5 The AMS/EPA Regulatory Model AERMOD

AERMOD is a steady-state plume model in that it assumes that concentrations at all

distances during a modeled hour are governed by the temporally averaged meteorology of the

hour. The steady state assumption yields useful results since the statistics of the concentration

distribution are of primary concern rather than specific concentrations at particular times and

locations. AERMOD has been designed to handle the computation of pollutant impacts in both

flat and complex terrain within the same modeling framework. In fact, with the AERMOD

structure, there is no need for the specification of terrain type (flat, simple, or complex) relative

to stack height since receptors at all elevations are handled with the same general methodology.

To define the form of the AERMOD concentration equations, it is necessary to simultaneously

discuss the handling of terrain.

In the stable boundary layer (SBL), the concentration distribution is assumed to be Gaussian

in both the vertical and horizontal. In the convective boundary layer (CBL), the horizontal

distribution is assumed to be Gaussian, but the vertical distribution is described with a bi-

Gaussian probability density function (pdf). This behavior of the concentration distributions in

the CBL was demonstrated by Willis and Deardorff (1981) and Briggs (1993). Additionally, in

the CBL, AERMOD treats “plume lofting,” whereby a portion of plume mass, released from a

buoyant source, rises to and remains near the top of the boundary layer before becoming

vertically mixed throughout the CBL. The model also tracks any plume mass that penetrates into

an elevated stable layer, and then allows it to re-enter the boundary layer when and if






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