NonMethane

Nonmethane, in atmospheric chemistry, refers to nonmethane hydrocarbons (NMHCs), a broad class of volatile organic compounds consisting of carbon and hydrogen with at least two carbon atoms. By definition, NMHCs exclude methane (CH4). They comprise a wide range of chemical species, including alkanes, alkenes, alkynes, and aromatics, and cover both biogenic and anthropogenic sources. Biogenic NMHCs such as isoprene and monoterpenes are emitted by many plants, while anthropogenic NMHCs include benzene, toluene, ethylbenzene, xylene, and other hydrocarbons released from vehicle exhaust, fuel evaporation, solvents, and industrial processes.

NMHCs are typically categorized by their carbon number and reactivity. They include short-chain alkanes and alkenes as well as larger aromatic and cyclic hydrocarbons. Their atmospheric fate depends on local chemistry; more reactive NMHCs rapidly participate in photochemical reactions, whereas less reactive species can persist longer and be transported over regional or continental distances. NMHCs contribute to the formation of ozone and secondary organic aerosols, affecting air quality and climate. Some NMHCs, such as benzene, pose health risks and are subject to regulatory controls in many regions.

Measurement and regulation of NMHCs rely on air sampling and analytical techniques such as gas chromatography with flame ionization detection or mass spectrometry. In practice, NMHC monitoring is often reported as total NMHC or as concentrations of key individual species. Regulatory frameworks may target NMHC emissions from transportation, solvent use, and industrial sources to mitigate ozone formation, reduce health risks, and improve air quality.

See also: volatile organic compounds, tropospheric ozone, secondary organic aerosol.