downwash will not be accurately 1 accounted for in AERMOD.


3 Emission source type characterization within the modeling environment is also important.

4 As stated in the AERMOD User’s Guide (U.S. EPA, 2004a; U.S. EPA, 2012a), emissions

5 sources can be characterized as several different source types: POINT sources, capped stacks

6 (POINTCAP), horizontal stacks (POINTHOR), VOLUME sources, OPENPIT sources, LINE

7 sources, rectangular AREA sources, circular area sources (AREACIRC), and irregularly shaped

8 area sources (AREAPOLY). Note that POINTCAP and POINTHOR are not part of the

9 regulatory default option in AERMOD because the user must invoke the BETA option in the

10 model options keyword MODELOPT while not including the “DFAULT” modeling option for

11 these options to work properly. While most sources can be characterized as POINT sources,

12 some sources, such as fugitive releases or nonpoint sources (emissions from ports/ships, airports,

13 or smaller point sources with no accurate locations), may be best characterized as VOLUME or

14 AREA type sources. Sources such as flares can be modeled in AERMOD using the parameter

15 input methodology described in Section 2.1.2 of the AERSCREEN User’s Guide (U. S. EPA,

16 2011b). If questions arise about proper source characterization or typing, users should consult

17 the appropriate EPA Regional Modeling Contact.

2.4. Urban/rural determination


21 For any dispersion modeling exercise, the urban or rural determination of a source is

22 important in determining the boundary layer characteristics that affect the model’s prediction of

23 downwind concentrations. Figure B-1 gives example maximum 24-hour concentration profiles

24 for a 10 meter stack (Figure B-1a) and a 100 m stack (Figure B-1b) based on urban vs. rural

25 designation. The urban population used for the examples is 100,000. In Figure 1a, the urban

26 concentration is much higher than the rural concentration for distances less than 750 m from the

27 stack but then drops below the rural concentration beyond 750 m. For the taller stack in Figure

28 1b, the urban concentration is much higher than the rural concentration even as distances

29 increase from the source. These profiles show that the urban or rural designation of a source can

30 be quite important.


32 Determining whether a source is urban or rural can be done using the methodology

33 outlined in Section 7.2.3 of Appendix W and recommendations outlined in Sections 5.1 through

34 5.3 in the AIG (U.S. EPA, 2009). In summary, there are two methods of urban/rural

35 classification described in Section 7.2.3 of Appendix W.



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