AirWare User Manual

AERMOD: A Dispersion Model
for Industrial Source Applications

This documentation is extracted from Perry et al. (1998).

Terrain Handling

ISC2 accounts for elevated terrain by assuming that the reflecting surface, which extends from the source to the receptor, is located at the height of the receptor. This assumption ignores the fact that under many atmospheric conditions a plume is displaced vertically as it senses rising terrain. As a result, ISC2 has been shown to significantly overestimate concentration as terrain exceeds stack top. This result caused EPA to restrict the application of ISC2 to terrain at or below stack top. For terrain above stack top, EPA guidance allows for the use of either:

  1. a conservative screening model (eg., COMPLEX I or RTDM) 0,0 coupled with the application of the "intermediate terrain policy0", or
Both options present significant implementation issues (e.g. intermediate terrain postprocessing, significant input requirements of CTDMPLUS, etc) and may introduce discontinuities in concentration estimates in complex terrain.

In its treatment of elevated terrain AERMOD represents significant improvement over current regulatory modeling methods. Contrary to ISC2, AERMOD provides a consistent and continuous approach for application at all terrain heights thus removing the discontinuities and much of the implementation burden of the present system. AERMOD assumes that the concentration at a receptor is a weighted combination of two concentration estimates: a purely horizontal plume, and a plume that is vertically displaced by the terrain. This simplified approach (relative to CTDMPLUS/CTSCREEN) allows an automated and objective scheme for characterizing terrain (see section on terrain preprocessor). It is important to note that with the AERMOD approach, the regulatory need for a definition of complex terrain (eg. terrain above stack top) becomes moot.

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