Dry deposition refers to the removal of trace substances from the atmosphere without the aid of precipitation. Dry deposition cleanses the air and delivers substances to the Earth's surface. In air, the vertical transfer is accomplished primarily by turbulent mixing; for particles whose diameters are greater than 1 to 2 |im, vertical transfer is aided by gravitational settling. As air comes into contact with surface elements, gas molecules can react with surface materials or dissolve in them. Particles can be captured by interception or impaction with the surface elements. In comparison to wet deposition, in which substances are carried in precipitation to the surface, dry deposition is a slower, more continuous process and is profoundly affected by the physical, chemical, and biological properties of the surface.
The downward vertical mass flux density (deposition rate per unit area), divided by concentration C at a specified height, is conventionally known as the deposition velocity Vd. The value of this velocity can vary greatly depending on the properties of the substance of interest and local atmospheric and surface conditions. Nevertheless, the concept of a deposition velocity is highly useful in many applications because it produces the deposition rate when multiplied by a measured or modeled concentration. As a general guideline, a deposition velocity of 0.1 cm/s is small, 1.0 cm/s is moderately large, and several centimeters per second is near the limit that is physically possible on the basis of turbulent mixing alone. The corresponding residence times of the constituents in the lower atmosphere can be weeks, days, or hours, respectively, when they are controlled only by dry deposition. Relatively inert gases with very small deposition velocities can have lifetimes of several years in the atmosphere if other sources of removal, such as chemical transformations, are weak.
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Because dry deposition is a surface process, it can be treated as a boundary condition in models that compute atmospheric chemical budgets. In numerical models, the deposition rate is usually estimated on the basis of deposition velocity fairly close to the surface, typically at a height less than 50 m, with the assumption that the vertical flux below such small heights does not change substantially with height. This assumption is valid if the lower atmosphere has concentrations and mixing properties that are horizontally uniform and steady with time over the period of a few hours. Another requirement for nearly constant fluxes with height is that the substance of interest does not undergo chemical and physical changes that are rapid in comparison to the time scales of local vertical mixing. When these requirements are met, the vertical flux can be estimated as where we have adopted the convention that a flux directed downward is negative. This type of formulation is intended only for the case of a flux being directed downward, when Vd is positive; it has little merit for substances that are emitted from the surface because ambient concentration often has little effect on emission rates.
This chapter provides a brief, somewhat introductory, overview of dry deposition. The reader can find considerable additional information in the scientific literature. For example, a review of the state of the science after considerable research on acidic deposition during the 1980s in the United States was provided by Hicks et al. (1989), and measurement techniques were reviewed by Businger (1986). Some European perspectives on acidic deposition were provided by Erisman and Draaijers (1995), and reviews of the status of dry deposition knowledge were conducted recently by Lovett (1994), Seinfeld and Pandis (1998), and Wesely and Hicks (2000).
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