AERMOD Tech Guide

Gaussian Plume Air Dispersion Model

1.3 Model Development Process

AERMOD was developed in the following stages: 1) initial model formulation; 2) developmental evaluation; 3) internal peer review and beta testing; 4) revised model formulation; 5) performance evaluation and sensitivity testing; 6).9 external peer review; and 7) submission to EPA’s Office of Air Quality Planning and Standards (OAQPS) for consideration as a regulatory model. We are currently at stage 7.

The initial formulations of AERMOD are summarized in Perry, et al. (1994) and Cimorelli, et al. (1996). Once formulated, the model was tested (developmental evaluation) against a variety offield measurements in order to identify areas needing improvement. The developmental evaluation provided a basis for selecting formulation options.

This developmental evaluation was conducted using five data bases. Three consisted of event-based tracer releases, while the other two each contain up to a full year of continuous SO2 measurements. These data bases cover elevated and surface releases, complex and simple terrain, and rural and urban boundary layers. A description of the early developmental evaluation is presented in Lee, et al., (1995) and in a later report by Lee et al. (1998a). Many revisions to the original early formulation have resulted from this evaluation as well as comments received during peer review, beta testing, and the public forum at the EPA’s Sixth Modeling conference (in 1995). Lee et al. (1998a, 1998b) describe the developmental evaluation repeated with the current model.

In addition, AERMOD has undergone a comprehensive performance evaluation (PES, 1998), designed to assess how well AERMOD‚Äôs concentration estimates compare against a variety of independent data bases and to assess the adequacy of the model for use in regulatory decision making. That is, to assess how well the model predicts concentrations at the high end of the concentration distribution. AERMOD was evaluated against five independent data bases (two in simple terrain and three in complex terrain), each containing one full year of continuous SO2 measurements. Additionally, AERMOD‚Äôs performance was compared against the performance of four other applied, regulatory models: they are: ISC3, CTDMPLUS (Perry, 1992), RTDM (Paine and Egan,1987) and HPDM (Hanna and Paine, 1989: Hanna and Chang, 1993). The performance of these models against AERMOD has been compared using the procedures in EPA‚Äôs ‚ÄúProtocol for Determining the Best Performing Model‚ÄĚ (EPA, 1992). Finally, upon completion of the peer review and presentation of the model at EPA‚Äôs Seventh Modeling Conference in early 1999, AERMOD will be submitted to EPA‚Äôs Office of Air Quality Planning and Standards (OAQPS) for inclusion in the Guideline on Air Quality Models (Code of Federal Regulations, 1997).