On November 9, 2006, the AMS/U.S. EPA Regulatory Model (AERMOD) replaced the Industrial Source Complex (ISC) model as the U.S. EPA-approved regulatory model for analyses in the nearfield (less than 50 kilometers from a site). The CALPUFF modeling system, another U.S. EPA-approved model typically regarded as a long-range transport model, can also be utilized in the nearfield on a case-by-case basis according to Appendix W of Title 40 of the Code of Federal Regulations Part 51 (the Guideline). Use of either model in the nearfield is essentially at the discretion of the permitting authority and/or facility.
Regardless of the model utilized for an air dispersion modeling analysis, one of the more challenging aspects of applying either of these air quality dispersion models to aggregate handling operations, such as those located at Portland cement or lime manufacturing plants, is determining the most appropriate method of representing fugitive particulate matter (PM) emission sources in the model. In particular, these models tend to overestimate impacts from low-level fugitive PM emission sources that are typically present at aggregate handling facilities. While numerous state regulatory agencies provide guidance on how to model fugitive particulate emission sources to reduce the inherent conservatism and best represent the emissions from these sources, these source types still prove problematic in regulatory modeling exercises such as those associated with Prevention of Significant Deterioration (PSD) permitting.
This analysis focused on typical PM emission sources such as storage piles, material transfers, and haul roads located at aggregate handling facilities. Specifically, this paper compares the performance of the CALPUFF and AERMOD modeling systems when modeling fugitive PM sources in the nearfield (i.e., distances under 50 km).
Performance of AERMOD vs. CALPUFF on Fugitive Emission Sources in the Nearfield