AERMOD Tech Guide

Gaussian Plume Air Dispersion Model

2. Model Overview

This section provides a general overview of the most important features of AERMOD. With the exception of applications involving wet and dry deposition, AERMOD serves as a replacement for ISC3. Thus, it 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), 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 (p.d.f.). 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 elevated stable layer, and then allows it to re-enter the boundary layer when and if appropriate.

AERMOD incorporates, with a new simple approach, 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 is required by present regulatory models. As a result, AERMOD removes the need for defining complex terrain regimes; all terrain is handled in a consistent, and continuous manner that is simple while still considering the dividing streamline concept (Snyder, et al., 1985) in stably-stratified conditions.

One of the major improvements that AERMOD brings to applied dispersion modeling is Its ability to characterize the PBL through both surface and mixed layer scaling. AERMOD constructs vertical profiles of required meteorological variables based on measurements and extrapolations of those measurements using similarity (scaling) relationships. Vertical profiles of wind speed, wind direction, turbulence, temperature, and temperature gradient are estimated using all available meteorological observations. AERMOD was designed to run with a minimum of observed meteorological parameters. As a replacement for the ISC3 model AERMOD can operate using data of a type that is readily available from an NWS station. AERMOD requires only a single surface (generally, 10m) measurement of wind speed (reference wind speed (between 7 z0 and 100m)), direction and ambient temperature (reference temperature). Like o ISC3, AERMOD also needs observed cloud cover. However, AERMOD also requires the full morning upper air sounding (RAWINSONDE). ISC3 required only the morning and afternoon mixing heights derived form that sounding. In addition, AERMOD needs surface characteristics (surface roughness, Bowen ratio, and albedo) in order to construct its PBL profiles.

Recommended minimum meteorological data requirements for AERMOD will be published in a future revision to the Guideline on Air Quality Models.

Unlike existing regulatory models, AERMOD accounts for the vertical inhomogeneity of the PBL. This is accomplished by “averaging “ the parameters of the actual PBL into “effective” parameters of an equivalent homogenous PBL.

Data Flow in the AERMOD Modeling System
Figure 1: Data Flow in the AERMOD Modeling System

Figure 1 shows the flow and processing of information in AERMOD. The modeling system consists of one main program (AERMOD) and two pre-processors (AERMET and AERMAP). The major purpose of AERMET is to calculate boundary layer parameters for use by AERMOD. The meteorological INTERFACE, internal to AERMOD, uses these parameters to generate profiles of the needed meteorological variables. In addition, AERMET passes all meteorological observations to AERMOD.

Surface characteristics in the form of albedo, surface roughness and Bowen ratio, plus standard meteorological observations (wind speed, wind direction, temperature, and cloud cover), are input to AERMET. AERMET then calculates the PBL parameters: friction velocity (u* ), Monin-Obukhov length (L), convective velocity scale (w* ), temperature scale (* ), mixing height (z i), and surface heat flux (H) These parameters are then passed to the INTERFACE (which is within AERMOD) where similarity expressions (in conjunction with measurements) are used to calculate vertical profiles of wind speed (u), lateral and vertical turbulent fluctuations (v , w ), potential temperature gradient (d/dz), potential temperature ( ), and the horizontal Lagrangian time scale (TLy ).

The AERMIC terrain pre-processor AERMAP uses gridded terrain data to calculate a representative terrain-influence height (hc ), also referred to as the terrain height scale. The terrain c height scale h , which is uniquely defined for each receptor location, is used to calculate the c dividing streamline height. The gridded data needed by AERMAP is selected from Digital Elevation Mapping (DEM) data. AERMAP is also used to create receptor grids. The elevation for each specified receptor is automatically assigned through AERMAP. For each receptor, AERMAP passes the following information to AERMOD: the receptor’s location (xr , yr), its height above mean sea level (zr ), and the receptor specific terrain height scale (hc ).

A comprehensive description of the basic formulation of the AERMOD dispersion model including the INTERFACE, AERMET, and AERMAP is presented in this document. Included are: 1) a complete description of the AERMET algorithms that provide quantitative hourly PBL parameters; 2) the general form of the concentration equation with adjustments for terrain; 3) plume rise and dispersion algorithms appropriate for both the convective and stable boundary layers; 4) handling of boundary layer inhomogeneity; 5) algorithms for developing vertical profiles of the necessary meteorological parameters; and 6) a treatment of the nighttime urban boundary layer. The model described here represents the version of AERMOD that has been submitted to OAQPS for regulatory considerations. In addition, all of the symbols used for the many parameters and variables that are referred to in this document are defined, with their appropriate units, in the section titled “List of Symbols.”