ISCST3 Tech Guide

Gaussian Plume Air Dispersion Model

6.4 The ISC Short-term Wet Deposition Model

A scavenging ratio approach is used to model the deposition of gases and particles through wet removal. In this approach, the flux of material to the surface through wet deposition (Fw) is the product of a scavenging ratio times the concentration, integrated in the vertical:


where the scavenging ratio (7) has units of s-1. The concentration value is calculated using Equation (1-1). Since the precipitation is assumed to initiate above the plume height, a wet deposition flux is calculated even if the plume height exceeds the mixing height. Across the plume, the total flux to the surface must equal the mass lost from the plume so that


Solving this equation for Q(x), the source depletion relationship is obtained as follows:


where t = x/u is the plume travel time in seconds. As with dry deposition (Section 1.3), the ratio Q(x)/Qo is computed as a wet depletion factor, which is applied to the flux term in Equation (1-89). The wet depletion calculation is also optional. Not considering the effects of wet depletion will result in conservative estimates of both concentration and deposition, since material deposited on the surface is not removed from the plume.

The scavenging ratio is computed from a scavenging coefficient and a precipitation rate (Scire et al., 1990):


where the coefficient 8 has units (s-mm/hr)-1, and the precipitation rate R has units (mm/hr). The scavenging coefficient depends on the characteristics of the pollutant (e.g., solubility and reactivity for gases, size distribution for particles) as well as the nature of the precipitation (e.g., liquid or frozen). Jindal and Heinold (1991) have analyzed particle scavenging data reported by Radke et al. (1980), and found that the linear relationship of Equation (1-90) provides a better fit to the data than the non-linear assumption 7 = 8Rb. Furthermore, they report best-fit values for 8 as a function of particle size. These values of the scavenging rate coefficient are displayed in Figure 1-11. Although the largest particle size included in the study is 10 µm, the authors suggest that 8 should reach a plateau beyond 10 µm, as shown in Figure 1-11. The scavenging rate coefficients for frozen precipitation are expected to be reduced to about 1/3 of the values in Figure 1-11 based on data for sulfate and nitrate (Scire et al., 1990). The scavenging rate coefficients are input to the model by the user.

The wet deposition algorithm requires precipitation type (liquid or solid) and precipitation rate, which is prepared for input to the model through the meteorological preprocessor programs (PCRAMMET or MPRM).