Composition of Atmosphere
The atmosphere contains a number of gases. The actual composition of dry air near sea level is given in Table 3.3. The components can be divided into three groups viz. major components, minor components and components present in trace amounts.
The components are expressed in percent by volume or ppm. The value in ppm is obtained by multiplying the percent volume by 104.Table 3.3 The Composition of Dry Air Near Sea Level
| Components | Amount | |
| %(by volume) | ∏m | |
| i. Major Components | ||
| Nitrogen (N2) | 78.09 | 7,80,900 |
| Oxygen (O2) | 20.98 | 2,09,400 |
| Water vapour (H2O) | ~0.15 | 1,000-50,000 |
| ii. Minor Components | ||
| Argon (Ar) | 0.934 | 9,340 |
| Carbon dioxide (CO2) | 0.032 | 320 |
| iii. Trace Components | ||
| Neon (Ne) | 0.00182 | 18.2 |
| Helium (He) | 0.000524 | 5.24 |
| Methane (CH4) | 0.00018 | 1.8 |
| Krypton (Kr) | 0.00018 | 1.8 |
| Nitrous oxide (N2O) | 0.000025 | 0.25 |
| Hydrogen (H2) | 0.00005 | 0.5 |
| Xenon (Xe) | 0.0000087 | 0.087 |
| Sulphur dioxide (SO2) | 0.0000002 | 0.002 |
| Nitrogen dioxide (NO2) | 0.0000001 | 0.001 |
| Ammonia (NH3) | 0.000001 | 0.01 |
| Carbon monoxide (CO) | 0.000012 | 0.12 |
| Ozone (O3) | 0.000002 | 0.02 |
| Iodine (I2) | traces | traces |
The main constituents of the atmosphere which are responsible for sustaining life on earth are nitrogen, oxygen and carbon dioxide along with water vapour. The amounts of these constituents in the atmosphere remain more or less the same (see nitrogen cycle, oxygen cycle and carbon dioxide cycle).
The inert gases are present in the atmosphere in very small amounts. The oxides of nitrogen i.e. nitrous oxide and nitrogen dioxide are formed in the atmosphere by the combination of nitrogen and oxygen during lightening. Ammonia and methane are obtained in the atmosphere by bacterial decomposition of organic matter on the earth. Ozone is obtained from oxygen of the atmosphere by lighting. The burning of sulphur compounds and release of vehicular exhaust gases to the atmosphere release sulphur dioxide in the atmosphere. Carbon dioxide results by the burning of different fuels and respiration processes. Oxygen results from the photosynthetic activity of plants.The percentage composition of the gases remains more or less constant in the atmosphere, at least up to a height of about 20 km. However, it does not hold true in case of air around large cities and industrial centres where there is an increase in carbon dioxide and a decrease in oxygen due to combustion of fuel in power plants and automobiles and also due to large population of human beings.
3.4
More on the topic Composition of Atmosphere:
- Introduction
- Major Regions of Atmosphere
- Heterotroph diets can be determined from the isotopic composition of food sources
- NPP changes during ecosystem development
- Index
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