Land-air exchange

The land surface is an efficient sink for many trace gases. These are absorbed or decomposed on contact with plants or soil surfaces. Plants can be particularly active because of their large surface area and ability to absorb water-soluble gases. The deposition process is crudely described by the deposition velocity, Vd, 

The term flux is analogous to a flow of material, in this case expressed as micrograms of substance depositing per square metre of ground surface per unit time. In the case of rough surfaces the square metre of area refers to the area of hypothetical horizontal flat surface beneath the true surface rather than the sum of the area of all the rough elements such as plant leaves, which make up the true surface. 

Since the deposition process itself causes a gradient in atmospheric concentration, Vd is deflned in relation to a reference height, usually 1 m, at which the atmospheric concentration is measured. For reasons described later, Vd is not constant for a given substance, but varies according to atmospheric and surface conditions. However, some typical values are given in Table 2, which exemplify the massive variability. 

For some trace gases, for example, nitric acid vapour, dry deposition represents a major sink mechanism. In this case the process may have a major impact upon atmospheric lifetimes. 

Dry deposition is frequently the main sink for ozone in the rural atmospheric boundary layer. What is the lifetime of ozone with respect to this process  

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