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.
The load of pollutants discharged in the air is also diluted, chemically modified and is finally brought back to earth's surface to be eliminated. 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. The temperature and pressure patterns determine precipitation which in turn regulates the abundance and composition of biotic communities on earth's surface.
4. 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.
5. 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.