Dispersion models: Toolbox basics

• By R.W. McManus, PE
• Jun 01, 1999

Air dispersion models are used to estimate the downwind concentration of pollutants emitted by various pollution sources such as industrial facilities and regional public traffic. Dispersion models play an important role in the industrial and regulatory communities. They are typically used to demonstrate compliance with National Ambient Air Quality Standards as part of new source review, prevention of significant deterioration and non-attainment permitting efforts. Other uses include Superfund Amendments and Reauthorization Act (SARA) Title II planning, evaluations of the effects of process changes or control technologies, siting analyses and as input to other scientific studies or models.

To conduct a dispersion modeling analysis, a user enters data in the following four major categories:

• Meteorological conditions, such as wind speed, wind direction, stability class, temperature and mixing height;
• Emissions parameters, such as source location, source height, stack diameter, gas exit velocity, gas exit temperature and emission rate;
• Terrain elevations; and
• Building parameters, such as location, height and width.

Specific requirements vary by model and source type, and the more refined models, such as the U.S. Environmental Protection Agency's (EPA) Industrial Source Complex-Short Term, Version 3 (ISCST3), typically require more inputs than simpler screening models, such as EPA's SCREEN3. Typical model output includes highest, second highest and average period concentrations at each modeled receptor. Many commercially available Windows versions of the models also include enhanced 2-D and 3-D graphical output of concentration isopleths (see Figure 1).

Perhaps the most widely used model for estimating near-field concentrations of non-reactive pollutants is ISCST3. It is a steady-state Gaussian plume model, meaning that dispersion parameters such as meteorological conditions and emission rate are assumed constant while the plume is en route from source to receptor. It is generally used to analyze continuous emissions and can account for multiple sources and receptors, varied land use, averaging times from one hour to one year, flat or elevated terrain, emission rates that vary with time and building downwash effects.

The model can also account for dry and wet depletion, and can estimate pollutant deposition. These features make ISCST3 a very versatile model, one that is popular both in the United States and abroad. It has been the workhorse model for many regulatory modeling applications since the 1970s, and has been revised numerous times, most recently in December 1998.

The American Meteorological Society/U.S. Environmental Protection Agency Regulatory Model (AERMOD) is a refined model currently under review by the EPA as a possible supplement or replacement to ISCST3 for regulatory purposes. AERMOD, also a steady-state plume model, improves estimates of dispersion in the planetary boundary layer by accounting for varying dispersion rates with height, refined turbulence based on current planetary boundary layer (PBL) theory and advanced treatment of mixing height, plume rise and complex terrain. AERMOD input and output, however, remain very similar to ISCST3.

For certain scenarios, environmental professionals may prefer to conduct a preliminary screening analysis using a conservative technique such as EPA's SCREEN3 model to determine if a more refined analysis is required. Although a simple screening procedure for use with a single stationary source (or a group of sources that are merged to simulate one source), SCREEN3 can account for elevated terrain, building downwash and various wind speed and stability class combinations.

In addition to the models and uses mentioned here, there are a variety of specialized applications available for accidental release planning, roadway modeling, offshore sources and regional transport modeling, among others. Dispersion modeling is a powerful tool that enables environmental professionals to examine virtually every aspect of an air pollution emission source quickly and cost-effectively.

IMAGE COURTESY OF BREEZE GRAPHICS®, TRINITY CONSULTANTS

This article originally appeared in the 06/01/1999 issue of Environmental Protection.