Section 204.1120  Air Quality Models

a)         All estimates of ambient concentrations required under this Section shall be based on applicable air quality models, databases, and other requirements specified in 40 CFR 51, appendix W (Guideline on Air Quality Models) (incorporated by reference in 35 Ill. Adm. Code 204.100).

b)         When an air quality model specified in 40 CFR 51, appendix W (Guideline on Air Quality Models) (incorporated by reference in Section 204.100) 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 USEPA 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 set forth in 35 Ill. Adm. Code 252.