Trace gas

The term Trace gas refers to a gas or gases which make up less than 1% by volume of the earth's atmosphere, and therefore includes all gases except nitrogen (78.1%) and oxygen (20.9%). The most abundant trace gas at 0.934% is argon, which is being continually produced by radioactive decay of 40K in the earth's rocks. Water vapor also occurs in the atmosphere with highly variable abundance.

Several atmospheric trace gases such as ozone, sulfur dioxide and nitrogen oxides  are anthropogenic, chemically reactive factors of air quality at a regional level. Others such as carbon dioxide and methane  are important greenhouse gases and are produced anthropogenically but also by plants, microorganisms and from natural geothermal sources.