## Facts

- The Clean Air Act (CAA) requires EPA to set National Ambient Air Quality Standards (NAAQS) for pollutants, including ozone, and to designate areas as attainment, nonattainment, or unclassifiable after a NAAQS revision.
- In 2008, EPA tightened the primary and secondary ozone NAAQS and initiated a nationwide area-designation process within the statutory timeframe.
- States submitted recommended designations; EPA reviewed them and, where it disagreed, modified the recommendations as it deemed necessary.
- EPA had previously used a nine-factor, nonbinding guidance framework for setting nonattainment boundaries, and for the 2008 ozone designations distilled that approach into five factors: air quality data, emissions data, meteorology, geography/topography, and jurisdictional boundaries.
- Using this multi-factor, weight-of-the-evidence approach, EPA designated Shelby County, Tennessee (Memphis area), as nonattainment and included part of DeSoto County, Mississippi, concluding DeSoto County contributed to Shelby County’s ozone violation.
- Mississippi and other state, local, and industry petitioners sought review, arguing EPA’s methodology and specific designations were arbitrary and capricious and contrary to the CAA and the Administrative Procedure Act (APA).

## Issues

1. Whether EPA’s use of a five-factor, weight-of-the-evidence approach to ozone-area designations under the 2008 ozone NAAQS was arbitrary, capricious, or contrary to the CAA and APA.
2. Whether EPA acted arbitrarily or capriciously in designating DeSoto County, Mississippi, as part of the Memphis-area nonattainment region on a “contribution” rationale.

## Decision

- The court largely upheld EPA’s 2008 ozone nonattainment designations and denied the principal petitions for review relevant to Mississippi’s challenge.
- The court held EPA reasonably exercised its discretion under CAA § 107 to modify state recommendations and to apply a multi-factor, record-based method to set nonattainment boundaries.
- The court sustained EPA’s determination that DeSoto County contributed to Shelby County’s ozone nonattainment based on emissions, transport, and growth indicators, even if DeSoto County did not independently record a NAAQS violation.

## Legal Principles

- CAA § 107 grants EPA substantial discretion to modify state designation recommendations; these technical and predictive determinations are reviewed under the APA’s narrow arbitrary-and-capricious standard.
- EPA may employ a flexible multi-factor, weight-of-the-evidence methodology to determine nonattainment boundaries and may refine or consolidate nonbinding guidance if it reasonably explains the approach and grounds it in the record.
- An area may be designated nonattainment based on contribution to a nearby NAAQS violation; the CAA does not require the contributing area itself to be independently violating the standard.

## Conclusion

The D.C. Circuit upheld EPA’s methodology and, as relevant to Mississippi, affirmed EPA’s inclusion of part of DeSoto County in the Memphis-area ozone nonattainment designation, concluding EPA’s record-based, multi-factor reasoning satisfied the APA and the CAA.


Mississippi Comm’n on Env’t Quality v. EPA, 790 F.3d 138 (D.C. Cir. 2015)