Non-methane volatile organic compound

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.[1] NMVOCs include a large variety of chemically different compounds, such as benzene, ethanol, formaldehyde, cyclohexane, 1,1,1-trichloroethane and acetone.[2] Essentially, NMVOCs are identical to volatile organic compounds (VOCs), but with methane excluded.[3] Methane is excluded in air-pollution contexts because it is not toxic. It is however a very potent greenhouse gas, with low reactivity and thus a long lifetime in the atmosphere.[1] 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).

The major sources of NMVOCs include vegetation, biomass burning, geogenic sources, and human activity.[4][5]

A molecular diagram of trimethylbenzene
Trimethylbenzene is an important NMVOC due to its high photochemical reactivity to form ozone in the atmosphere.[4]

Importance of atmospheric chemistry

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.[1] 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.[6] 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.[4] Ground level ozone 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, and nitrogen oxides.[7]

Biogenic emission

In some non-urban areas, biogenic emissions of NMVOCs meet or exceed anthropogenic emissions of NMVOCs.[8]

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.[8] These atmospherically important NMVOCs include compounds such as terpenoids, hexenals, alkenes, aldehydes, organic acids, alcohols, ketones, and alkanes). These NMVOCs which are emitted by vegetation can be divided by source as having originated from one of seven processes:[8]

  • 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.[8]

Soil microbe emissions

Many NMVOCs are produced by soil microorganisms (such as methane, ethane, and isoprene). 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.[8]

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.[5]

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.[8] The main NMVOCs emitted from burning are ethane, propane, propene, and acetylene.[8]

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.[9]

Anthropogenic emissions

In the European Database for Global Atmospheric Research (EDGAR), anthropogenic sources of NMVOCs are divided into the following categories:[4]

  1. Power generation
  2. Combustion for manufacturing
  3. Energy for buildings
  4. Road transportation
  5. Transformation Industry
  6. Fugitive emissions from fuel exploitation
  7. Emissions from production processes
  8. Oil Refineries
  9. Agricultural waste burning
  10. Shipping
  11. Railways, pipelines, and off-road transport
  12. Fossil Fuel Fires
  13. Solid waste and wastewater
  14. 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.[10] The reported emissions are provided by sector as follows:

NMVOC Emissions by Sector[10]
Sector NMVOC Emissions (tons)
Agriculture 9,450,016.04
Power Industry 856,907.07
Waste 3,066,094.19
Buildings 24,948,773.51
Transport 32,729,144.19
Other Industrial Combustion 48,505,685.26

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.[4] 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.[4] Reductions in emissions from America and Europe are largely attributed to use of greener fuels for transport and changing emissions standards.[4]

References
  1. Koppmann, Ralf, ed. (2007). Volatile Organic Compounds in the Atmosphere. Oxford, UK: Blackwell Publishing Ltd. doi:10.1002/9780470988657. ISBN 978-0-470-98865-7.
  2. Nesaratnam, Suresh T.; Taherzadeh, Shahram; Barratt, Rod (2014), "Section 2: Meteorology and Air Pollutants", Air Quality Management, Chichester, UK: John Wiley & Sons, Ltd, pp. 15–98, doi:10.1002/9781118863886.ch2, ISBN 9781118863886
  3. "System of Registries". sor.epa.gov. US EPA.
  4. Huang, Ganlin; Brook, Rosie; Crippa, Monica; Janssens-Maenhout, Greet; Schieberle, Christian; Dore, Chris; Guizzardi, Diego; Muntean, Marilena; Schaaf, Edwin; Friedrich, Rainer (2017). "Speciation of anthropogenic emissions of non-methane volatile organic compounds: a global gridded data set for 1970–2012". Atmospheric Chemistry and Physics. 17 (12): 7683–7701. doi:10.5194/acp-17-7683-2017. ISSN 1680-7324. S2CID 55072182.
  5. Guenther, Alex; Hewitt, C. Nicholas; Erickson, David; Fall, Ray; Geron, Chris; Graedel, Tom; Harley, Peter; Klinger, Lee; Lerdau, Manuel; Mckay, W. A.; Pierce, Tom (1995). "A global model of natural volatile organic compound emissions". Journal of Geophysical Research: Atmospheres. 100 (D5): 8873–8892. doi:10.1029/94JD02950. ISSN 2156-2202. S2CID 42852605.
  6. Goldstein, Allen H.; Galbally, Ian E. (2007). "Known and Unexplored Organic Constituents in the Earth's Atmosphere". Environmental Science & Technology. 41 (5): 1514–1521. doi:10.1021/es072476p. ISSN 0013-936X. PMID 17396635.
  7. "Tropospheric ozone". Climate & Clean Air Coalition. Retrieved 2021-11-20.
  8. Guenther, A (2000). "Natural emissions of non-methane volatile organic compounds, carbon monoxide, and oxides of nitrogen from North America". Atmospheric Environment. 34 (12–14): 2205–2230. doi:10.1016/s1352-2310(99)00465-3. ISSN 1352-2310. S2CID 55449924.
  9. Lamb, Brian; Guenther, Alex; Gay, David; Westberg, Hal (1987). "A national inventory of biogenic hydrocarbon emissions". Atmospheric Environment. 21 (8): 1695–1705. doi:10.1016/0004-6981(87)90108-9. ISSN 0004-6981. S2CID 55462971.
  10. Crippa, Monica; Janssens-Maenhout, Greet; Dentener, Frank; Guizzardi, Diego; Sindelarova, Katerina; Muntean, Marilena; Van Dingenen, Rita; Granier, Claire (2016). "Forty years of improvements in European air quality: regional policy-industry interactions with global impacts". Atmospheric Chemistry and Physics. 16 (6): 3825–3841. doi:10.5194/acp-16-3825-2016. ISSN 1680-7324.
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