Air is mainly composed of nitrogen, oxygen, and argon, which together constitute the "major gases" of the atmosphere. The remaining gases often are referred to as "trace gases,"
among which are the greenhouse gases such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many other chemical compounds. Many natural substances may be present in tiny amounts in an unfiltered air sample, including dust, pollen and spores, sea spray, volcanic ash, and meteoroids. Various industrial pollutants also may be present, such as chlorine (elementary or in compounds), fluorine (in compounds), elemental mercury, and sulfur (in compounds such as sulfur dioxide [SO2])
| ppmv: parts per million by volume (note: volume fraction is equal to mole fraction for ideal gas only, see Gas volume#Partial volume) | |
| Gas | Volume |
|---|---|
| Nitrogen (N2) | 780,840 ppmv (78.084%) |
| Oxygen (O2) | 209,460 ppmv (20.946%) |
| Argon (Ar) | 9,340 ppmv (0.9340%) |
| Carbon dioxide (CO2) | 387 ppmv (0.0387%) |
| Neon (Ne) | 18.18 ppmv (0.001818%) |
| Helium (He) | 5.24 ppmv (0.000524%) |
| Methane (CH4) | 1.79 ppmv (0.000179%) |
| Krypton (Kr) | 1.14 ppmv (0.000114%) |
| Hydrogen (H2) | 0.55 ppmv (0.000055%) |
| Nitrous oxide (N2O) | 0.3 ppmv (0.00003%) |
| Xenon (Xe) | 0.09 ppmv (9x10−6%) |
| Ozone (O3) | 0.0 to 0.07 ppmv (0% to 7x10−6%) |
| Nitrogen dioxide (NO2) | 0.02 ppmv (2x10−6%) |
| Iodine (I) | 0.01 ppmv (1x10−6%) |
| Carbon monoxide (CO) | 0.1 ppmv |
| Ammonia (NH3) | trace |
| Not included in above dry atmosphere: | |
| Water vapor (H2O) | ~0.40% over full atmosphere, typically 1%-4% at surface |
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