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

=== 2.2.6 Aerosols ===

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The AR5 assessed large-scale aerosol optical depth (AOD) trends over 2000–2009, concluding that there was ''low confidence'' in a global trend, but that AOD ''very likely'' decreased from 1990 onwards over Europe and the eastern USA, and increased since 2000 over eastern and southern Asia. The ERF associated with aerosol–radiation interactions for 2011 (relative to 1750) was estimated to be –0.45 ± 0.5 W m <sup>–2</sup> and of aerosol–cloud interaction estimated as –0.45 [–1.2 to 0.0] W m <sup>–2</sup> . Aerosol ERF uncertainty was assessed as the largest contributor to the overall ERF uncertainty since 1750.

This section assesses the observed large-scale temporal evolution of tropospheric aerosols. Aerosol-related processes, chemical and physical properties, and links to air quality, are assessed in Chapter 6. An in-depth assessment of aerosol interactions with radiation and clouds is provided in Section 7.3.3.

Aerosol proxy records of improved temporal resolution and quality are now available ( [[#Kylander--2016|Kylander et al., 2016]] ; [[#Stevens--2016|Stevens et al., 2016]] , 2018; [[#Jacobel--2017|Jacobel et al., 2017]] ; [[#Dornelas--2018|Dornelas et al., 2018]] ; [[#Middleton--2018|Middleton et al., 2018]] ), which further advance synthesis of new global compilations of aerosol loadings ( [[#Lambert--2015|Lambert et al., 2015]] ; [[#Albani--2016|Albani et al., 2016]] ). Estimates of the glacial/interglacial ratio in global dust deposition are within the range of 2–4 ( [[#Albani--2015|Albani et al., 2015]] ; [[#Lambert--2015|Lambert et al., 2015]] ). New reconstructions indicate a ratio of 3–5 for the glacial/interglacial loadings for mid- and high-latitude ocean of both hemispheres ( [[#Lamy--2014|Lamy et al., 2014]] ; [[#Martinez-Garcia--2014|Martinez-Garcia et al., 2014]] ; [[#Serno--2015|Serno et al., 2015]] ). Improved quantification of changes in dust deposition from North Africa and North Atlantic sediment records confirms dust deposition rates lower by a factor 2–5 during the African Humid Period (10–5 ka) compared to the late Holocene ( [[#McGee--2013|McGee et al., 2013]] ; [[#Albani--2015|Albani et al., 2015]] ; [[#Middleton--2018|Middleton et al., 2018]] ; [[#Palchan--2019|Palchan and Torfstein, 2019]] ). During the Holocene, biogenic emissions and volcanic activity drove significant variability (up to one order of magnitude) in sulphate concentrations ( [[#Schüpbach--2018|Schüpbach et al., 2018]] ).

Ice cores allow for estimation of multi-centennial trends in mid- and high-latitude aerosol deposition, including those for sulphate and black carbon (Figure 2.9a,b). Sulphate in ice cores increased by a factor of 8 from the end of the 19th century to the 1970s in continental Europe, by a factor of 4 from the 1940s to the 1970s in Russia, and by a factor of 3 from the end of the 19th century to 1950 in the Arctic (Svalbard). In all regions studied, concentrations have declined by about a factor of 2 following their peak (around 1970 in Europe and Russia, and 1950 in the Arctic). Strong increases of black carbon (BC) were observed in the 20th century over Europe, Russia, Greenland (primarily originating from emissions from North America), and in the Arctic (Svalbard). South America exhibits a small positive trend (Figure 2.9). BC concentrations in various Antarctic ice cores were below 1 ng g <sup>–1</sup> without a clear trend.

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[[File:d620688ec652c435c624d8090aff573c IPCC_AR6_WGI_Figure_2_9.png]]

