Long-term, this approval could influence how western states address transport amid climate-driven changes like wildfires, which complicate PM2.5 attribution. It also underscores the role of federal oversight in balancing state autonomy with national air quality goals.
In summary, the EPA's proposed approval affirms Arizona's compliance with interstate transport requirements for the 2012 PM2.5 standards. Key takeaways include the state's non-linkage to downwind receptors and the effectiveness of its existing measures. Looking ahead, potential next steps involve monitoring for new data that might alter projections, addressing any future nonattainment designations, and navigating ongoing debates over the good neighbor provision's application in a changing regulatory landscape. Challenges may arise from litigation or revised standards, while debates continue on equity in cross-state pollution management.