(a) The following shall apply when dispersion or other air quality modeling is used to meet the requirements of this subpart.
(b) General. All applications of air quality modeling involved in this subpart shall be based on the applicable models, data bases, and other requirements specified in Appendix W of 40 C.F.R. pt. 51, Guideline on Air Quality Models.
(c) Substitution.
(1) Where an air quality model specified in the Guideline on Air Quality Models is inappropriate, the model may be modified or another model substituted.
(2) Such a modification or substitution of a model may be made on a case-by-case basis or, where appropriate, on a generic basis for a specific pollutant or type of stationary source.
(3) Written approval of the Administrator of the United States Environmental Protection Agency must be obtained for any modification or substitution.