'''Figure 2.9''' '''|''' '''Aerosol evolution from ice-core measurements.''' Changes are shown as 10-year averaged time series '''(a, b)''' and trends in remote-sensing aerosol optical depth (AOD) and AODf '''(c, d). (a)''' Concentrations of non-sea salt (nss) sulphate (ng g <sup>–1</sup> ). '''(b)''' Black carbon (BC) in glacier ice from the Arctic (Lomonosovfonna), Russia (Belukha), Europe (Colle Gnifetti), South America (Illimani), Antarctica (stacked sulphate record, and BC from the B40 core), and BC from Greenland (stacked rBC record from Greenland and eastern Europe (Elbrus)). '''(c)''' Linear trend in annual mean AOD retrieved from satellite data for the 2000–2019 period (% yr <sup>–1</sup> ). The average trend from MODerate Resolution Imaging Spectroradiometer (MODIS) and Multi-Angle Imaging Spectroradiometer (MISR) is shown. Trends are calculated using OLS regression with significance assessed following AR(1) adjustment after [[#Santer--2008|Santer et al. (2008)]] . Superimposed are the trends in annual-mean AOD from the AERONET surface sunphotometer network for 2000–2019. '''(d)''' Linear trend in 2000–2019 as in (c), but for fine-mode AOD, AODf, and using only MISR over land. ‘×’ marks denote non-significant trends. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Spatially resolved trends of AOD derived from Aqua/Terra MISR and MODIS instruments over 2000–2019 range between –2% and +2% per year (Figure 2.9c). Ground-based solar attenuation networks help to constrain and improve the satellite-derived retrievals of AOD, and trends derived from the AERONET network (Figure 2.9c,d) corroborate satellite results ( [[#Georgoulias--2016|Georgoulias et al., 2016]] ; [[#Wei--2019|Wei et al., 2019]] ; [[#Bauer--2020|Bauer et al., 2020]] ; H. [[#Yu--2020|]] [[#Yu--2020|Yu et al., 2020]] ) in particular for declines over Europe ( [[#Stjern--2011|Stjern et al., 2011]] ; [[#Cherian--2014|Cherian et al., 2014]] ; [[#Li--2014|Li et al., 2014]] ) and the USA ( [[#Li--2014|Li et al., 2014]] ; [[#Jongeward--2016|Jongeward et al., 2016]] ). The tendency in AOD over East Asia reversed from positive (2000–2010) to negative (since 2010) ( [[#Sogacheva--2018|Sogacheva et al., 2018]] ; [[#Filonchyk--2019|Filonchyk et al., 2019]] ; [[#Ma--2019|Ma et al., 2019]] ; [[#Samset--2019|Samset et al., 2019]] ). Over southern Asia, however, AOD from satellite (MODIS/MISR) and AERONET retrievals show continuing increases ( [[#Li--2014|Li et al., 2014]] ; [[#Zhao--2017|Zhao et al., 2017]] ), with similar trends from UV-based aerosol retrievals from the Ozone Monitoring Instrument (OMI) on the Aura satellite ( [[#Dahutia--2018|Dahutia et al., 2018]] ; [[#Hammer--2018|Hammer et al., 2018]] ). A comparison of MODIS and MISR radiometric observations with the broadband CERES satellite instrument ( [[#Corbett--2015|Corbett and Loeb, 2015]] ) showed that drifts in calibration are ''unlikely'' to affect the satellite derived trends. CERES shows patterns for clear-sky broadband radiation consistent with the aerosol spatio-temporal changes ( [[#Loeb--2018|Loeb et al., 2018]] ; [[#Paulot--2018|Paulot et al., 2018]] ).

Satellite-derived trends are further supported by in situ regional surface concentration measurements, operational since the 1980s (sulphate) and 1990s (PM2.5) from a global compilation ( [[#Collaud%20Coen--2020|Collaud Coen et al., 2020]] ) of networks over Europe ( [[#Stjern--2011|Stjern et al., 2011]] ), North America ( [[#Jongeward--2016|Jongeward et al., 2016]] ), and China ( [[#Zheng--2018|Zheng et al., 2018]] ). [[#Collaud%20Coen--2020|Collaud Coen et al. (2020)]] report from surface observations across the NH mid-latitudes that aerosol absorption coefficients decreased since the first decade of the 21st century.

Anthropogenic aerosol is predominantly found in the fraction of particles with radii &lt;1 µm that comprise the fine-mode AOD (AODf; Figure 2.9d; [[#Kinne--2019|Kinne, 2019]] ). A significant decline in AODf of more than 1.5% per year from 2000 to 2019 has occurred over Europe and North America, while there have been positive trends of up to 1.5% per year over Southern Asia and East Africa. The global-scale trend in AODf of –0.03% per year (Figure 2.9) is significant. The results are consistent with trend estimates from an aerosol reanalysis ( [[#Bellouin--2020|Bellouin et al., 2020]] ), and the trends in satellite-derived cloud droplet number concentrations are consistent with the aerosol trends ( [[#Cherian--2020|Cherian and Quaas, 2020]] ). Cloudiness and cloud radiative properties trends are, however, less conclusive possibly due to their large variability ( [[#Norris--2016|Norris et al., 2016]] ; [[#Cherian--2020|Cherian and Quaas, 2020]] ). Further details on aerosol-cloud interactions are assessed in Section 7.3.3.2.

To conclude, atmospheric aerosols sampled by ice cores, influenced by northern mid-latitude emissions, show positive trends from 1700 until the last quarter of the 20th century and decreases thereafter ( ''high confidence'' ), but there is ''low confidence'' in observations of systematic changes in other parts of the world in these periods. Satellite data and ground-based records indicate that AOD exhibits predominantly negative trends since 2000 over NH mid-latitudes and SH continents, but increased over South Asia and East Africa ( ''high confidence'' ). A globally deceasing aerosol abundance is thus assessed with ''medium confidence'' . This implies increasing net positive ERF, since the overall negative aerosol ERF has become smaller.

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