Collectively, all the hydrocarbons other than methane. Methane is fairly long-lived in the atmosphere and has a large and relatively constant mixing ratio in the troposphere. The other hydrocarbons have lifetimes factors of 30–30 000 times shorter than methane, thus showing much greater variability, and tend to have more localized sources. Since such sources are often anthropogenic in nature, the total concentration of NMHCs is often used as a measure of the degree of pollution of an air mass.