Dry Deposition

Dry deposition, as the name suggests, refers to the removal of pollutants that are not absorbed into liquid or ice, but rather are removed as either dry particles or gases. Dry deposition involves three stages known as the aerodynamic, surface, and transfer stages. The aerodynamic stage involves the transport of pollutants through the surface layer of the atmosphere down to within a few millimeters of the surface onto which the pollutant is deposited. The turbulence of the atmosphere and the aerodynamic resistance of the pollutant control the success of this stage of dry deposition.

The surface stage refers to the diffusion of the pollutant through this remaining thin layer to make contact with the surface. Any resistance through this layer occurs on the molecular level rather than due to turbulent diffusion, as in the aerodynamic stage.

The transfer stage, refers to the actual removal of the pollutant from the atmosphere. A pollutant is not considered to be removed by dry deposition until it is taken up or absorbed by the surface it contacts. Removal occurs, for example, when a pollutant is dissolved into a body of water or surface moisture, incorporated into soil, or taken up by plants. The transfer stage is controlled by the reactivity between the pollutant and the surface. Some inert gases, such as helium, are rarely, if ever, removed by dry deposition because they are non-reactive and are never absorbed into a surface.