air, mixture of gases comprising the Earth’s atmosphere. The mixture contains a group of gases of nearly constant concentrations and a group with concentrations that are variable in both space and time. The atmospheric gases of steady concentration (and their proportions in percentage by volume) are as follows:
| nitrogen (N2) | 78.084 |
| oxygen (O2) | 20.946 |
| argon (Ar) | 0.934 |
| neon (Ne) | 0.0018 |
| helium (He) | 0.000524 |
| methane (CH4) | 0.0002 |
| krypton (Kr) | 0.000114 |
| hydrogen (H2) | 0.00005 |
| nitrous oxide (N2O) | 0.00005 |
| xenon (Xe) | 0.0000087 |
The uniformity of composition is maintained by mixing associated with atmospheric motions; but, above a height of about 90 km (55 miles), diffusional processes become more important than mixing, and the lighter gases (hydrogen and helium, in particular) are more abundant above that level.
Of the gases present in variable concentrations, water vapour, ozone, carbon dioxide, sulfur dioxide, and nitrogen dioxide are of principal importance. The typical concentration ranges of these gases (in percentage by volume) are as follows:
| water vapour (H2O) | 0 to 7 |
| carbon dioxide (CO2) | 0.01 to 0.1 (average about 0.032) |
| ozone (O3) | 0 to 0.01 |
| sulfur dioxide (SO2) | 0 to 0.0001 |
| nitrogen dioxide (NO2) | 0 to 0.000002 |
Although present in relatively small amounts, these variable constituents may be very important for maintaining life on Earth’s surface. Water vapour is the source for all forms of precipitation and is an important absorber and emitter of infrared radiation. Carbon dioxide, besides being involved in the process of photosynthesis, is also an important absorber and emitter of infrared radiation. Ozone, which is present mainly in the atmospheric region 10 to 50 km (6 to 30 miles) above the Earth’s surface, is an effective absorber of ultraviolet radiation from the Sun and effectively shields the Earth from all radiation of wavelengths less than 3,000 angstroms.