Dry deposition is the portion of the total deposition that is deposited in dry weather through such processes as settling, impaction, and adsorption. Dry deposition is usually quantified in terms of the flux F, which is a function of the dry deposition velocity (i^) and concentration of a chemical in the air (C):
Resistance models for dry deposition are used to calculate the dry deposition velocity based on the resistance to transport for each step in the process:
where ra is the resistance to aerodynamic transport, rb is the resistance to the transport through the boundary sublayer, and rc is resistance to collection by the surface.
Particle deposition depends largely on atmospheric, surface, and particle characteristics. The most important atmospheric parameters include wind speed, humidity, stability, and temperature. There are also micrometeoro-logical parameters that have an impact on dry deposition of particles which are related to the surface, such as friction velocity, roughness height, and zero-plane displacement. The most important surface properties that have an impact on the dry deposition process include chemical and biological reactivity, geometry of roughness elements, terrain characteristics, and wetness. Finally, the size, shape, density, reactivity, hygroscopicity, and solubility of particles also have a very important effect on their dry deposition.
The above-mentioned parameters are brought together with the information on deposition mechanisms when developing models for dry deposition velocities for various size particles onto different surfaces. Three major groups of models are developed for dry deposition of (1) fine particles to vegetative canopies, (2) fine particles to water surfaces, and (3) coarse particles.
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