4.3 DISPERSION COEFFICIENTS

        The MEPAS atmospheric pathway component uses six classes of atmospheric stability to characterize the dispersion rates. The atmospheric stability classes are designated by the letters A to F (Slade 1968) and are commonly referred to as the Pasquill Stability Categories (Pasquill and Smith 1983). The classes A, B, and C stand for very unstable, unstable, and slightly unstable conditions, respectively; D stands for a neutral condition; and E and F stand for stable and very stable conditions, respectively. Dispersion varies from being fastest for very unstable conditions to being slowest for very stable conditions.

        The Pasquill dispersion curves used in the atmospheric component of MEPAS are computed as a function of elapsed plume travel time. The conversion from the distance dependence to the time dependence is based on equivalent wind speeds. The Pasquill curves are applied as a function of time for the conditions for which the curves were originally developed. Following Hasse and Weber (1985), the Pasquill dispersion curves are assumed to apply over rural English countryside (z_o = 10 cm). Equations 51 and 52 are used to compute wind speeds. The plume travel time is computed as the sum of travel times over various surfaces, thus allowing for local wind shear effects in the dispersion computation.

        The MEPAS formulation that accounts for local roughness influences is particularly important in cases where the wind data are from a location with a quite different surface roughness than the site. The MEPAS formulation also accounts for changes in surface roughness in the region.

        In terms of the computed dilution rate in the air, 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 and the wind speed profile. At a given distance from the source, a site with a smoother surface will have smaller dispersion rates and larger near-surface wind speeds than a site with a rougher surface. The equation for computing sector-average air concentrations (Equation 47) contains the product of the vertical dispersion rate and the wind speed. All other factors being equal, these two local surface influences almost exactly cancel each other. The result is that the computed dilution at a potential receptor location does not vary with the surface roughness.

        Even though the dilution does not vary with surface roughness, the computed concentrations and deposition rates will vary with surface roughness. The roughness-induced changes in dispersion rate and wind speed directly change the estimated deposition rates. Deposition rates indirectly change computed air concentrations by the plume depletion rate.