The Brownian diffusivity of the pollutant (in cm/s) is computed from the following relationship:
where Ta is the air temperature (°K).
The first term of Eqn. (1-83), involving the Schmidt number, parameterizes the effects of Brownian motion. This term controls the deposition rate for small particles. The second term, involving the Stokes number, is a measure of the importance of inertial impaction, which tends to dominate for intermediate‑sized particles in the 2‑20 μm diameter size range.
The deposition algorithm also allows a small adjustment to the deposition rates to account for possible phoretic effects. Some examples of phoretic effects (Hicks, 1982) are:
THERMOPHORESIS: Particles close to a hot surface experience a force directed away from the surface because, on the average, the air molecules impacting on the side of the particle facing the surface are hotter and more energetic.
DIFFUSIOPHORESIS: Close to an evaporating surface, a particle is more likely to be impacted by water molecules on the side of the particle facing the surface. Since the water molecules have a lower molecular weight than the average air molecule, there is a net force toward the surface, which results in a small enhancement of the deposition velocity of the particle.
A second effect is that the impaction of new water vapor molecules at an evaporating surface displaces a certain volume of air. For example, 18 g of water vapor evaporating from 1 m2 will displace 22.4 liters of air at standard temperature and pressure (STP) conditions (Hicks, 1982). This effect is called Stefan flow. The Stefan flow effect tends to reduce deposition fluxes from an evaporating surface. Conversely, deposition fluxes to a surface experiencing condensation will be enhanced.
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