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
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
use a non-Gaussian distribution in the vertical direction. The concentration is highest at the plume centerline
decreases with distance from the centerline.
Air dispersion models require input data in four main categories: meteorological conditions, source/emission
parameters, land use/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 - 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: