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 abundance of methane on Earth makes ...
.
NMVOCs include a large variety of chemically different compounds, such as
benzene
Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
,
ethanol
Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...
,
formaldehyde
Formaldehyde ( , ) (systematic name methanal) is an organic compound with the chemical formula and structure , more precisely . The compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde. It is stored as ...
,
cyclohexane
Cyclohexane is a cycloalkane with the molecular formula . Cyclohexane is non-polar. Cyclohexane is a colourless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cyclohexan ...
,
1,1,1-trichloroethane
1,1,1-Trichloroethane, also known as methyl chloroform and chlorothene, is a chloroalkane with the chemical formula CH3CCl3. It is an isomer of 1,1,2-trichloroethane. A colourless and sweet-smelling liquid, it was once produced industrially i ...
and
acetone
Acetone (2-propanone or dimethyl ketone) is an organic compound with the chemical formula, formula . It is the simplest and smallest ketone (). It is a colorless, highly Volatile organic compound, volatile, and flammable liquid with a charact ...
. Essentially, NMVOCs are identical to
volatile organic compound
Volatile organic compounds (VOCs) are organic compounds that have a high vapor pressure at room temperature. They are common and exist in a variety of settings and products, not limited to Indoor mold, house mold, Upholstery, upholstered furnitur ...
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
Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. Unlike other gases, greenhouse gases absorb the radiations that a planet emits, resulting in the greenhouse effect. T ...
, 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
Smog, or smoke fog, is a type of intense air pollution. The word "smog" was coined in the early 20th century, and is a portmanteau of the words ''smoke'' and ''fog'' to refer to smoky fog due to its opacity, and odour. The word was then inte ...
or
indoor air pollution
Indoor air quality (IAQ) is the air quality within buildings and structures. Poor indoor air quality due to indoor air pollution is known to affect the health, comfort, and well-being of building occupants. It has also been linked to sick build ...
).
The major sources of NMVOCs include vegetation,
biomass
Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...
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 that studies the chemistry of the Earth's atmosphere and that of other planets. This multidisciplinary approach of research draws on environmental chemistry, physics, meteorology, comput ...
, 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 compound, 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 , break ...
.
Ground level ozone
Ground-level ozone (), 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 poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
, and
nitrogen oxides
In atmospheric chemistry, is shorthand for nitric oxide () and nitrogen dioxide (), the nitrogen oxides that are most relevant for air pollution.
These gases contribute to the formation of smog and acid rain, as well as affecting tr ...
.
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
terpenoid
The terpenoids, also known as isoprenoids, are a class of naturally occurring organic compound, organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeabl ...
s,
hexenals,
alkene
In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins.
The Internationa ...
s,
aldehyde
In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...
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 re ...
s,
alcohols
In chemistry, an alcohol (), is a type of organic compound that carries at least one hydroxyl () functional group bound to a Saturated and unsaturated compounds, saturated carbon atom. Alcohols range from the simple, like methanol and ethanol ...
,
ketone
In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...
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 whi ...
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 a naturally occurring Organic compound, organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is List of purification methods ...
, 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. It is produced by many plants and animals (including humans) and its polymers ar ...
). 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 (Chemical nomenclature, systematic name: ethyne) is a 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 u ...
.
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 t ...
s from fuel exploitation
# Emissions from production processes
# Oil Refineries
#
Agricultural waste
Agricultural waste are plant residues from agriculture. These waste streams originate from arable land and horticulture. Agricultural waste are all parts of crops that are not used for human or animal food. Crop residues consist mainly of stems ...
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 (or waste water) 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 do ...
# 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*10
8 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
Indoor air pollution