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).

2 Model Overview

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

conditions.

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