A State Implementation Plan (SIP), as developed by a given State, defines regulations and their scope within that State and also spells out consequences for National Ambient Air Quality Standards (NAAQS) when implemented. A SIP may only be approved by the appropriate U.S. EPA Regional Office for that State.

A SIP typically uses environmental models to demonstrate NAAQS attainment. Specifically, Air Quality Modeling (AQM) is used to predict the chemical interaction, atmospheric transport, and deposition of Criteria Pollutants in out-door air State-wide. The model used in the AQM is first validated by application to historical episodes and only then is it applied to future scenarios to demonstrate compliance with the NAAQS.

Thus with the aid of AQM, an approved SIP must demonstrate either:

• NAAQS attainment by the predetermined date, or
• NAAQS non-attainment

In the case of NAAQS attainment, a 10 year Maintenance Plan is imposed. Whereas in the case of non-attainment, a Federal Implementation Plan (FIP) is developed at the local U.S. EPA Regional Office and Federal sanctions may be applied on the State in question.

Further information on the CAA and U.S. EPA is available at the following sites: