downwash will not be accurately1 accounted for in AERMOD.
3Emission source type characterization within the modeling environment is also important.
4As stated in the AERMOD User’s Guide (U.S. EPA, 2004a; U.S. EPA, 2012a), emissions
5sources can be characterized as several different source types: POINT sources, capped stacks
6(POINTCAP), horizontal stacks (POINTHOR), VOLUME sources, OPENPIT sources, LINE
7sources, rectangular AREA sources, circular area sources (AREACIRC), and irregularly shaped
8area sources (AREAPOLY). Note that POINTCAP and POINTHOR are not part of the
9regulatory default option in AERMOD because the user must invoke the BETA option in the
10model options keyword MODELOPT while not including the “DFAULT” modeling option for
11these options to work properly. While most sources can be characterized as POINT sources,
12some sources, such as fugitive releases or nonpoint sources (emissions from ports/ships, airports,
13or smaller point sources with no accurate locations), may be best characterized as VOLUME or
14AREA type sources. Sources such as flares can be modeled in AERMOD using the parameter
15input methodology described in Section 2.1.2 of the AERSCREEN User’s Guide (U. S. EPA,
162011b). If questions arise about proper source characterization or typing, users should consult
17the appropriate EPA Regional Modeling Contact.
2.4. Urban/rural determination
21For any dispersion modeling exercise, the urban or rural determination of a source is
22important in determining the boundary layer characteristics that affect the model’s prediction of
23downwind concentrations. Figure B-1 gives example maximum 24-hour concentration profiles
24for a 10 meter stack (Figure B-1a) and a 100 m stack (Figure B-1b) based on urban vs. rural
25designation. The urban population used for the examples is 100,000. In Figure 1a, the urban
26concentration is much higher than the rural concentration for distances less than 750 m from the
27stack but then drops below the rural concentration beyond 750 m. For the taller stack in Figure
281b, the urban concentration is much higher than the rural concentration even as distances
29increase from the source. These profiles show that the urban or rural designation of a source can
30be quite important.
32Determining whether a source is urban or rural can be done using the methodology
33outlined in Section 7.2.3 of Appendix W and recommendations outlined in Sections 5.1 through
345.3 in the AIG (U.S. EPA, 2009). In summary, there are two methods of urban/rural
35classification described in Section 7.2.3 of Appendix W.
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