4.8 WET DEPOSITION


        The detailed calculation of the scavenging of contaminants from individual plumes requires a complex model with a number of inputs that are difficult to define. The MEPAS calculation of climatological scavenging of contaminants is accomplished using a simpler approach (Slinn 1976). The climatological calculation used in MEPAS provides estimates of wet deposition rates. This computation accounts for the major factors changing the wet deposition for the various combinations of releases and receptors between sites.

        The wet deposition involves integration of the scavenged material over height. Hanna et al. (1982) expresses the integrated wet flux (Fwet) for rain falling completely through a Gaussian plume as

(64)


 
where
Equation 64, converted to a sector-averaged form for the total deposition, is expressed as

(65)


          This relationship for Wisk(x,z) is input for Equation 58. The contaminant removal term, Rk, is determined from Equation 51. The scavenging coefficient for a specified volume of a plume is defined as the airborne contaminant removal by precipitation scavenging. Hanna et al. (1982) point out that the scavenging coefficient varies with the rainfall type and rate, saturation conditions, and contaminant characteristics. The MEPAS implementation of this model assumes a neutral stability for all precipitation conditions. The wet deposition plume depletion term (w in Equation 51) is obtained using

(66)


        Hanna et al. (1982) points out that this method applies to monodisperse particles or to highly reactive gases that are irreversibly scavenged. As such this method is limited to providing upper-limit estimates that maximize the near-source wet-removal rates.