Natural Resources Defense Council v. Environmental Protection Agency, 414 U.S. App. D.C. 89

EPA revised the ozone NAAQS in 2008 (from 0.08 to 0.075 ppm, 8‑hour) and issued an Implementation Rule on May 21, 2012, applying Subpart 2 classifications to nonattainment areas.
For attainment deadlines, instead of measuring Table 1 attainment periods from the designation effective date (July 20, 2012), EPA delayed the trigger to December 31 of the designation year, effectively giving some areas an extra ozone season (e.g., Marginal areas get to Dec. 31, 2015 rather than July 20, 2015).
EPA also revoked the 1997 ozone NAAQS only for transportation conformity purposes while leaving 1997 designations and maintenance obligations otherwise in place.
NRDC petitioned for review arguing (1) EPA lacked statutory authority to lengthen attainment deadlines and (2) EPA unlawfully revoked transportation conformity requirements for the 1997 NAAQS.
The D.C. Circuit majority vacated the portions of the Implementation Rule at issue, holding (1) the December 31 trigger is inconsistent with Subpart 2 and prior agency positions, and (2) EPA cannot revoke conformity obligations alone because the statute mandates conformity apply to nonattainment and maintenance areas.

Issue	Plaintiff's Argument (NRDC)	Defendant's Argument (EPA)	Held
Whether EPA may defer the Subpart 2 attainment trigger to Dec. 31 of the designation year (thereby lengthening deadlines)	Statute and Table 1 require measuring attainment periods from the designation/redesignation date; deferral unlawfully extends deadlines beyond statutory bounds	Chevron‑deferable: Dec. 31 trigger reasonably preserves the same number of post‑designation ozone seasons and is a permissible interpretation of the timing gap	Held unlawful: court rejects EPA’s Dec. 31 trigger at Chevron step two — statutory text, structure, and prior EPA practice point to using the designation date as the trigger
Whether EPA may revoke the 1997 NAAQS transportation conformity requirements while keeping 1997 designations/maintenance obligations	Statute plainly mandates that conformity requirements “shall apply” to nonattainment and maintenance areas; EPA cannot eliminate conformity obligations alone	EPA contends it can partially revoke prior NAAQS for conformity purposes to avoid duplicative/complex conformity obligations and relies on its authority to revoke a NAAQS in full (South Coast)	Held unlawful: court finds at Chevron step one that the statute unambiguously requires conformity to apply to nonattainment/maintenance areas and disallows eliminating conformity obligations while leaving designations in place

Whitman v. Am. Trucking Ass’ns, Inc., 531 U.S. 457 (2001) (identified a timing gap in Subpart 2 when EPA revises ozone NAAQS and held courts must defer to reasonable agency gap-filling)
South Coast Air Quality Mgmt. Dist. v. EPA, 472 F.3d 882 (D.C. Cir. 2006) (addressed EPA’s implementation of a revised ozone NAAQS and limits under anti‑backsliding; recognized EPA may revoke a prior NAAQS in full subject to statutory constraints)
Sierra Club v. EPA, 129 F.3d 137 (D.C. Cir. 1997) (held conformity requirements categorically apply to nonattainment and maintenance areas; agency may not create grace‑period exemptions)
ATA III (Am. Trucking Ass’ns, Inc. v. EPA), 283 F.3d 355 (D.C. Cir. 2002) (describes EPA/state implementation framework under NAAQS and Subpart interaction)
ATA I (Am. Trucking Ass’ns, Inc. v. EPA), 175 F.3d 1027 (D.C. Cir. 1999) (held Subpart 2 governs revised primary ozone NAAQS in principle)
Util. Air Regulatory Grp. v. EPA, 134 S. Ct. 2427 (2014) (framework for judicial review and limits on agency reasoning divorced from statutory text)
EPA v. EME Homer City Generation, L.P., 134 S. Ct. 1584 (2014) (contextual authority on NAAQS implementation litigation)