Dispersion Model Parameters that Affect the Impacts from Area Sources using AERMOD

Previous studies have cited differences in predicted impacts from the Industrial Source Complex (ISC) model and the AERMOD model (Diosey, 2002 and Porter, 2003). These studies compared the overall difference of the models when predicting impacts from treatment process as a whole. More recent studies (Porter, 2004 and 2007) have cited the difference in predictions for point or area sources when the different dispersion models were used.One particular issue identified in the U.S. Environmental Protection Agency's (EPA) model performance evaluation was that low level point sources from process units tended to be under-predicted by AERMOD relative to ISC, while area sources tended to be over-predicted. This suggests that odor impacts using AERMOD would shift the control strategies from reducing offsite odor impacts from enclosed process areas (headworks facilities and solids processing) to open tanks and basins (primary clarifiers and aeration basins).This study examines the point and area source algorithms in the AERMOD model and discusses how differences occur. While EPA used field studies to demonstrate the performance of the AERMOD model, these field studies were not representative of short process stacks or open basins. Thus, the differences may in part be due to the changes in the dispersion algorithm or in the way the atmospheric boundary layer is characterized in the more refined AERMOD dispersion model.Key to both the characterization of the surface boundary layer and dispersion algorithm is the way in which the local land use parameters, albedo, surface roughness, and Bowen ratio are defined. A comparison is made that highlights the sensitivity of the AERMOD model to the selection of land use parameters for predicting offsite odor impacts. The treatment of vertical temperature and wind speed lapse rates is also discussed because lower wind speeds along the near surface layer could result in higher offsite odor impacts.While the AERMOD model will generally predict impacts that are 30 to 40 percent lower than ISCST3, some impacts can be 25 percent higher when subject to the influences of building cavity and wake effects or more than 60 percent lower when impacting elevated terrain.