Composition of Earth's atmosphere as of Dec. 1987. The lower pie represents the least common gases that compose 0.038% of the atmosphere. Values normalized for illustration.
Filtered air includes trace amounts of many of the chemical elements. Substantial amounts of argon, nitrogen, and oxygen are present as elementary gases. Note the major greenhouse gases: water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Many additional elements from natural sources may be present in tiny amounts in an unfiltered air sample, including contributions from dust, pollen and spores, sea spray, vulcanism, and meteoroids. Various industrial pollutants are also now present in the air, such as chlorine (elementary or in compounds), fluorine (in compounds), elementary mercury, and sulfur (in compounds such as sulfur dioxide [SO2]).
| ppmv: parts per million by 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) | 383 ppmv (0.0383%) |
| Neon (Ne) | 18.18 ppmv (0.001818%) |
| Helium (He) | 5.24 ppmv (0.000524%) |
| Methane (CH4) | 1.745 ppmv (0.0001745%) |
| 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%-7x10-6%) |
| Nitrogen dioxide (NO2) | 0.02 ppmv (2x10-6%) |
| Iodine (I) | 0.01 ppmv (1x10-6%) |
| Carbon monoxide (CO) | trace |
| Ammonia (NH3) | trace |
| Not included in above dry atmosphere: | |
| Water vapor (H2O) | ~0.40% over full atmosphere, typically 1%-4% at surface |
ppmv
The parts per million by volume figures above are by volume-fraction (V%), which for ideal gases is equal to mole-fraction (that is, the fraction of total molecules). Although the atmosphere is not an ideal gas, nonetheless the atmosphere behaves enough like an ideal gas that the volume-fraction is the same as the mole-fraction for the precision given.
By contrast, mass-fraction abundances of gases will differ from the volume values. The mean molar mass of air is 28.97 g/mol, while the molar mass of helium is 4.00, and krypton is 83.80. Thus helium is 5.2 ppm by volume-fraction, but 0.72 ppm by mass-fraction ([4/29] × 5.2 = 0.72), and krypton is 1.1 ppm by volume-fraction, but 3.2 ppm by mass-fraction ([84/29] × 1.1 = 3.2).
Heterosphere
Below the turbopause, at an altitude of about 100 km (62 mi; 330,000 ft) (not far from the mesopause), the Earth's atmosphere has a more-or-less uniform composition (apart from water vapor) as described above; this constitutes the homosphere.[6] However, above the turbopause, the Earth's atmosphere begins to have a composition which varies with altitude. This is because, in the absence of mixing, the density of a gas falls off exponentially with increasing altitude but at a rate which depends on the molar mass. Thus higher mass constituents, such as oxygen and nitrogen, fall off more quickly than lighter constituents such as helium and hydrogen. Thus there is a layer, called the heterosphere, in which the Earth's atmosphere has varying composition. The precise altitude of the heterosphere and the layers it contains varies significantly with temperature.
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