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1.3 THE ISC SHORT‑TERM DRY DEPOSITION MODEL

1.3.1 General

This section describes the ISC Short Term dry deposition model, which is used to calculate the amount of material deposited (i.e., the deposition flux, Fd) at the surface from a particle plume through dry deposition processes.

The Short Term dry deposition model is based on a dry deposition algorithm (Pleim et al., 1984) contained in the Acid Deposition and Oxidant Model (ADOM).  This algorithm was selected as a result of an independent model evaluation study (EPA, 1994).

The deposition flux, Fd, is calculated as the product of the concentration, χd, and a deposition velocity, vd, computed at a reference height zd:

The concentration value, χd, used in Equation (1-79) is calculated according to Equation (1-1) with deposition effects accounted for in the vertical term as described in Section 1.1.6.3.  The calculation of deposition velocities is described below.

1.3.2 Deposition Velocities

A resistance method is used to calculate the deposition velocity, vd.  The general approach used in the resistance methods for estimating vd is to include explicit parameterizations of the effects of Brownian motion, inertial impaction, and gravitational settling.  The deposition velocity is written as the inverse of a sum of resistances to pollutant transfer through various layers, plus gravitational settling terms (Slinn and Slinn, 1980; Pleim et al., 1984):

where,    vd   =the deposition velocity (cm/s),

vg   =the gravitational settling velocity (cm/s),

ra   =the aerodynamic resistance (s/cm), and,

rd   =the deposition layer resistance (s/cm).

Note that for large settling velocities, the deposition velocity approaches the settling velocity (vd ® vg), whereas, for small settling velocities, vd tends to be dominated by the ra and rd resistance terms.

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