technical formulation of AERMOD and its preprocessors. This document is intended to provide
many of the details that are not included in the published journal articles (Cimorelli et al. 2004;
Perry et al. 2003).
This document does not include information related to model performance. As mentioned
above, a description of the performance of the model that is described in this document can be
found in Perry et al. (2003) and Brode (2002).
This section provides a general overview of the most important features of AERMOD.
With the exception of treating pollutant deposition, AERMOD serves as a complete replacement
for ISC3. However, it is the intention of AERMIC to incorporate both dry and wet particle and
gaseous deposition as well as source or plume depletion. Once this is accomplished this report
will be revised to include a description of the deposition formulation. Thus, the AERMOD
model described here is applicable to rural and urban areas, flat and complex terrain, surface and
elevated releases, and multiple sources (including, point, area and volume sources). Every effort
has been made to avoid model formulation discontinuities wherein large changes in calculated
concentrations result from small changes in input parameters.
AERMOD is a steady-state plume model. In the stable boundary layer (SBL), it assumes
the concentration distribution to be Gaussian in both the vertical and horizontal. In the
convective boundary layer (CBL), the horizontal distribution is also 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 mixed into the CBL. AERMOD also tracks any plume mass
that penetrates into the elevated stable layer, and then allows it to re-enter the boundary layer
when and if appropriate. For sources in both the CBL and the SBL AERMOD treats the
enhancement of lateral dispersion resulting from plume meander.
Using a relatively simple approach, AERMOD incorporates current concepts about flow
and dispersion in complex terrain. Where appropriate the plume is modeled as either impacting
and/or following the terrain. This approach has been designed to be physically realistic and
simple to implement while avoiding the need to distinguish among simple, intermediate and
complex terrain, as required by other regulatory models. As a result, AERMOD removes the
need for defining complex terrain regimes. All terrain is handled in a consistent and continuous
manner while considering the dividing streamline concept (Snyder et al. 1985) in stablystratified
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