Non-methane volatile organic compound
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Essentially, NMVOCs are identical to volatile organic compounds (VOCs), but with methane excluded. An important subset of NMVOCs are the non-methane hydrocarbons (NMHCs). Methane is excluded in air-pollution contexts because it is not harmful. Its low reactivity and thus long lifetime in the atmosphere, however, makes it an important greenhouse gas.
The study of NMVOCs is important in atmospheric chemistry, where it can be used as a proxy to study the collective properties of reactive atmospheric VOCs. The exclusion of methane is necessary due to its relatively high ambient concentration in comparison to other atmospheric species and its relative inertness. NMVOCs is an umbrella term which encompasses all speciated and oxygenated biogenic, anthropogenic, and pyrogenic organic molecules present in the atmosphere, minus the contribution of methane. The necessity of this term is also governed by current estimates which suggest that somewhere between 10,000 and 100,000 NMVOCs are present in the atmosphere, most with concentrations in the realm of parts per billion or parts per trillion. The aggregation of these compounds and their collective properties are easier to study than the individual components.
Sometimes NMVOC is also used as a sum parameter for emissions, where all NMVOC emissions are added up per weight into one figure. In absence of more detailed data, this can be a very coarse parameter for pollution (e.g. for summer smog or indoor air pollution).