Air Modeling Group
Air quality dispersion modeling is a tool used to predict the pollutant concentrations in the ambient atmosphere resulting from the release of air pollutants. Models incorporate the influences of physical processes through mathematical equations derived from fundamental scientific principles and are useful to study the consequences of new or modified sources of air pollution. The equations account for the movement of pollutants by wind and dispersion of pollutants by mixing with ambient air. Pre-construction permitting under the Clean Air Act often requires the use of models to estimate the air quality impact from proposed new and modified sources.
Most dispersion models have been based on the Gaussian (or normal) distribution where the concentration profile through the plume has the shape of the normal bell curve in both the vertical and horizontal direction. Newer models use a non-Gaussian distribution in the vertical direction. The concentration is highest at the plume centerline and decreases with distance from the centerline.
Air dispersion models require input data in four main categories: meteorological conditions, source/emission parameters, landuse/terrain information and receptor information. Specific requirements vary by source type and the model used. Input data can include:
Meteorological data -
Meteorological data is used by the model to simulate transport and dispersion and include wind speed, wind direction, ambient temperature, mixing height, and atmospheric stability. Data are recorded hourly and are typically obtained from National Weather Service (NWS) stations or from on-site collection stations. Five years of NWS data or one year of on-site data are typically used as input to a model.
Source/emission parameters -
Source/emission parameters define how the emissions are released into the atmosphere and include stack gas temperature, stack gas velocity, height and diameter of state, and emission rates of pollutants.
Land use/terrain information -
Land use and terrain elevations are necessary to help predict ambient concentrations of pollutants. Land use surrounding a source of emissions influences the atmospheric turbulence while surrounding elevated terrain heights can increase pollutant concentrations.
Receptors are locations where the model computes concentration estimates. Receptors are typically located using a grid system with enough locations to identify maximum impacts. Also, locations of particular interest are often included as receptors.
Applicants for major modifications or for major sources of criteria air pollutants in an attainment or unclassified area must perform air quality dispersion modeling under the Prevention of Significant Deterioration (PSD) program. Also, under certain circumstances, minor sources of air pollutants may be required to model. Sources use the air dispersion model to predict the ground level impact, in terms of concentration of a particular pollutant, for comparison to a standard.
As part of the Clean Air Act, Federal Land Managers have an affirmative responsibility to protect the natural and cultural resources of Class I areas from the adverse impacts of air pollution. Federal Class I areas are defined in the Clean Air Act as national parks over 6,000 acres and wilderness areas and memorial parks over 5,000 acres, established as of 1977. All other federally managed areas are designed as Class II. The Federal Land Managers' Quality Related Values Work Group (FLAG) was formed to develop a more consistent approach for the Federal Land Managers (FLM's) to evaluate air pollution effects on their resources. Of particular importance is the New Source Review (NSR) program, especially in the review of PSD air quality permit applications. The goals of FLAG are to provide consistent policies and procedures both for identifying air quality related values (AQRV's) and for evaluating the effects of air pollution on AQRVs, primarily those in Federal Class I areas, but in some instances, in Class II areas. The AQRVs that are studied for PSD permits are: visibility, regional and direct plume impaction, and deposition. Air quality modeling is used to predict impacts of a proposed source for comparison to National Ambient Air Quality Standards (NAAQS), PSD standards, and AQRV target values.
The Class I areas in West Virginia are Otter Creek and Dolly Sods: