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1.1.6.3  The Vertical Term With Dry Deposition.

Particulates are brought to the surface through the combined processes of turbulent diffusion and gravitational settling.  Once near the surface, they may be removed from the atmosphere and deposited on the surface.  This removal is modeled in terms of a deposition velocity (vd), which is described in Section 1.3.1, by assuming that the deposition flux of material to the surface is equal to the product vdχd, where χd is the airborne concentration just above the surface.  As the plume of airborne particulates is transported downwind, such deposition near the surface reduces the concentration of particulates in the plume, and thereby alters the vertical distribution of the remaining particulates.  Furthermore, the larger particles will also move steadily nearer the surface at a rate equal to their gravitational settling velocity (vg).  As a result, the plume centerline height is reduced, and the vertical concentration distribution is no longer Gaussian.

A corrected source-depletion model developed by Horst (1983) is used to obtain a "vertical term" that incorporates both the gravitational settling of the plume and the removal of plume mass at the surface.  These effects are incorporated as modifications to the Gaussian plume equation.  First, gravitational settling is assumed to result in a "tilted plume", so that the effective plume height (he) in Equation

(1-50) is replaced by

where hv = (x/us)vg is the adjustment of the plume height due to gravitational settling.  Then, a new vertical term (Vd) that includes the effects of dry deposition is defined as:

V(x,z,hed) is the vertical term in the absence of any deposition--it is just Equation (1-50), with the tilted plume approximation.   FQ(x) is the fraction of material that remains in the plume at the downwind distance x (i.e., the mass that has not yet been deposited on the surface).  This factor may be thought of as a source depletion factor, a ratio of the "current" mass emission rate to the original mass emission rate.  P(x,z) is a vertical profile adjustment factor, which modifies the reflected Gaussian distribution of Equation

(1-50), so that the effects of dry deposition on near-surface concentrations can be simulated.

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