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Answer -
The composition of theatmosphere can be understood by considering 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 iscomposed of gases, water vapour and dust particles. Nitrogen constitutes 78.8%,oxygen contributes 20.94% and argon contributes 0.93% in atmosphere. Othergases include carbon dioxide, helium, ozone, methane, hydrogen, krypton, xenonand neon, etc. Nitrogen and oxygen together constitute 99% of the atmosphere.Neon, krypton, xenon are scarce gases. The proportion of gases changes in thehigher layers of the atmosphere in such a way that oxygen will be almost innegligible quantity at the height of 120 km. Similarly, carbon dioxide andwater vapour are found only up to 90 km from the surface of the earth.
Carbon dioxide ismeteorologically a very important gas as it is transparent to the incomingsolar radiation but opaque to the outgoing terrestrial radiation. It absorbs apart of terrestrial radiation and reflects back some part of it towards theearthтАЩs surface. It is largely responsible for the green house effect. Ozone isanother important component of the atmosphere. It is found between 10 and 50 kmabove the earthтАЩs surface and acts as a filter. It absorbs the ultra-violetrays radiating from the sun. It prevents them from reaching the surface of theearth.
Water vapour is such avariable gas in the atmosphere, which decreases with altitude. In the warm andwet tropics, it may account for four per cent of the air by volume, while inthe dry and cold areas of desert and polar regions, it may be less than one percent of the air. Atmosphere has a sufficient capacity to keep small solid particles,which may originate from, different sources and include sea salts, fine soil,smoke-soot, ash, pollen, dust and disintegrated particles of meteors.