## Facts

- Phoenix, Arizona was designated a moderate PM-10 nonattainment area under the Clean Air Act and was subject to deadlines and additional SIP requirements.
- Arizona submitted, and EPA approved, a Phoenix-area PM-10 State Implementation Plan intended to attain the national annual and 24-hour PM-10 standards.
- Two Phoenix residents petitioned for review, alleging EPA’s approval failed to ensure timely attainment and failed to require and evaluate statutorily required control measures.
- The SIP treated impracticability of attaining the annual PM-10 standard as sufficient to justify approval without a separate, express analysis of how the 24-hour standard would be met (or why it could not be met).
- Petitioners also challenged EPA’s reasoning and aspects of the rulemaking process under the Administrative Procedure Act.

## Issues

1. Whether EPA may approve a PM-10 SIP and an impracticability demonstration that does not independently address both the annual and 24-hour PM-10 national standards.
2. Whether EPA acted arbitrarily and capriciously by approving a SIP that inadequately required or evaluated “reasonably available control measures” and other Clean Air Act measures for a PM-10 nonattainment area.
3. Whether EPA’s SIP approval was procedurally defective under the APA due to deficiencies in notice-and-comment and the agency’s response to significant comments.

## Decision

- The Ninth Circuit granted the petition for review.
- The court vacated EPA’s approval of the Phoenix PM-10 SIP and remanded to EPA for further proceedings.
- The court held that inability to attain the annual PM-10 standard did not excuse the SIP from independently examining the 24-hour PM-10 standard.
- The court found EPA’s approval inconsistent with Clean Air Act requirements and arbitrary and capricious in relevant respects, including inadequacies in evaluating and ensuring required control measures.
- The court stated its ruling did not bar EPA from reclassifying Phoenix as a serious PM-10 nonattainment area.

## Legal Principles

- A PM-10 SIP must separately address each applicable NAAQS (annual and 24-hour) because the standards serve different public-health purposes and may involve different sources and control strategies.
- Under Chevron, courts may accept an agency’s reasonable construction of ambiguous Clean Air Act provisions, but will set aside actions that misapply the statute or lack a reasoned basis.
- EPA must ensure and explain that SIPs include required control measures for the area’s classification (including RACM where applicable) and provide reasoned explanations for rejecting feasible measures or claiming impracticability.
- EPA SIP approvals are reviewable under the APA’s arbitrary-and-capricious standard and must be supported by a rational explanation grounded in the administrative record and procedurally adequate public participation.

## Conclusion

The Ninth Circuit vacated EPA’s approval of Arizona’s Phoenix PM-10 SIP because the Clean Air Act required an independent treatment of the 24-hour PM-10 standard and a adequately reasoned showing that required control measures were implemented and evaluated; the matter was remanded to EPA without precluding reclassification of Phoenix to serious nonattainment status.


Ober v. U.S. Env’t Prot. Agency, 84 F.3d 304 (9th Cir. 1996)