The concentration of trace constituents in the atmosphere would rise quickly if sink mechanisms did not assure thecleansing of the atmosphere. Thus, Hales (1978) calculated from the sulfur source strength and the tropospheric volume that the average sulfur concentration would increase annually about 70/igm-3 without the action of removal processes. This means that within 1 year remote tropospheric air would become at least as polluted as dirty urban locations are today.

We have seen in Chapter 3 that in the case of several trace gases (H2, CH4, CO, 03, N20) these sink mechanisms are provided by chemical reactions producing components that are utilized by the biosphere. Another possibility for the removal of some gaseous species (sulfur, nitrogen and organic compounds) is their transformation into particulate matter (see Chapters 3 and 4).

The aim of this chapter is to present briefly our ideas on the removal of aerosol particles and water soluble gases (e.g. S02, NH3, N02), both during dry weather conditions (d ry removal) and during periods with cloud and precipitation formation (wet removal). Because of wet removal, precipitation water contains many soluble (and insoluble) materials, as we will see at the end of this chapter.

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