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

 

where the coefficient λ 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 Λ = λRb.  Furthermore, they report best-fit values for λ 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 λ 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).

 

 

 

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