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Non-methane volatile organic compounds (NMVOCs) are a set of organic compounds that are typically photochemically reactive in the atmosphere—marked by the exclusion of
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
. NMVOCs include a large variety of chemically different compounds, such as
benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
,
ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...
,
formaldehyde Formaldehyde ( , ) ( systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section ...
,
cyclohexane Cyclohexane is a cycloalkane with the molecular formula . Cyclohexane is non-polar. Cyclohexane is a colorless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cyclohe ...
,
1,1,1-trichloroethane The organic compound 1,1,1-trichloroethane, also known as methyl chloroform, is a chloroalkane. This colorless, sweet-smelling liquid was once produced industrially in large quantities for use as a solvent. It is regulated by the Montreal Prot ...
and
acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscibl ...
. Essentially, NMVOCs are identical to
volatile organic compound Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a ...
s (VOCs), but with methane excluded. Methane is excluded in air-pollution contexts because it is not toxic. It is however a very potent
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), methane ...
, with low reactivity and thus a long lifetime in the atmosphere. An important subset of NMVOCs are the non-methane hydrocarbons (NMHCs). 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 Indoor air quality (IAQ) is the air quality within and around buildings and structures. IAQ is known to affect the health, comfort, and well-being of building occupants. Poor indoor air quality has been linked to sick building syndrome, reduce ...
). The major sources of NMVOCs include vegetation,
biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bio ...
burning, geogenic sources, and human activity.


Importance of atmospheric chemistry

The study of NMVOCs is important in
atmospheric chemistry Atmospheric chemistry is a branch of atmospheric science in which the chemistry of the Earth's atmosphere and that of other planets is studied. It is a multidisciplinary approach of research and draws on environmental chemistry, physics, meteorol ...
, 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. Many NMVOCs carry importance due to their influence on atmospheric
ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the l ...
.
Ground level ozone Ground-level ozone (O3), also known as surface-level ozone and tropospheric ozone, is a trace gas in the troposphere (the lowest level of the Earth's atmosphere), with an average concentration of 20–30 parts per billion by volume (ppbv), with c ...
is not directly emitted, but is instead formed by the reaction of sunlight with various other emitted compounds, including NMHCs (a type of NMVOC), methane,
carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...
, and
nitrogen oxides Nitrogen oxide may refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds: Charge-neutral *Nitric oxide (NO), nitrogen(II) oxide, or nitrogen monoxide *Nitrogen dioxide (), nitrogen(IV) oxide * Nitrogen trioxide (), or ...
.


Biogenic emission

In some non-urban areas, biogenic emissions of NMVOCs meet or exceed anthropogenic emissions of NMVOCs.


Vegetation emissions

There are estimated to be 40 or less NMVOC classified compounds emitted from vegetation that actively influence atmospheric composition, as many NMVOCs are either weakly volatile or are unlikely to be emitted at high volume into the atmosphere. These atmospherically important NMVOCs include compounds such as terpenoids, hexenals,
alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic ...
s,
aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl gro ...
s,
organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are ...
s,
alcohols In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl () functional group bound to a saturated carbon atom. The term ''alcohol'' originally referred to the primary alcohol ethanol (ethyl alcohol), which is ...
,
ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double b ...
s, and
alkane In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in wh ...
s). These NMVOCs which are emitted by vegetation can be divided by source as having originated from one of seven processes: * Emissions from chloroplast activity * Emissions from specialized defense tissues * Emissions from defense processes not related to defense specialized tissues * Emissions of plant growth hormones * Emissions from cut and drying vegetation * Emissions of floral scents * Other vegetation related emissions Of these processes, chlorophyll related emissions and emissions from specialized defense tissues are understood to the point of numerical description. This has led to the characterization of all other emissions processes (besides chlorophyll related emissions) using the model of emissions from specialized defense tissues.


Soil microbe emissions

Many NMVOCs are produced by soil microorganisms (such as methane,
ethane Ethane ( , ) is an organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petroc ...
, and
isoprene Isoprene, or 2-methyl-1,3-butadiene, is a common volatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. Isoprene is an unsaturated hydrocarbon. It is produced by many plants and animals ...
). However, due to the ability for many other soil microorganisms to metabolize these compounds, soils sometimes act as a sink for NMVOCs, leading to the belief that NMVOC flux from soil is negligible.


Biomass burning

Biomass burning, other than for use as fuel, is considered to be a biogenic source. These emissions are modeled based on the area burned, the ratio of above ground biomass to total biomass, the density of the burned organic matter, and combustion efficiency. The chemical composition of emissions from biomass burning varies across different stages of burning, but total NMVOCs emitted from burning is estimated to be 4.5 grams of Carbon per kilogram. The main NMVOCs emitted from burning are ethane, propane, propene, and
acetylene Acetylene ( systematic name: ethyne) is the chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure ...
.


Geogenic sources

Major geogenic sources of NMVOCs include volcanism and seepage resulting from natural gas. Volcanism results in the emissions of many NMVOCs, but at negligible rates. Natural gas seepage is estimated to result in emissions of approximately 0.06 o 2.6 μg m−2 h−1.


Anthropogenic emissions

In the European Database for Global Atmospheric Research (EDGAR), anthropogenic sources of NMVOCs are divided into the following categories: # Power generation # Combustion for manufacturing # Energy for buildings # Road transportation # Transformation Industry #
Fugitive emission Fugitive emissions are leaks and other irregular releases of gases or vapors from a pressurized containment – such as appliances, storage tanks, pipelines, wells, or other pieces of equipment – mostly from industrial activities. In addition ...
s from fuel exploitation # Emissions from production processes # Oil Refineries # Agricultural waste burning # Shipping # Railways, pipelines, and off-road transport # Fossil Fuel Fires #
Solid waste Municipal solid waste (MSW), commonly known as trash or garbage in the United States and rubbish in Britain, is a waste type consisting of everyday items that are discarded by the public. "Garbage" can also refer specifically to food waste, ...
and
wastewater Wastewater is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of domestic, industrial ...
# Aviation EDGAR measures that in 2015, the amount of NMVOCS from the six most contributing sectors (agriculture, power industry, waste, buildings, transport, and other industrial combustion) was 1.2*108 tons. The reported emissions are provided by sector as follows: Global NMVOC emissions from anthropogenic sources have been increasing over time, with the emissions amount rising from 119,000kt to 169,000kt between 1970 and 2010. Regionally, trends vary, with America and Europe reducing their emissions in the same time period, while Africa and Asia increased their NMVOC emissions in this period. Reductions in emissions from America and Europe are largely attributed to use of greener fuels for transport and changing emissions standards.


References

{{Reflist Smog Solvents