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

==== 6.3.5.3 Carbonaceous Aerosols ====

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Carbonaceous aerosols are black carbon (BC) <sup>[[#footnote-002|3]]</sup> , which is soot made almost purely of carbon, and organic aerosols <sup>[[#footnote-001|4]]</sup> (OA), which also contain hydrogen and oxygen and can be of both primary (POA) or secondary (SOA) origin. BC and a fraction of OA called brown carbon (BrC) absorb solar radiation. The various components of carbonaceous aerosols have different optical properties, so the knowledge of their partition, mixing, coating and ageing is essential to assess their climate effect ( [[IPCC:Wg1:Chapter:Chapter-7#7.3.3.1.2|Section 7.3.3.1.2]] ).

Carbonaceous aerosols receive attention in the scientific and policy arena due to their radiative forcing, and their sizeable contribution to PM in an air-quality context (Rogelj et al. , 2014b; Harmsen et al. , 2015; Shindell et al. , 2016; Haines et al. , 2017; Myhre et al. , 2017) . BC exerts a positive forcing, but the forcing from carbonaceous aerosol as a whole is negative ( [[#Bond--2013|Bond et al., 2013]] ; [[#Thornhill--2021b|Thornhill et al., 2021b]] ). On average, carbonaceous aerosols account for 50–70% of PM with a diameter lower than 1 µm in polluted and pristine areas (Zhang et al. , 2007; Carslaw et al. , 2010; Andreae et al. , 2015; Monteiro dos Santos et al. , 2016; Chen et al. , 2017) .

An extensive review on BC ( [[#Bond--2013|Bond et al., 2013]] ) discussed limitations in inferring its atmospheric abundance and highlighted inconsistencies between different terminology and related measurement techniques ( [[#Petzold--2013|Petzold et al., 2013]] ; [[#Sharma--2017|Sharma et al., 2017]] ). Due to a lack of global observations, AR5 only reported declining total carbonaceous aerosol trends from the USA and a declining BC trend from the Arctic based on data available up to 2008. Since AR5, the number of observation sites has grown worldwide (Figure 6.7) but datasets suitable for global trend analyses remain limited ( [[#Reddington--2017|Reddington et al., 2017]] ; [[#Laj--2020|Laj et al., 2020]] ). Locally, studies based on observations from rural and background sites have reported decreasing surface carbonaceous aerosol trends in the Arctic, Europe, the USA, Japan and India (Table 6.6). Increases in carbonaceous aerosol concentrations in some rural sites of the western USA have been associated with wildfires ( [[#Hand--2013|Hand et al., 2013]] ; [[#Malm--2017|Malm et al., 2017]] ). Long-term OA observations are scarce, so their trends outside of the USA are difficult to assess. Ice-core analysis has provided insight into carbonaceous aerosol trends predating the satellite and observation era over the Northern Hemisphere ( [[IPCC:Wg1:Chapter:Chapter-2#2.2.6|Section 2.2.6]] , Figure 2.9b).

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'''Table 6.6 |''' '''Summary of the regional carbonaceous aerosol trends at background observation sites.'''

{| class="wikitable"
|-
| Species

| Analysis Period

| Change/Trends

| References

|-
| rowspan="6"| BC

| 1990–2009

| Arctic Sites (Alert, Barrow, Ny-Alesund)

−2% yr <sup>−1</sup>

| [[#Sharma--2013|Sharma et al. (2013)]]

|-
| 1970–2010

| Finland (Kevo remote site)

−1.8% yr <sup>–1</sup>

| [[#Dutkiewicz--2014|Dutkiewicz et al. (2014)]]

|-
| 2005–2014

| Germany (rural site)

−2% yr <sup>–1</sup>

| [[#Kutzner--2018|Kutzner et al. (2018)]]

|-
| 2009–2016

| United Kingdom (Harwell rural site)

−8% yr <sup>–1</sup>

| [[#Singh--2018|Singh et al. (2018)]]

|-
| 2009–2019

| Japan (Fukue Island)

−5.8 ± 1.5% yr <sup>–1</sup>

| [[#Kanaya--2020|Kanaya et al. (2020)]]

|-
| 2009–2015

| India (Darjeeling mountain site)

−5% yr <sup>–1</sup>

| [[#Sarkar--2019|Sarkar et al. (2019)]]

|-
| OA

| 2001–2015

| USA (IMPROVE sites east of 100°W)

−2% yr <sup>–1</sup>

| [[#Malm--2017|Malm et al. (2017)]]

|-
| rowspan="2"| Total Carbon (EC + OC)

| 1990–2010

| USA (IMPROVE sites)

Western USA: −4 to −5% yr <sup>–1</sup>

Eastern USA: −1 to −2% yr <sup>–1</sup>

| [[#Hand--2013|Hand et al. (2013)]]

|-
| 2002–2010

| Spain (Montseny rural site)

