Another consideration that may need attention by the user, 1 and is discussed in Section 5.1

2 of the AIG, relates to tall stacks located within or adjacent to small to moderate size urban areas.

3 In such cases, the stack height or effective plume height for very buoyant sources may extend

4 above the urban boundary layer height. The application of the urban option in AERMOD for

5 these types of sources may artificially limit the plume height. The use of the urban option may

6 not be appropriate for these sources, since the actual plume is likely to be transported over the

7 urban boundary layer. Section 5.1 of the AIG gives details on determining if a tall stack should

8 be modeled as urban or rural based on comparing the stack or effective plume height to the urban

9 boundary layer height. The 100 m stack illustrated in Figure B-1b, may be such an example as

10 the urban boundary layer height for this stack would be 189 m (based on a population of

11 100,000) and equation 104 of the AERMOD formulation document (Cimorelli, et al., 2004).


14 where ziuo is a reference height of 400 m corresponding to a reference population Po of 2,000,000

15 people.


17 Given that the stack is a buoyant release, the plume may extend above the urban

18 boundary layer and may be best characterized as a rural source, even if it were near an urban

19 complex. Exclusion of these elevated sources from application of the urban option would need

20 to be justified on a case-by-case basis in consultation with the appropriate reviewing authority.



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