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

==== 2.4.2.1 Carbonaceous aerosol precursors of short-lived climate forcers from land ====

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OC is a major component of aerosol mass concentration, and it originates from different anthropogenic (combustion processes) and natural (natural biogenic emissions) sources (Robinson et al. 2007 <sup>[[#fn:r841|841]]</sup> ). A large fraction of OC in the atmosphere has a secondary origin, as it can be formed in the atmosphere through condensation to the aerosol phase of low vapour pressure gaseous compounds emitted as primary pollutants or formed in the atmosphere. This component is SOA (Hodzic et al. 2016 <sup>[[#fn:r842|842]]</sup> ). A third component of the optically active aerosols is the so-called brown carbon (BrC), an organic material that shows enhanced solar radiation absorption at short wavelengths (Wang et al. 2016b <sup>[[#fn:r843|843]]</sup> ; Laskin et al. 2015 <sup>[[#fn:r844|844]]</sup> ; Liu et al. 2016a <sup>[[#fn:r845|845]]</sup> ; Bond et al. 2013 <sup>[[#fn:r846|846]]</sup> ; Saturno et al. 2018 <sup>[[#fn:r847|847]]</sup> ).

OC and EC have distinctly different optical properties, with OC being important for the scattering properties of aerosols and EC central for the absorption component (Rizzo et al. 2013 <sup>[[#fn:r848|848]]</sup> ; Tsigaridis et al. 2014 <sup>[[#fn:r849|849]]</sup> ; Fuzzi et al. 2015 <sup>[[#fn:r850|850]]</sup> ). While OC is reflective and scatters solar radiation, it has a cooling effect on climate. On the other side, BC and BrC absorb solar radiation and they have a warming effect in the climate system (Bond et al. 2013 <sup>[[#fn:r851|851]]</sup> ).

OC is also characterised by a high solubility with a high fraction of water-soluble organic compounds (WSOC) and it is one of the main drivers of the oxidative potential of atmospheric particles. This makes particles loaded with oxidised OC an efficient cloud condensation nuclei (CCN) in most of the conditions (Pöhlker et al. 2016 <sup>[[#fn:r852|852]]</sup> ; Thalman et al. 2017 <sup>[[#fn:r853|853]]</sup> ; Schmale et al. 2018 <sup>[[#fn:r854|854]]</sup> ).

Biomass burning is a major global source of carbonaceous aerosols (Bowman et al. 2011 <sup>[[#fn:r855|855]]</sup> ; Harrison et al. 2010 <sup>[[#fn:r856|856]]</sup> ; Reddington et al. 2016 <sup>[[#fn:r857|857]]</sup> ; Artaxo et al. 2013 <sup>[[#fn:r858|858]]</sup> ). As knowledge of past fire dynamics improved through new satellite observations, new fire proxies’ datasets (Marlon et al. 2013 <sup>[[#fn:r859|859]]</sup> ; van Marle et al. 2017a <sup>[[#fn:r860|860]]</sup> ), process-based models (Hantson et al. 2016 <sup>[[#fn:r861|861]]</sup> ) and a new historic biomass burning emissions dataset starting in 1750 have been developed (van Marle et al. 2017b <sup>[[#fn:r862|862]]</sup> ) (Cross-Chapter Box 3 in this chapter). Revised versions of OC biomass burning emissions (van Marle et al. 2017b <sup>[[#fn:r863|863]]</sup> ) show, in general, reduced trends compared to the emissions derived by Lamarque et al. (2010) <sup>[[#fn:r864|864]]</sup> for CMIP5. CMIP6 global emissions pathways (Gidden et al. 2018 <sup>[[#fn:r865|865]]</sup> ; Hoesly et al. 2018 <sup>[[#fn:r866|866]]</sup> ) estimate global BC emissions in 2015 at 9.8 MtBC yr <sup>–1</sup> , while global OC emissions are 35 MtOC yr <sup>–1</sup> .

Land use change is critically important for carbonaceous aerosols, since biomass-burning emissions consist mostly of organic aerosol, and the undisturbed forest is also a large source of organic aerosols (Artaxo et al. 2013 <sup>[[#fn:r867|867]]</sup> ). Additionally, urban aerosols are also mostly carbonaceous because of the source composition (traffic, combustion, industry, etc.) (Fuzzi et al. 2015 <sup>[[#fn:r868|868]]</sup> ). Burning of fossil fuels, biomass- burning emissions and SOA from natural BVOC emissions are the main global sources of carbonaceous aerosols. Any change in each of these components directly influence the radiative forcing (Contini et al. 2018 <sup>[[#fn:r869|869]]</sup> ; Boucher et al. 2013 <sup>[[#fn:r870|870]]</sup> ; Bond et al. 2013 <sup>[[#fn:r871|871]]</sup> ).

One important component of carbonaceous aerosols is the primary biological aerosol particles (PBAP), also called bioaerosols, that correspond to a significant fraction of aerosols in forested areas (Fröhlich-Nowoisky et al. 2016 <sup>[[#fn:r872|872]]</sup> ; Pöschl and Shiraiwa 2015 <sup>[[#fn:r873|873]]</sup> ). They are emitted directly by the vegetation as part of the biological processes (Huffman et al. 2012 <sup>[[#fn:r874|874]]</sup> ). Airborne bacteria, fungal spores, pollen, archaea, algae and other bioparticles are essential for the reproduction and spread of organisms across various terrestrial ecosystems. They can serve as nuclei for cloud droplets, ice crystals and precipitation, thus influencing the hydrological cycle and climate (Whitehead et al. 2016 <sup>[[#fn:r875|875]]</sup> ; Scott et al. 2015 <sup>[[#fn:r876|876]]</sup> ; Pöschl et al. 2010 <sup>[[#fn:r877|877]]</sup> ).

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