EPA has clarified that air quality “modeling” - the use of modeling techniques, databases or computer models to assess impacts to the National Ambient Air Quality Standards - is sufficient and appropriate for a demonstration of General Conformity when following the EPA 2017 Guideline on Air Quality Models. Federal agencies may rely upon the recommendations in the Guideline to comply with EPA’s General Conformity regulations when their actions are proposed to occur within nitrogen dioxide (NO2), ozone (O3), or fine particulate matter (PM2.5) nonattainment or maintenance areas.

Historically, EPA has not recommended that federal agencies use modeling to demonstrate General Conformity for these areas because the science and technology of the models and their capabilities were not sufficient to handle the complexities of photochemical reactions in the atmosphere from a relatively small source. Recent advances in modeling, and EPA’s innovative, alternative modeling techniques, can now support the use of modeling to demonstrate General Conformity using the recommendations of the Guideline. 

Read the Modeling Clarification Memorandum below.

• Use of Modeling Techniques to Demonstrate General Conformity for Ozone, Fine Particulate Matter and Nitrogen Dioxide (PDF)(6 pp, 3 MB)