9.0 SHORT-TERM AIR CONCENTRATIONS

        In addition to the evaluation of potential longer-term chronic impacts, many situations require consideration of potential acute impacts over a shorter time period. The processes are the same as discussed for the long-term impacts; the released material is transported and dispersed by air movement. The difference is the concern for a release occurring over a relatively short time—resulting in a greater range of possible plume concentrations and impacts over a smaller area.

        MEPAS computes acute normalized dispersion values applicable to releases of approximately an one hour duration. The standard straight-line, Gaussian model expressed by Equation 45 is used to compute hourly concentrations for a matrix of wind speed, wind direction, and stability conditions. Both statistical and maximum value summaries of the short-term normalized concentrations are provided. For a near-ground-level release, the computed hourly concentrations combined with the frequencies of occurrence for each case (from the climatological joint frequency dispersion summaries) are used to define the 95% and 50% normalized air concentrations at 100 m from the source. For all types of atmospheric releases, the maximum hourly normalized air concentration and its location are provided for each direction. These summaries of short-term air concentrations normalized to a unit release are provided as supplemental information for evaluation of acute exposures.

        The roughness influences on dispersion rates and wind speed tend to cancel each other. As a result of surface-induced mechanical mixing, the local surface roughness influences local dispersion rates. All other factors being equal, a site with a smoother surface will appear to have smaller dilution rates than a site with a rougher surface. However the surface roughness will also change the near-surface wind speed; there will be faster near-surface winds over a smooth surface compared to a rough surface. The surface-roughness induced changes in the vertical and horizontal dilution rates tend to be greater than the opposing change in the near-surface wind speeds. The result is that, all other factors being equal, the estimated acute normalized air concentrations are larger over smooth surfaces than over rough surfaces.

        The computed hourly normalized dispersion values are estimates for potential ambient conditions that result in the maximum acute exposures. The Gaussian model applies best to a site located on a uniform flat plane. The model is used only as an approximation for sites located on other types of terrain.