Editing IPCC:AR6/WGI/Chapter-6 (section)

==== 6.3.3.3 Non-Methane Volatile Organic Compounds (NMVOCs) ====

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NMVOCs encompass thousands of compounds with lifetimes from hours to days to months, and abundances and chemical composition highly variable with respect to space and time. Although the biogenic source (Section 6.2.2) dominates the global NMVOC budget, anthropogenic activities are the main driver of long-term trends in the abundance of many compounds.

Information on the global distribution of individual NMVOCs is scarce, except for the less reactive compounds having lifetimes of several days to months. Based on measurements from polar firn air samples and ground-based networks, AR5 reported that the abundances of the predominantly anthropogenic light alkanes (C <sub>2</sub> -C <sub>5</sub> ) increased until 1980 and declined afterwards. The decline was attributed to air-quality emissions controls and to fugitive emissions decreases following the collapse of the Soviet Union ( [[#Simpson--2012|Simpson et al., 2012]] ). Since AR5, scarce ground-based measurements have shown that the decline in C <sub>2</sub> -C <sub>3</sub> alkanes ended around 2008 and their abundances are since growing again, which is primarily attributed to increasing North American emissions (Section 6.2.1). Furthermore, since AR5 the evolution of ethane levels during the past millennium was made accessible by analysis of ice-core samples ( [[#Nicewonger--2016|Nicewonger et al., 2016]] ). The large observed interpolar ratio of ethane in pre-industrial times (3.9) corroborates a large geologic source of ethane previously put forward by ( [[#Etiope--2009|Etiope and Ciccioli, 2009]] ), and narrows down its likely global magnitude ( [[#Nicewonger--2018|Nicewonger et al., 2018]] ) ( ''low to medium confidence'' ). The incorporation of geologic emissions in CCMs is not yet systematic though a one-model study has shown improved agreement of the results with observations ( [[#Dalsøren--2018|Dalsøren et al., 2018]] ).

Formaldehyde (HCHO) is a short-lived, high-yield product of NMVOC oxidation, and formaldehyde column data from satellite instruments can therefore inform on trends in anthropogenic NMVOC abundances over very industrialized regions. The AR5 reported significant positive trends in formaldehyde between 1997 and 2009 over northeastern China (4% yr <sup>–1</sup> ) and negative trends over northeastern US cities. Since AR5, there is ''robust evidence'' and ''high agreement'' of an upward trend of HCHO over eastern China, though large regional disparities exist in the trends ( [[#De%20Smedt--2015|De Smedt et al., 2015]] ; [[#Shen--2019|Shen et al., 2019]] ) with a possible negligible or decreasing trend over Beijing and the Pearl River Delta. In other world regions, in particular North America, there is ''limited'' to ''medium evidence'' for significant changes in the HCHO columns, except in regions where the trend is particularly strong (e.g., the Houston area: –2.2% yr <sup>–1</sup> over 2005–2014) and the Alberta oil sands (+3.8% yr <sup>–1</sup> ; <sup></sup> [[#Zhu--2017|Zhu et al., 2017]] ). Over the northeastern USA, even the sign of the trend differs between studies (De Smedt et al. , 2015; Zhu et al. , 2017) for reasons that are unclear.

In summary, after a decline between 1980 and 2008, abundances of light NMVOCs have increased again over the Northern Hemisphere due to the extraction of oil and gas in North America ( ''high confidence'' ). Trends in satellite HCHO observations, used as a proxy of anthropogenic NMVOC over industrialized areas, show a significant positive trend over eastern China ( ''high confidence'' ) but also indicate large regional disparities in the magnitude of the trends over China and even in their signs over North America.

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