Mixture of gases constituting the earth's atmosphere. Some gases occur in steady concentrations. The most important are molecular nitrogen (N2), 78% by volume, and molecular oxygen (O2), 21%. Small amounts of argon (Ar; 1.9%), neon (Ne), helium (He), methane (CH4), krypton (Kr), hydrogen (H2), nitrous oxide (N2O), and xenon (Xe) are also present in almost constant proportions.
Other gases occur in variable concentrations: water vapour (H2O), ozone (O3), carbon dioxide (CO2), sulfur dioxide (SO2), and nitrogen dioxide (NO2). Air also contains trace amounts of ammonia and hydrogen sulfide. The variable constituents are important for maintaining life. Water vapour is the source for all forms of precipitation and is an important absorber and emitter of infrared radiation. Carbon dioxide is necessary for photosynthesis and is also an important absorber and emitter of infrared radiation. Ozone in the stratosphere (&see; ozone layer) is an effective absorber of ultraviolet radiation from the Sun but at ground-level is a corrosive pollutant and a major constituent of smog.
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