trace gas

trace gas: Definition and Much More from Answers.com:

"Wikipedia: trace gas

The term trace gas refers to a gas or gases which make up less than 1% of the earth's atmosphere, which encompasses all occurring gasses except nitrogen and oxygen. Several are chemically reactive factors of air quality at a regional level, and others are important greenhouse gases.[1] Most are anthropogenic, but plants and microorganisms are important sources in remote areas.[2][3] The most common trace gas is argon and trace compounds include water vapor, carbon dioxide, methane, nitrogen oxides, and ozone."