Requirements under the federal CAA, state law enacted pursuant to the federal CAA, and other state law designed to protect air quality.

Ellis v. Gallatin Steel Co., 390 F.3d 461, 473 (6th Cir. 2004) (noting plaintiff in citizen suit intervened in EPA enforcement action).Res judicata applies even though the plaintiff in the citizen suit gave notice of intent to file the citizen suit before EPA filed its civil enforcement action.

Latino Issues Forum v. EPA, 558 F.3d 936, 941 (9th Cir. 2009) (derives standard from APA).The arbitrary and capricious test applies to judicial review of EPA's exercise of discretion.

Clean Air Act § 186(b)(2), 42 U.S.C. § 7512(b)(2) (CO nonattainment).The next step in the consequences for nonattainment depends on which pollutant is involved and, for pollutants for which areas are classified, what the classification of the area is.