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The atmosphere is the gaseous realm. It consists of a mixture of gases that completely surround the earth. Of much greater significance to metabolism, however, are the gaseous cycles of the atmosphere.
The air consists mainly of oxygen (about 20 per cent), carbon dioxide (about 0.03 per cent), nitrogen (about 79 per cent), water vapour, and minute traces of inert gases.
Except the inert Gases, all these components of air serve as metabolites and each circulates through a cycle in which the organisms play an important role.
As all gases are dissolved in natural waters, the hydrosphere maintains equilibrium with the atmosphere. The metabolic role played by important gases of the atmosphere and their cycles are discussed under biogeochemical cycles. Atmosphere is usually divided into four layers called the troposphere, stratosphere, mesosphere, and the thermosphere (Fig. 6.2). 
1. Troposphere:
The troposphere or lower atmosphere is in contact with the earth’s surface, and is about 8 km high at the poles and 16 km at the equator. It consists of, in the unpolluted state, nitrogen (78.08% by volume of dry air), oxygen (20.95% by volume of dry air) and traces of other gases such as Argon (0.93%), Carbon dioxide (0.0325%), Neon (0.0018%), Helium (0.0005%), Krypton (0.0001%) and Ozone (0.000002%). The troposphere also contains varying quantities of water vapour (1-4% by volume), with a maximum concentration at about 10-15 km altitude.
Water vapour can condense into clouds and fog and also as snow and hail. Troposphere is the most important layer of the atmosphere since the changes in weather and climate, which affect man and other organisms, take place in this part of the atmosphere.
2. Stratosphere:
The second layer of the atmosphere is called Stratosphere. It lies above the troposphere and extends an average height of about 12 to 50 km above the earth. It differs from the troposphere in that it contains very little water vapours and no clouds. Though its gaseous composition is similar, but the gaseous mass is only 15% of the total atmosphere, and it contains more ozone. In troposphere, the average rate of temperature decrease is 6.4°C per km of ascent, but in Stratosphere the temperature shows a gradual increase.
3. Mesosphere:
The third layer, or mesosphere, extends from about 50 to 80 km above the earth. It consists of less gaseous mass, no water vapour, and ozone is also present in it. In mesosphere, the temperature decreases with height.
4. Thermosphere:
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The fourth layer or the thermosphere or heterosphere starts at an altitude of about 80 km. In this layer the gases are present in the atomic state and exhibit a sort of layering. From 80 to 120 km oxygen and nitrogen are abundant, but at 500 km helium and hydrogen are the main gases. Beyond 500 km the thermosphere becomes increasingly rarefied and extends to the extreme outer limit of the atmosphere at about 40,000 km.
The heterosphere contains no water vapour or ozone. Lying between the upper mesosphere and the lower thermosphere is the so-called ionosphere because free electrons and ions exist in this region. These are formed by the photochemical action of ultraviolet radiation upon oxygen and nitrogen gases.
