Composition
Air is mainly composed of nitrogen, oxygen, and argon, which together constitute the major gases of the atmosphere. The remaining gases are often 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, and volcanic ash. Various industrial pollutants also may be present, such as chlorine (elementary or in compounds), fluorine compounds, elemental mercury, and sulfur compounds such as sulfur dioxide .
| ppmv: parts per million by volume (note: volume fraction is equal to mole fraction for ideal gas only, see volume (thermodynamics)) | |
| 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) | 394.45 ppmv (0.039445%) |
| 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) | 0.325 ppmv (0.0000325%) |
| Carbon monoxide (CO) | 0.1 ppmv (0.00001%) |
| Xenon (Xe) | 0.09 ppmv (9×10−6%) (0.000009%) |
| Ozone (O3) | 0.0 to 0.07 ppmv (0 to 7×10−6%) |
| Nitrogen dioxide (NO2) | 0.02 ppmv (2×10−6%) (0.000002%) |
| Iodine (I2) | 0.01 ppmv (1×10−6%) (0.000001%) |
| Ammonia (NH3) | trace |
| Not included in above dry atmosphere: | |
| Water vapor (H2O) | ~0.40% over full atmosphere, typically 1%–4% at surface |
Read more about this topic: Earth's Atmosphere
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—Henry David Thoreau (1817–1862)