Comparison of AERMOD and ISCST3 Emissions Factors for PM From Cotton Harvesting

Emission factors (EFs) and results from dispersion models are key components in the air pollution regulatory process. The EPA preferred regulatory model changed from ISCST3 to AERMOD in November, 2007. This change in regulatory models has had an impact on the regulatory process and the industries regulated.

In this study, the measured concentrations of regulated particulate matter (PM) including TSP, PM₁₀ and PM_2.5 reported by Wanjura (2008) were the data source. Wanjura utilized the previous preferred model (ISCST3) in conjunction with the Texas A&M protocol to back-calculate PM EFs for cotton harvesting. These data were used to back-calculate EFs for cotton harvesting with the more recent preferred AERMOD dispersion model. The goal of this research was to document differences in emission factors as a consequence of the models used. The PM₁₀ EFs developed for two-row and six-row pickers were 154 ± 43 kg/km² and 425 ± 178 kg/km², respectively.  From the comparison between AERMOD and ISCST3, it was observed that AERMOD EFs were 1.8 times higher than ISCST3 EFs for a six-row harvester.

A new approach to model PM emissions from harvesting operations was introduced in this research. This approach included modeling harvesting as a series of line sources, with each line corresponding to a harvester pass in the field. In this approach, we used 5-min averages of met. data and GPS data to better simulate field conditions.  It was hypothesized that this approach would lead to improved accuraies of EFs. However, the final results indicated that the more sophisticated approach did not result in improved EFs. There were no significant differences between EFs developed by this new approach and the conventional Texas A&M protocol.