Components of an Air Quality Analysis
Generating the source inventory for modeling is intertwined with the creation of the pollutant inventory. Each emission source and the type of pollutants it emits must be specifically identified. For dispersion modeling, each source must be classified as a point source (stack, vent), area source (landfill, lagoon), volume source (tank farm), or line source (paved, unpaved road).
Meteorological conditions govern the transport and dispersion of contaminants and, in the case of some fugitive sources such as lagoons or landfills, can affect the amount of contaminant that becomes airborne. It is important, therefore, to use meteorological data that are representative of the site area and vicinity. A minimum of one year of on-site data or five years of off-site (NWS) data is required to run refined dispersion models.
Wind Speed and Direction
In dispersion modeling, wind speed is used in determining: (1) plume rise; (2) plume dilution; and (3) mass transfer rate into the atmosphere (used mostly in fugitive dust and evaporation rate models). Wind direction is used to approximate the direction of transport of the plume. Most wind data are collected near ground level (the standard height for wind measurement is 10 meters [m]).
Dispersion models currently use stability categories as indicators of atmospheric turbulence. Based on the work of Pasquill and Gifford, six stability categories have been defined: Category A representing extremely unstable conditions thru Category F representing moderately stable conditions. The amount of turbulence in the atmosphere has a major impact on the rise of stack gas plumes, and upon subsequent plume dispersion by diffusion.
Ambient Temperature, Relative Humidity, and Pressure
Ambient temperature is routinely used in dispersion models to calculate the amount of rise of a buoyant plume and to calculate evaporation rates. Relative humidity affects the amount of energy available in the atmosphere for plume mixing within the atmosphere. Atmospheric pressure data are used in calculating gas and liquid release rates from storage and process vessels, and from pipes.
The mixing height defines the depth through which pollutants released to the atmosphere are mixed by dispersive processes. The mixing height determines the vertical extent of dispersion for releases occurring below that height. Releases occurring above that height are assumed to have no ground-level impact.
For point sources, high impacts can be predicted due to plume impaction on terrain at elevations greater than or equal to plume centerline. Incorporating the effect of elevated terrain in the vicinity of the site may, therefore, be significant for point emission sources. Incorporating terrain is generally not a consideration when modeling fugitive releases (i.e. non-stack releases such as coal piles) because these releases are typically neutrally buoyant, with no plume rise to consider. Maximum impacts are thus expected to occur at the nearest downwind location.
In dispersion modeling, receptors are defined as the locations where impacts are predicted. Various types of receptor grids can be used by defining points on a polar coordinate system, a Cartesian coordinate system, or a combination of both systems. Development of the receptor grid is facilitated by some refined models through an option to automatically generate a grid based on some user specifications, such as desired interval spacing.
For the purpose of dispersion modeling, sites are classified as being in a predominantly "urban" or "rural" area. This determination is typically based on the land use in the area surrounding the site to be modeled. The general effect of an urban area is to create enough additional turbulence, due to the buildings and urban "heat island" effects, which enhance plume dispersion. Sources located in an area classified as urban should be modeled using urban dispersion coefficients, while sources located in an area classified as rural should be modeled using rural dispersion coefficients.
Air quality modeling of point sources with stack heights less than a certain height defined by good engineering practice (GEP) should consider the impacts associated with building wake effects. These effects cause the pollutant plume to fall to ground-level quicker. Incorporating building downwash for stacks with heights less than GEP will, thus, increase model-predicted concentrations. Building wake effects are not considered for area or volume sources.
Averaging Time Considerations
Depending on the time periods of the applicable regulations or action levels, several averaging periods may be of interest for any given analysis. These include instantaneous, 15-minute, 1-hour, 24-hour, monthly, and annual averages.