Another consideration that may need attention by the user,1 and is discussed in Section 5.1
2of the AIG, relates to tall stacks located within or adjacent to small to moderate size urban areas.
3In such cases, the stack height or effective plume height for very buoyant sources may extend
4above the urban boundary layer height. The application of the urban option in AERMOD for
5these types of sources may artificially limit the plume height. The use of the urban option may
6not be appropriate for these sources, since the actual plume is likely to be transported over the
7urban boundary layer. Section 5.1 of the AIG gives details on determining if a tall stack should
8be modeled as urban or rural based on comparing the stack or effective plume height to the urban
9boundary layer height. The 100 m stack illustrated in Figure B-1b, may be such an example as
10the urban boundary layer height for this stack would be 189 m (based on a population of
11100,000) and equation 104 of the AERMOD formulation document (Cimorelli, et al., 2004).
14where ziuo is a reference height of 400 m corresponding to a reference population Po of 2,000,000
17Given that the stack is a buoyant release, the plume may extend above the urban
18boundary layer and may be best characterized as a rural source, even if it were near an urban
19complex. Exclusion of these elevated sources from application of the urban option would need
20to be justified on a case-by-case basis in consultation with the appropriate reviewing authority.
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