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 O3, sulfur dioxide SO2 and nitrogen oxides NOx are anthropogenic, chemically reactive factors of air quality at a regional level. Others such as carbon dioxide CO2 and methane CH4 are important greenhouse gases and are produced anthropogenically but mainly by plants, microorganisms and from natural geothermal sources.
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