AERMOD Tech Guide

Gaussian Plume Air Dispersion Model

1.2 The AERMIC Focus: A Replacement for the ISC Model

AERMIC’s initial focus has been on the regulatory models that are designed for estimating near-field impacts from a variety of industrial source types. EPA’s present regulatory platform for near-field modeling has, with few exceptions, remained fundamentally unchanged since the beginning of the air programs, some 25 years ago. ISC3 is the workhorse of current regulatory tools with code structure that is conducive to change. Therefore, AERMIC selected the EPA’s ISC3 Model for a major overhaul. AERMIC’s objective is to develop a complete replacement for ISC3 by: 1) adopting ISC3's input/output computer architecture; 2) updating, where practical, antiquated ISC3 model algorithms with newly developed or current state-of-the-art modeling.8 techniques; and 3) insuring that the source and atmospheric processes presently modeled by ISC3 will continue to be handled by the AERMIC Model (AERMOD), albeit in an improved manner. Although the current model described here does not fully satisfy this third objective (the wet and dry deposition algorithms in AERMOD are not complete), it is AERMIC’s intent to continue the development of AERMOD, beyond this work, until all objectives are satisfied.

The AERMOD modeling system consists of two pre-processors and the dispersion model. The AERMIC meteorological preprocessor (AERMET) provides AERMOD with the meteorological information it needs to characterize the PBL. The AERMIC terrain pre-processor (AERMAP) both characterizes the terrain and generates receptor grids and elevations for the dispersion model (referred to simply as AERMOD).

AERMET uses meteorological data and surface characteristics to calculate boundary layer parameters (e.g. mixing height, friction velocity, etc.) needed by AERMOD. This data, whether measured off-site or on-site, must be representative of the meteorology in the modeling domain. AERMAP uses gridded terrain data for the modeling area to calculate a representative terrain-influence height associated with each receptor location. At the present time, the gridded data must be supplied to AERMAP in the format of the Digital Elevation Mapping (DEM) data (USGS, 1994). The terrain preprocessor can also be used to compute elevations for both discrete receptors and receptor grids.

In developing AERMOD, AERMIC adopted design criteria to yield a model with desirable regulatory attributes. We felt that the model should: 1) provide reasonable concentration estimates under a wide variety of conditions with minimal discontinuities; 2) be user friendly and require reasonable input data and computer resources as is the current ISC3 model; 3) capture the essential physical processes while remaining fundamentally simple; and, 4) accommodate modifications with ease as the science evolves.

Relative to ISC3, AERMOD currently contains new or improved algorithms for: 1) dispersion in both the convective and stable boundary layers; 2) plume rise and buoyancy; 3) plume penetration into elevated inversions; 4) computation of vertical profiles of wind, turbulence, and temperature; 5) the urban boundary layer; and 6) the treatment of receptors on all types of terrain from the surface up to and above the plume height. AERMET contains an improved approach to characterizing the fundamental boundary layer parameters. High priority items for future efforts include new or improved algorithms dealing with building downwash and both wet and dry deposition.