ABSTRACT

The air pollution regulatory process involves the permitting of sources of regulated pollutants. Part of the permit process requires sources to demonstrate that the National Ambient Air Quality Standards (NAAQS) are not exceeded as a result of pollutant release. While ambient air sampling may be used to demonstrate compliance with the NAAQS, a source's right to operate is becoming increasingly dependent primarily on dispersion modeling results. Currently used Gaussian based dispersion models do not adequately account for pollutant dispersion resulting from sub-hourly variations in wind speed and direction. This can result in over-estimates of downwind concentration, which may result in costly additional control measures or operating permit being denied. Low elevation releases and close property line boundaries cause low-level agricultural sources, such as cotton gins, to be very sensitive to inappropriate modeling results. This research focused on the issue of developing a methodology to analyze the theoretical degree of dispersion within sub-hourly intervals. The degree of dispersion was based on small time-interval (15 second) meteorological data. Using the results of this analysis a new model that accounts for plume dispersion due to sub-hourly variation in wind speed and direction, was developed. The new model (FTAM) was compared to the currently used regulatory models ISC3 and AERMOD, using concentration predictions down-wind of a source typical of a cotton gin. FTAM 24-hour concentrations were generally lower that ISC3 predicted 24-hour concentrations, and similar or slightly higher than AERMOD predicted 24-hour concentrations.