Some models handle various factors that affect the plume. These options include stack-tip downwash, buoyancy-induced dispersion, gradual plume rise, and building or topography downwash factors as described in previous sections. Other factors include pollutant decay (half-life) and deposition. Pollutant decay considers chemical changes or scavenging of the pollutant in the atmosphere, typically through the use of a half-life duration factor. Deposition accounts for the settling of the pollutant from the plume. This deposition can either occur through dry (gravitational) settling or wet (rainwash) removal. Dry deposition modeling usually requires particle size and settling velocities for the pollutant emitted; wet deposition also requires meteorological data about precipitation frequency and characteristics.
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