(1) All estimates of ambient concentrations required under this subchapter must be based on the applicable air quality models, data bases, and other requirements specified in the Guideline on Air Quality Models, 40 CFR Part 51, Appendix W.
(2) Where an air quality impact model specified in 40 CFR Part 51, Appendix W is inappropriate, the model may be modified or another model substituted. 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 state program. Written approval of the administrator must be obtained for any modification or substitution. In addition, use of a modified or substituted model must be subject to notice and opportunity for public comment under procedures developed in accordance with ARM 17.8.826.