Biogeochemical Cycling of Macroelements in the Atmosphere

The mass of the atmosphere is about 5.14-5.27 x 10 tons (Walker, 1977 Voitke-vich, 1986). The major part of gaseous matter (about 80%) occurs in the troposphere whose upper equatorial boundary reaches as high as about 17 km going down to 8-10 km at the poles. The troposphere is restricted by tropopause, which is characterized by a sharp temperature drop and the absence of water vapors. The active physical, chemical, and hiogeochemical interactions of the atmosphere with ocean and land occur mainly in the troposphere. It contains also the bulk of water vapors and air-borne particulate matter. It is also well known that many important biospheric photochemical reactions take place in the troposphere. 

Above the troposphere, in the stratosphere and mesosphere, the gas density becomes increasingly more important, and the thermal conditions become subject to complex variations, both temporal and spatial. At a height of 25-35 km, the oxygen molecules are split by the influence of solar UV radiation to form ozone in accordance with Chapman s mechanism, which is responsible for the absorption of 97% of harmful ultraviolet solar radiation. Devoid of this shield, life on the terrestrial surface would have been doomed to extinction. 

Up to 80-100 km from the Earth s surface is the ionosphere, a region of highly rarefied and ionized gas molecules. The exosphere, the outermost belt of the gas shield, might extend as far as 1800 km. The losses of the lightest gases, hydrogen and helium, into space from the Earth s atmosphere occur in the exosphere. 

The concept of mean residence time (MRT) is useful in the consideration of average atmospheric composition shown in Table 2. For any biogeochemical reservoir, its MRT 

Const tuems/spedes Chemical formula Content % by volume) Total mass, 10 tons 

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