Question

Describe the composition of the atmosphere.

Answer 1

The composition of the atmosphere can be understood by considering the following table.

          Gases of the Atmosphere

Constituent	Formulae	% by Volume
Nitrogen	N2	78.08
Oxygen	O	20.95
Argon	Ar	0.93
Carbon dioxide	CO2	0.036
Neon	Ne	0.002
Helium	He	0.0005
Krypton	Kr	0.001
Xenon	Xe	0.00009
Hydrogen	H2	0.00005

The atmosphere is composed of gases, water vapour and dust particles. Nitrogen constitutes 78.8%, oxygen contributes 20.94% and argon contributes 0.93% in atmosphere. Other gases include carbon dioxide, helium, ozone, methane, hydrogen, krypton, xenon and neon, etc. Nitrogen and oxygen together constitute 99% of the atmosphere. Neon, krypton, xenon are scarce gases. The proportion of gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost in negligible quantity at the height of 120 km. Similarly, carbon dioxide and water vapour are found only up to 90 km from the surface of the earth.

Carbon dioxide is meteorologically a very important gas as it is transparent to the incoming solar radiation but opaque to the outgoing terrestrial radiation. It absorbs a part of terrestrial radiation and reflects back some part of it towards the earth’s surface. It is largely responsible for the greenhouse effect. Ozone is another important component of the atmosphere. It is found between 10 and 50 km above the earth’s surface and acts as a filter. It absorbs the ultra-violet rays radiating from the sun. It prevents them from reaching the surface of the earth.

Water vapour is such a variable gas in the atmosphere, which decreases with altitude. In the warm and wet tropics, it may account for four per cent of the air by volume, while in the dry and cold areas of desert and polar regions, it may be less than one per cent of the air. The atmosphere has a sufficient capacity to keep small solid particles, which may originate from, different sources and includes sea salts, fine soil, smoke-soot, ash, pollen, dust and disintegrated particles of meteors.