# ISCST3 Tech Guide

## 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 (F_{w}) 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)/Q_{o} 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 =
8R^{b}. 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).