Air Quality Modeling: Solutions Through Simulation

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Air quality monitoring and regulation is critically important, especially to potential sources of air pollutants such as the chemical, mineral and energy industries. Manufacturing these basic building blocks of our modern society requires the careful monitoring and control of air pollution. Maintaining government emission standards can be challenging, and the first step is accurately measuring pollutants as they disperse through the air. To accomplish this, we need a model, and GeoEngineers usually uses AERMOD, a standardized system for analyzing air quality.

Air dispersion modeling is simply the numerical simulation of how a pollutant disperses, and in some cases reacts, following release in the atmosphere from an emission source. The models use source pollutant emission rates, meteorological data, and terrain data to make an estimate of pollutant concentrations at downwind receptor locations. The results of the modeling analyses are used for evaluating the maximum expected impact from new sources permitted under government regulations, establishing stack emission limits, developing control strategies to move areas into compliance with the National Ambient Air Quality Standards (NAAQS), and determining impact areas for risk management plans.

Most modeling studies are concerned with the maximum impact near a specific source. The vast majority of these types of analyses will be completed with a plume model like the AERMOD modeling system. The AERMOD system was developed by AERMIC, a collaboration of the American Meteorological Society (AMS) and the US Environmental Protection Agency (EPA). The model includes the treatment of both surface and elevated sources and can be used to model both simple and complex terrain.

In addition to the dispersion model itself, referred to simply as AERMOD, the modeling system includes two pre-processors for meteorological and terrain data. The meteorological pre-processor processes hourly surface and upper air meteorological data to produce wind profile and surface data files that characterize the PBL for the dispersion model. A terrain pre-processor processes digital elevation data to calculate elevations as well as a representative terrain-influence height associated with each receptor location.  AERMOD is the EPA’s preferred dispersion model for estimating air impacts in close proximity to stationary sources.

Despite the numerous improvements that have been made to the AERMOD modeling system over the years, it still provides a relatively simple approximation of a very complex system. As models become more complex they usually become more realistic, however, the increased complexity can also increase model uncertainty and computational resource requirements. The AERMOD modeling system includes a number of simplifying assumptions which may, or may not, significantly impact the quality of the model results for a given application. For example, AERMOD assumes a straight-line plume trajectory based on the one-hour average wind direction. Sub-hourly changes in wind direction and turbulence are accounted for in the model by increasing the spread of the plume from the mean trajectory. Additionally, it is assumed that the emissions are continuous and constant, the plume is assumed to be perfectly reflected when it comes into contact with the ground, the wind speed is assumed constant over the hour, and the pollutant concentrations are assumed to follow a Gaussian distribution.

While the simplifying assumptions utilized by the AERMOD modeling system may be appropriate for most situations, it is important to validate the model using actual measured concentration data. The results of the validation analysis will provide additional confidence in the model results and make it very difficult for the study to be discredited by outside parties.

For any potential sources of air pollutants, planning now to stay compliant with air quality regulations in the future can save money and reduce the risk of litigation. Using air modeling systems such as AERMOD to develop defensible and realistic modeling simulations can help in the development of control strategies and emission limits that are protective of air quality and cost effective. GeoEngineers personnel have performed hundreds of analyses using the AERMOD modeling system and have extensive experience with validating model results using actual measured ambient concentration data. We’d be happy to answer any questions you might have about air quality regulations and modeling strategies.

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