An official website of the United States government.

Due to a lapse in appropriations, EPA websites will not be regularly updated. In the event of an environmental emergency imminently threatening the safety of human life or where necessary to protect certain property, the EPA website will be updated with appropriate information. Please note that all information on the EPA website may not be up to date, and transactions and inquiries submitted to the EPA website may not be processed or responded to until appropriations are enacted.

We've made some changes to If the information you are looking for is not here, you may be able to find it on the EPA Web Archive or the January 19, 2017 Web Snapshot.

Support Center for Regulatory Atmospheric Modeling (SCRAM)

Clean Air Act Permit Modeling Guidance

This section provides recommendations on modeling techniques and guidance for estimating pollutant concentrations in order to assess control strategies and determine emission limits. These recommendations were originally published in April 1978 as the "Guideline on Air Quality Models" and were incorporated by reference in the regulations for the Prevention of Significant Deterioration of Air Quality, Title 40, Code of Federal Regulations (CFR) sections 51.166 and 52.21 in June 1978 [Federal Register, 43 (118), 26 382-26 388]. The purpose of these guidelines is to promote consistency in the use of modeling within the air quality management process. These guidelines are periodically revised to ensure that new model developments or expanded regulatory requirements are incorporated. This section provides guidance associated with permit modeling and is divided into three components: Appendix W Guidance, Screening Guidance, and Other Permit Modeling Guidance.

You may need a PDF reader to view some of the files on this page. See EPA’s About PDF page to learn more.

Guideline on Air Quality Models ("Appendix W" to 40 CFR Part 51)

Appendix W (PDF)(54 pp, 742 K) - Guideline on Air Quality Models ("Appendix W" to 40 CFR Part 51), January 2017

Air Quality Analysis Checklist (PDF)(13 pp, 74 K, 2016) -  The Air Quality Analysis Checklist assists the regulatory reviewer in assuring that important aspects of the NSR/PSD compliance demonstrations for NAAQS and PSD increments are accounted for and appropriately examined. Additionally, this checklist is useful for those conducting these regulatory demonstrations by identifying the necessary pieces of information required to assure an appropriate air quality assessment.  A previous version of the Checklist (14 pp, 111 K ) formerly appeared as Appendix C of the "Guideline on Air Quality Models" as explained in the Explanatory Note (TXT)(1 K).

Clarification Memos - Provides access to memoranda issued by EPA as clarifications of Appendix W and technical aspects of dispersion modeling in general.

Screening Guidance
Screening Procedures for Estimating the Air Quality Impact of Stationary Sources (PDF) (93 pp, 2.6MB, 1992)

Screening Models - SCRAM webpage providing information on screening models, including model code and documentation
Other Permit Modeling Guidance
Guidance for PM2.5 Permit Modeling (PDF) (144 pp, 2.3MB) - May 2014. EPA 454/B-14-001. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.

Draft Guidance for PM2.5 Permit Modeling (116 pp, 4.6MB) - March 2013. EPA 454/D-13-001. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.