a) A permit application under this Subpart shall provide a comparison of the ambient air quality under existing requirements and the ambient air quality which would exist under the proposed ACS. This analysis shall include dispersion modeling based on the best and most appropriate models for the pollutant and emission sources involved, unless the Agency finds that:

1. 1) Due to the characteristics of the pollutant and emission source, dispersion modeling is inappropriate or unnecessary for determining effects on air quality; or
2. 2) The location of emission sources included in the ACS are not more than 250 meters apart, the effective plume height of the emission increases and decreases are not significantly different and the differences in the characteristics of the emission sources are not likely to affect ambient air quality; or

3. 3) Differences in location, plume height, operating practice, and other characteristics of the emission sources subject to the ACS are not likely to significantly affect ambient air quality. An effect on ambient air quality is significant if it equals or exceeds the levels specified in the following table:

   SIGNIFICANCE LEVELS
   Pollutant	Annual	24-Hour	8-Hour	3-Hour	1-Hour
   SO2	1.0 ug/m3	5 ug/m3		25 ug/m3
   TSP	1.0 ug/m3	5 ug/m3
   NOx	1.0 ug/m3
   CO			0.5 ug/m3		2 ug/m3