Editing IPCC:AR6/SRCCL/Chapter-2 (section)

==== 2.4.3.1 BVOC precursors of short-lived climate forcers from land ====

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BVOCs are rapidly oxidised in the atmosphere to form less volatile compounds that can condense and form SOA. In boreal and tropical forests, carbonaceous aerosols originate from BVOC oxidation, of which isoprene and terpenes are the most important precursors (Claeys et al. 2004 <sup>[[#fn:r916|916]]</sup> ; Hu et al. 2015 <sup>[[#fn:r917|917]]</sup> ; De Sá et al. 2017 <sup>[[#fn:r918|918]]</sup> ; de Sá et al. 2018 <sup>[[#fn:r919|919]]</sup> ; Liu et al. 2016b <sup>[[#fn:r920|920]]</sup> ). See the following sub-section for more detail.

BVOCs are the most important precursors of SOA. The transformation process of BVOCs affects the aerosol size distribution both by contributing to new particle formation and to the growth of larger pre-existing particles. SOA affects the scattering of radiation by the particles themselves (direct aerosol effect), but also changes the amount of CCN and the lifetime and optical properties of clouds (indirect aerosol effect).

High amounts of SOA are observed over forest areas, in particular in boreal and tropical regions where they have been found to mostly originate from BVOC emissions (Manish et al. 2017 <sup>[[#fn:r921|921]]</sup> ). In particular, isoprene epoxydiol-derived SOA (IEPOX-SOA) is being identified in recent studies in North America and Amazonian forest as a major component in the oxidation of isoprene (Allan et al. 2014 <sup>[[#fn:r922|922]]</sup> ; Schulz et al. 2018 <sup>[[#fn:r923|923]]</sup> ; De Sá et al. 2017 <sup>[[#fn:r924|924]]</sup> ). In tropical regions, BVOCs can be convected up to the upper atmosphere, where their volatility is reduced and where they become SOA. In some cases those particles are transported back to the lower atmosphere (Schulz et al. 2018 <sup>[[#fn:r925|925]]</sup> ; Wang et al. 2016a <sup>[[#fn:r926|926]]</sup> ; Andreae et al. 2018 <sup>[[#fn:r927|927]]</sup> ). In the upper troposphere in the Amazon, SOA are important CCN and are responsible for the vigorous hydrological cycle (Pöhlker et al. 2018 <sup>[[#fn:r928|928]]</sup> ). This strong link between BVOC emissions by plants and the hydrological cycle has been discussed in a number of studies (Fuentes et al. 2000 <sup>[[#fn:r929|929]]</sup> ; Schmale et al. 2018 <sup>[[#fn:r930|930]]</sup> ; Pöhlker et al. 2018 <sup>[[#fn:r931|931]]</sup> , 2016 <sup>[[#fn:r932|932]]</sup> ).

Changing BVOC emissions also affect the oxidant concentrations in the atmosphere. Their impact on the concentration of ozone depends on the NOx concentrations. In polluted regions, high BVOC emissions lead to increased production of ozone, followed by the formation of more OH and a reduction in the methane lifetime. In more pristine regions (NOx-limited), increasing BVOC emissions instead lead to decreasing OH and ozone concentrations, resulting in a longer methane lifetime. The net effect of BVOCs then can change over time if NOx emissions are changing.

BVOCs’ possible climate effects have received little attention because it was thought that their short lifetime would preclude them from having any significant direct influence on climate (Unger 2014a <sup>[[#fn:r933|933]]</sup> ; Sporre et al. 2019 <sup>[[#fn:r934|934]]</sup> ). Higher temperatures and increased CO <sub>2</sub> concentrations are (separately) expected to increase the emissions of BVOCs (Jardine et al. 2011 <sup>[[#fn:r935|935]]</sup> , 2015 <sup>[[#fn:r936|936]]</sup> ; Fuentes et al. 2016 <sup>[[#fn:r937|937]]</sup> ). This has been proposed to initiate negative climate feedback mechanisms through increased formation of SOA (Arneth et al. 2010 <sup>[[#fn:r938|938]]</sup> ; Kulmala 2004 <sup>[[#fn:r939|939]]</sup> ; Unger et al. 2017 <sup>[[#fn:r940|940]]</sup> ). More SOA can make clouds more reflective, which can provide a cooling effect. Furthermore, the increase in SOA formation has also been proposed to lead to increased aerosol scattering, resulting in an increase in diffuse radiation. This could boost GPP and further increase BVOC emissions (Kulmala et al. 2014 <sup>[[#fn:r941|941]]</sup> ; Cirino et al. 2014 <sup>[[#fn:r942|942]]</sup> ; Sena et al. 2016 <sup>[[#fn:r943|943]]</sup> ; Schafer et al. 2002 <sup>[[#fn:r944|944]]</sup> ; Ometto et al. 2005 <sup>[[#fn:r945|945]]</sup> ; Oliveira et al. 2007 <sup>[[#fn:r946|946]]</sup> ). This important feedback is starting to emerge (Sporre et al. 2019 <sup>[[#fn:r947|947]]</sup> ; Kulmala 2004 <sup>[[#fn:r948|948]]</sup> ; Arneth et al. 2017 <sup>[[#fn:r949|949]]</sup> ). However, there is evidence that this influence might be significant at different spatial scales, from local to global, through aerosol formation and through direct and indirect greenhouse effects (l ''imited evidence, medium agreement'' ). Most tropical forest BVOCs are primarily emitted from tree foliage, but soil microbes can also be a major source of some compounds including sesquiterpenes (Bourtsoukidis et al. 2018 <sup>[[#fn:r950|950]]</sup> ).

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