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The 2013 EPA Draft SO2 NAAQS Designations Modeling Technical Assistance Document states that an accurate characterization of the modeled facility is critical. The document also says that that if the building information is not accurate, downwash will not be accurately accounted for in AERMOD. This presentation will discuss two generic facilities, one with a 31 m high long narrow solid building and a single stack that is 1.5 times the building height. The second facility has two 50 m high porous structures located near a single stack of the same height. Accurate building information was assembled for these two facilities and input into BPIP. The BPIP AERMOD input file was analyzed and the following problems were found: 1) building widths and/or lengths outside the range of AERMOD theory; and 2) the porous structures were assumed to be solid. In spite of inputting accurate site information, BPIP generated building dimensions for AERMOD input will not result in accurate predictions. Consequently, an EPA “Source Characterization” study was conducted where “Equivalent Building Dimensions” were defined that more accurately model the dispersion for these two sites. AERMOD was then run using the original BPIP determined inputs and the refined inputs based on a more accurate “Source Characterization.” With refined BPIP inputs, the maximum 1-hr concentrations decreased by factors of 2 to 3.5 Due to the stringent nature of the 1-hr NAAQS, clearly a more accurate source characterization study should be high on the list of refined modeling options.