Atmosphere consists of a complex mixture of a number of gases, water vapour and a variety of fine particulate matter (Table). The gaseous mantle which surrounds the globe is constituted by about 5.15 x 10′5 metric tonnes of gas which exerts a pressure of about 1 kg per sq.cm. On earth’s crust most of these gases are compressed in the lowermost layer due to the pressure of lower layer above it. Pressure decreases as we move upward.

Structure of atmosphere

Based primarily on the temperature pattern, earth’s atmosphere can be roughly divided into four major zones:-

1. The troposphere:

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The zone immediately above earth’s surface is known as the troposphere which extends upto a height of about 20 kms above the equator and about 8 kms over the poles. The temperature in this zone drops with height being as low as – 60°C at its upper extremity.

2. The stratosphere:

The layer next to troposphere is called the stratosphere. It is about 30 kms in thickness and is a very important zone of atmosphere as it contains the vital ozone layer. The temperature in this zone rises from – 60°C to about 0°C in its upper layers. The rise in temperature is due to the formation of ozone under the influence of ultra-violet radiation.

3. The mesosphere:

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It is a zone of about 40 kms in thickness which lies next to the stratosphere. This zone is characterised by gradual decline in temperatures to about – 90°C.

4. The Thermosphere:

It is the zone next to mesosphere in which temperature increases with height. Most of the constituents of this zone are in ionized state.

Stability of atmosphere

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It is the typical structure and composition of earth’s atmosphere which is responsible for creating conditions suitable for the existence of a healthy biosphere on this planet. Atmosphere’s role in maintenance of the biosphere in a healthy state can be summed up as follows:

1. Atmosphere regulates the temperature of earth’s crust. The presence of gases capable of absorbing long wavelength radiations is responsible for maintaining the temperatures under which life activities are possible. Moon for example, has no atmosphere. At its equator, surface tempe­ratures rise to 101°C in the day. In nights they drop down to -180°C. No life can survive at these temperatures.

2. The incoming solar radiations are filtered high above earth’s surface. Harmful ultra-violet radiations are absorbed in the stratosphere by the vital ozone layer. These rays can severely damage the terrestrial life.

3. Constant mixing of the contents of atmosphere occurs due to air currents and the vertical temperature gradient. This prevents accumulation of harmful gases and vapours at any particular spot. Atmosphere is therefore, a quick and effective media for transfer, transport and dissemination of gaseous wastes.

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4. Pollutants in the atmosphere are removed by a much more effective mechanism than that operating on land or in water. As the troposphere derives its heat from earth’s surface, warm air being lighter rises and cools down adiabatically. Cooling condenses water vapours. The entire load of pollutants is brought down with snow, dew or rains which cleans the atmosphere.