This action finalizes revisions to the Guideline on Air Quality Models, published as Appendix W to 40 CFR part 51. The Guideline provides EPA-recommended models and other techniques for use in predicting ambient concentrations of pollutants for controlling air pollution sources in programs ranging from the Prevention of Significant Deterioration (PSD) permitting program to State Implementation Plans (SIPs). The Guideline fulfills a Clean Air Act mandate for the EPA to specify models with reasonable particularity to be used under specified conditions for purposes of the PSD permitting program. This action includes important enhancements to the EPA's AERMOD near-field dispersion modeling system that will establish AERSCREEN as the recommended screening level model for simple and complex terrain settings, significantly improve the model performance under stable/light wind conditions, allow for the use of meteorological input data derived from prognostic meteorological models, provide additional options for the modeling of nitrogen dioxide, incorporate the treatment of buoyant line sources within AERMOD, and incidental modifications to the modeling system that have received peer and external review. Additionally, these revisions would incorporate the use of photochemical modeling techniques to more adequately account for the secondary chemical formation of fine particulate matter and ozone associated with precursor emissions from single sources.