−5% yr <sup>–1</sup>

| [[#Querol--2013|Querol et al. (2013)]]

|}

Knowledge of carbonaceous aerosol atmospheric abundance continues to rely on global models due to a lack of global-scale observations. For BC, models agree within a factor of two with measured surface mass concentrations in Europe and North America, but underestimate concentrations at the Arctic surface by one to two orders of magnitude, especially in winter and spring ( [[#Lee--2013|Lee et al., 2013]] ; [[#Lund--2018a|Lund et al., 2018a]] ). For OA, AeroCom models underestimate surface mass concentrations by a factor of two over urban areas, as their low horizontal resolution prevents them from resolving local pollution peaks ( [[#Tsigaridis--2014|Tsigaridis et al., 2014]] ; [[#Lund--2018a|Lund et al., 2018a]] ). Models agree within a factor of two with OA surface concentrations measured at remote sites, where surface concentrations are more spatially uniform ( [[#Tsigaridis--2014|Tsigaridis et al., 2014]] ).

BC and OA lifetimes are estimated to be 5.5 days ± 35% and 6.0 days ± 29% (median ± 1 standard deviation), respectively, based on an ensemble of 14 models ( [[#Gliß--2021|Gliß et al., 2021]] ). Disagreement in simulated lifetime leads to horizontal and vertical variations in predicted carbonaceous aerosol concentrations, with implications for radiative forcing ( [[#Samset--2013|Samset et al., 2013]] ; [[#Lund--2018b|Lund et al., 2018b]] ). Airborne campaigns have provided valuable vertical-profile measurements of carbonaceous aerosol concentrations (Schwarz et al. , 2013; Freney et al. , 2018; Hodgson et al. , 2018; Schulz et al. , 2019; D. Zhao et al. , 2019; Morgan et al. , 2020). Compared to those measurements, models tend to transport BC too high in the atmosphere, suggesting that lifetimes are not larger than 5.5 days ( [[#Samset--2013|Samset et al., 2013]] ; [[#Lund--2018b|Lund et al., 2018b]] ). Newly developed size-dependent wet-scavenging parametrization for BC ( [[#Taylor--2014|Taylor et al., 2014]] ; [[#Schroder--2015|Schroder et al., 2015]] ; [[#Ohata--2016|Ohata et al., 2016]] ; G. [[#Zhang--2017|]] [[#Zhang--2017|Zhang et al., 2017]] ; [[#Ding--2019|Ding et al., 2019]] ; [[#Moteki--2019|Moteki et al., 2019]] ; [[#Motos--2019|Motos et al., 2019]] ) may lead to decreased BC lifetimes and improve agreement with observed vertical profiles.

Simulated BC burdens show a large spread among models ( [[#Gliß--2021|Gliß et al., 2021]] ), despite using harmonised primary emissions, because of differences in BC removal efficiency linked to different treatment of ageing and mixing, particularly in strong source regions. The multi-model median BC burden for the year 2010 from [[#Gliß--2021|Gliß et al. (2021)]] , based on 14 AeroCom models, is 0.131 ± 0.047 Tg (median ± 1 standard deviation). The range encompasses values reported by independent single-model estimates ( Huang et al. , 2013; Lee et al. , 2013; Sharma et al. , 2013; Q. Wang et al. , 2014; Tilmes et al. , 2019) .

Simulated OA burdens also show a large spread among global models, with [[#Gliß--2021|Gliß et al. (2021)]] reporting a multi-model median of 1.91 ± 0.65 Tg for the year 2010. The large spread reflects the wide range in the complexity of the OA parametrizations, particularly for SOA formation, as well as in the primary OA emissions ( [[#Tsigaridis--2014|Tsigaridis et al., 2014]] ; [[#Gliß--2021|Gliß et al., 2021]] ). The uncertainties are particularly large in model estimates of SOA production rates, which vary between 10 and 143 Tg yr <sup>–1</sup> ( [[#Tsigaridis--2014|Tsigaridis et al., 2014]] ; [[#Hodzic--2016|Hodzic et al., 2016]] ; [[#Tilmes--2019|Tilmes et al., 2019]] ). While the level of complexity in the representation of OA in global models has increased since AR5 ( [[#Shrivastava--2017|Shrivastava et al., 2017]] ; [[#Hodzic--2020|Hodzic et al., 2020]] ), limitations in process-level understanding of the formation, ageing and removal of organic compounds lead to uncertainties in the global model predictions of global OA burden and distribution as well as the relative contribution of POA and SOA to OA. [[#Jo--2016|Jo et al. (2016)]] estimated that BrC contributes about 20% of total OA burden. That would give BrC a burden similar to that of BC ( ''low confidence'' ), enhancing the overall forcing exerted by carbonaceous aerosol absorption ( [[#Zhang--2020|Zhang et al., 2020]] ).

In summary, the lack of global-scale observations of carbonaceous aerosols, the complexity of processes influencing them, and the large spread in their simulated global budget and burdens means that there is only ''low confidence'' in the quantification of the present-day atmospheric distribution of individual components of carbonaceous aerosols. Global trends in carbonaceous aerosols cannot be characterized due to limited observations, but sites representative of background conditions have reported multi-year declines in BC over several regions of the Northern Hemisphere.

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