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Implication of Applying CALPUFF to Demonstrate Compliance with the Regional Haze Rule

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The U.S. Environmental Protection Agency promulgated the final Regional Haze Rule in July 1999 to protect Class I areas from visibility impairment. Industrial sources may be required to conduct modeling analysis to demonstrate their potential impact on nearby Class I areas during air permit applications. Although the widely used Industrial Source Complex Short-Term model (ISCST3) can be applied (and it may be recommended as the first step screening tool by state agencies), its results may not pass the strict criteria, which are set to protect the Class I areas. Moreover, regional haze is generally a long-range transport phenomenon. Theoretically, the ISCST3 model is not appropriate for a long-range transport (e.g., greater than 50 kilometers from the emission sources). Variations of the meteorological and geophysical conditions in such a distance require sophisticated temporal and spatial treatments of meteorological data, geophysical data, and plume dispersions. The CALPUFF modeling system is designed to handle long-range transport of various pollutants with consideration of chemical transformation.

For regional haze analysis, different states may have different modeling approach and guidance. Colorado proposes a 3-tier approach: Tier 1 -ISCST3 modeling analysis; Tier 2 - CALPUFF Screen analysis; and Tier 3 -Full CALPUFF analysis. CALPUFF Screen analysis is a simplified approach with flat terrain assumption and meteorological data from one station. In this paper, a case study of applying the CALPUFF modeling for regional haze analysis is conducted for an industrial source with potential impact on a nearby Class I area. Results for a base case are discussed and influences of several variables (e.g., distance between the source and the Class I area, emission rates, stack parameters, and background ozone and ammonia concentrations) on modeling results are evaluated. Modeling results indicate that control of NOx and SO2 is most effective in reducing extinction change while stack parameters have small effects on extinction change. Extinction change may also be a strong function of distance for distances less than certain values (e.g., 80 km). In addition, it seems typical background ozone concentrations affect the extinction change more than typical background ammonia concentrations.

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