More information from the EPA document "40 CFR Part 51, Appendix W: Guideline on Air Quality Models" mentioned in a previous post. The following paragraph in section 8.2.9 discusses fumigation, a condition Calpine has modeled using a SCREEN3 computer model.

"Fumigation occurs when a plume (or multiple plumes) is emitted into a stable layer of air and that layer is subsequently mixed to the ground either through convective transfer of heat from the surface or because of advection to less stable surroundings. Fumigation may cause excessively high concentrations but is usually rather short-lived at a given receptor. There are no recommended refined techniques to model this phenomenon. There are, however, screening procedures (see ''Screening Procedures for Estimating the Air Quality Impact of Stationary Sources'' 18 ) that may be used to approximate the concentrations. Considerable care should be exercised in using the results obtained from the screening techniques."

I guess the key question here is what is meant by "considerable care". For those that are interested I found an EPA web site devoted to modeling of pollutants at http://www.epa.gov/ttn/scram/index.htm.

Sincerely,

Steven Nelson