It is important to realise the dynamic character of the atmosphere's composition. The total amount remains more or less unchanged, but only because the quantity entering almost equals what leaves. Individual atoms of carbon in carbon dioxide, for example, continually go through a cycle of change: the gas becomes part of plant tissue, with subsequent respiration by plants or animals, maybe solution in ocean water and then escape back into the atmosphere, as indicated in Figure 1.3. The time that a molecule spends as a gas is described in terms of its atmospheric residence time, the average period between a molecule entering and leaving the air (Note 1.D). Figure 1.3 shows how the difference between the annual amounts entering and leaving a stage equals the increment of storage there. (Such an equality is called a 'mass balance', mentioned in Chapter 5.) Similar diagrams of bio-geo-chemical cycles can be constructed for other components of air. These cycles are interrelated, e.g. the cycle of carbon meshes with that for oxygen, at the carbon dioxide stage.
A complete cycle of a carbon molecule may take several years, whereas nitrogen takes 10
million years for a cycle of fixation and electrification. 'Fixation' is the conversion of nitrogen into nitrates (chemicals containing both nitrogen and oxygen) by lightning or industrial processes, and by organisms in the soil or oceans. On the other hand, 'denitrification' is the liberation of nitrogen from decaying organic matter back into the air. The fixation-nitrification loop is supplemented by another involving the simple dissolving of nitrogen in the oceans and its subsequent release back into the atmosphere. It exists there as a stable constituent of the air (Table 1.3).
Similarly, there is a cycle of water (the hydrologic cycle), which involves evaporation, the formation of cloud, precipitation, stream flow, absorption into the oceans and then evaporation to form water vapour once more. This is discussed in detail in Chapters 6-11.
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