The "Guideline on Air Quality Models (Revised)" (hereinafter, the "Guideline"), as modified by supplement A (1987), sets forth air quality models and guidance for estimating ambient air concentrations due to sources of air pollutants. The Guideline is presently incorporated by reference into the prevention of significant deterioration (PSD) regulations under the Clean Air Act. On February 13, 1991, EPA issued a Notice of Proposed Rulemaking (NPR) to further clarify and update the Guideline, as well as to augment the Guideline with several new modeling techniques, and to codify the Guideline for all air quality planning purposes.

EPA takes final action in order to add new models to the Guideline and improve existing models. In addition, this action amends the CFR to incorporate supplement B as codified text, as well as giving regulatory status to long-standing EPA policy regarding the use of air quality models for other regulatory programs. Therefore, EPA is setting out the Guideline, revised by supplements A and B, as appendix W to 40 CFR part 51. Adoption of these new and refined modeling techniques and associated guidance should significantly improve the technical basis for impact assessment of air pollution sources.

• Requirements for Preparation, Adoption, and Submittal of Implementation Plans - Federal Register Notice, July 20, 1993 (PDF)(4 pp, 10 K, July 20, 1993, FR Vol. 58, No. 137)