The requirements placed on air quality control agencies by the Clean Air Act have dramatically increased the need for improved air quality modeling. The resulting increase in the use of models has also led to a substantial increase in the number and complexity of situations in which models are employed. The modeling guideline (Guideline on Air Quality Models, EPA-450/2-78-027, April 1978) addresses many of the problems in this relatively new and growing field, but much is left to the discretion of the reviewing agency since many complex problems are best solved on a case-by-case basis. However, because of the variety of technically correct solutions to any complex problem, different approaches with differing results have led to inconsistency in model applications from Region to Region. In an effort to improve consistency several workshops were held to provide a forum for the Regional Office and Headquarters groups to discuss common problem areas and arrive at generally acceptable solutions. Many recommendations were made in the course of the workshops. These were reviewed by OAQPS and some have necessarily been modified and supplemented to ensure consistency with other modeling policies. This report clarifies preferred data bases and procedures for the application of specific models and modeling techniques in situations where the guideline permits a case-by-case analysis.

"Revised 1982." Addendum, with revised and appended pages, released in October 1983. "EPA-450/4-82-015." "PB83-150573."