A trace gas is a gas which makes up less than 1% by volume of the Earth's atmosphere, and it includes all gases except nitrogen (78.1%) and oxygen (20.9%). The most abundant trace gas at 0.934% is argon. Water vapor also occurs in the atmosphere with highly variable abundance.
Several atmospheric trace gases such as ozone O
3, sulfur dioxide SO
2 and nitrogen oxides NO
x are anthropogenic, chemically reactive factors of air quality at a regional level. Others such as carbon dioxide CO
2 and methane CH
4 are important greenhouse gases and are produced anthropogenically but mainly by plants, microorganisms and from natural geothermal sources.
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