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.

In addition to the mass mean diameters (microns), particle densities (gm/cm3), and the mass fractions for each particle size category being modeled, the dry deposition model also requires surface roughness length (cm), friction velocity (m/s), and Monin-Obukhov length (m).  The surface roughness length is specified by the user, and the meteorological preprocessor (PCRAMMET or MPRM) calculates the friction velocity and Monin-Obukhov length for input to the model.

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