A citizen suit may not be commenced if the EPA or the State is already diligently prosecuting a civil action in a court to enforce compliance.

In a citizen suit, the court may grant an injunction, impose penalties, and award attorney fees and litigation costs to any prevailing or partially prevailing party whenever the court determines such an award is appropriate.  Attorney fees will not be assessed against a citizen unless the court determines the suit is frivolous.
Clean Air Act
Clean Air Act Outline

Clean Air Act: A Summary of the Act and Its Major Requirements
(CRS Report for Congress 2011)

The Plain English Guide to the Clean Air Act
(EPA 1993)
The Clean Air Act authorizes citizens to enforce compliance with emission standards or limitations and orders issued by the EPA Administrator or a State.  42 USC ยง 7604.  Citizens must be adversely affected by the violation and normally must give 60-days notice of the alleged violation to the alleged violator, State and EPA prior to filing suit.  40 CFR Part 54.  This notice and delay period are intended to allow the violator an opportunity to correct his violation and to give the EPA or State an opportunity to enforce compliance, thus making citizen enforcement unnecessary.