Formulation Of Deposition Velocity

A common simple method of evaluating Eq. (1) is by analogy to Ohm's Law, where F corresponds to current, Vd corresponds to the inverse of the total resistance, and C corresponds to the voltage, referenced to electrical ground corresponding to a concentration of zero that is assumed to occur somewhere in the surface. From this analogy, the deposition velocity can be expressed in terms of three resistances in series.

Here, Ra represents the aerodynamic resistance to transfer associated with turbulent mixing above the surface, Rh is the resistance of the quasilaminar sublayer of air in contact with surface elements, and Rc is the bulk resistance of the surface. The values of Ra and Rh can be estimated with readily available micrometeorological formulations. Various schemes exist in the literature for depicting and evaluating

particular surface as environmental conditions change. When leaf stomatal openings close at night, for example, the bulk resistance to deposition on the outer surfaces of leaves and the ground below vegetative canopies can be inferred. The resistance of the waxy cuticle is sometimes measured in the laboratory, and the resistance of the ground surface beneath the canopy is occasionally evaluated with flux measurements there. In the parameterizations that are generated, important variables include environmental factors (such as solar radiation, temperature, air humidity, wetness of the surface caused by dew and rainfall, and soil moisture content) and details of the surface (such as height of vegetative canopy, amount of leaf area, species of vegetation, and soil pH). For bodies of water, the structure and size of waves can be important.

For particle deposition, Rc is not commonly considered explicitly and the deposition velocity is expressed in terms of Ra, Rb, and gravitation settling velocity Vg. However, Rb embodies several somewhat complex processes involving transport through the quasilaminar sublayer, interception of particles by fine elements of the surface, and inertial impaction of particles on the surface. Theoretical formulations for both Rb and Vg usually include a strong dependency on particle size.

Although Ra, Rb, and aerodynamic resistances in canopies are considered separately, they are all strongly affected by turbulence parameters. The turbulent mixing induced by buoyancy forces associated with surface heating by solar radiation can directly alter Ra and in-canopy resistances. The roughness of the surface, a primary factor in evaluating Ra, is linked indirectly to the vegetative properties that affect the resistances of elements of a vegetative canopy. Somewhat more confusing is the fact that in-canopy resistances are implied in Figure 1 to be controlled purely by turbulence, but the distribution of "sinks" in canopies can alter the value of the in-canopy aerodynamic resistance because the latter is actually a composite of vertically distributed air-phase resistances to many surface elements. For this and other reasons noted below, the approach that uses Eq. (2) and Figure 1 is considered by many researchers to be oversimplified.

Several other difficulties exist with the resistance analogy for dry deposition. Some experiments have shown, for example, that weak mixing at night in tall canopies might lead to storage of chemicals in air in the sheltered areas. Deep snowpack might store some relatively insoluble substances. Gusty winds can resuspend particles. Relatively inert gases might temporarily dissolve in surface materials and later be reemitted. Some trace gases are emitted from natural surfaces, sometimes at greater rates when the ambient concentrations are small. To overcome such difficulties, experimental observations of deposition velocities sometimes provide the primary information used to evaluate Eq. (1), or more sophisticated models of air-surface exchange are developed.

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