Aqueous layer pollutants deposition

Wet atmospheric corrosion results from repeated wet and dry cycles, the presence of pollutants, and the formation of an aqueous layer in which the atmospheric pollutants dissolve. The wet cycles result from dew, fog, rain, or snow. In many cases the dew, fog, rain, or snow may already contain the dissolved corrodent, which then deposits on the surface. 

Atmospheric pollutants can be deposited into the aqueous layer by either wet or dry deposition. For wet deposition to take place it is necessary for rain, fog, dew, or snow to be present, whereas for dry deposition precipitation of any kind is not involved. Dry deposition is considered to be predominant indoors or in highly polluted areas close to emission sources. It is difficult to determine the relative importance of wet deposition because of the incidental nature of the precipitation. 

The incorporation of atmospheric species into the aqueous layer may occur through either dry or wet deposition. In dry deposition there is no involvement of any precipitation, whereas wet deposition requires, e.g., rain, dew, fog, or snow for atmospheric pollutants to deposit. Indoors or in highly polluted areas close to emission sources, dry deposition is considered to be dominating but the relative importance of wet deposition may be difficult to establish because of the incidental nature of precipitation. Controlled field studies combined with extensive laboratory exposures have been undertaken within the National Acid Precipitation Assessment Program (NAPAP) to explore the relative contribution of wet and dry deposition to increased corrosion rates of a number of metals [45]. 

Contaminant volatilization from subsurface solid and aqueous phases may lead, on the one hand, to pollution of the atmosphere and, on the other hand, to contamination (by vapor transport) of the vadose zone and groundwater. Potential volatihty of a contaminant is related to its inherent vapor pressure, but actual vaporization rates depend on the environmental conditions and other factors that control behavior of chemicals at the solid-gas-water interface. For surface deposits, the actual rate of loss, or the pro-portionahty constant relating vapor pressure to volatilization rates, depends on external conditions (such as turbulence, surface roughness, and wind speed) that affect movement away from the evaporating surface. Close to the evaporating surface, there is relatively little movement of air and the vaporized substance is transported from the surface through the stagnant air layer only by molecular diffusion. The rate of contaminant volatilization from the subsurface is a function of the equilibrium distribution between the gas, water, and solid phases, as related to vapor pressure solubility and adsorption, as well as of the rate of contaminant movement to the soil surface. 

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