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

==== 6.3.5.1 Sulphate (SO <sub>4</sub> <sup>2–</sup> ) ====

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Sulphate aerosols (or sulphate-containing aerosols) are emitted directly or formed in the atmosphere by gas- and aqueous-phase oxidation of precursor sulphur gases, including SO <sub>2</sub> , DMS and carbonyl sulphide (OCS), emitted from anthropogenic and natural sources (Section 6.2). Sulphate aerosols influence climate forcing directly by either scattering solar radiation or absorbing longwave radiation, and indirectly by influencing cloud micro- and macrophysical properties and precipitation ( [[#Boucher--2013|Boucher et al., 2013]] ; [[#Myhre--2013|Myhre et al., 2013]] ). Additionally, sulphate aerosols and sulphate deposition have a large impact on air quality and ecosystems ( [[#Reis--2012|Reis et al., 2012]] ). The majority of sulphate particles are formed in the troposphere, however, SO <sub>2</sub> and other longer-lived natural precursors, such as OCS, transported into the stratosphere, contribute to the background stratospheric aerosol layer ( [[#Kremser--2016|Kremser et al., 2016]] ). SO <sub>2</sub> emissions from volcanic eruptions are a significant source of stratospheric sulphate loading (see [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] for reconstruction of stratospheric aerosol optical depth and [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] for radiative forcing of volcanic aerosols). Furthermore, studies suggest sulphate contributions from anthropogenic SO <sub>2</sub> emissions transported into the stratosphere could have a consequent impact on radiative forcing ( [[#Myhre--2004|Myhre et al., 2004]] ; [[#Yu--2016|Yu et al., 2016]] ). However, there is significant uncertainty in the relative importance of this stratospheric sulphate source ( [[#Kremser--2016|Kremser et al., 2016]] ).

Process understanding of sulphate production pathways from SO <sub>2</sub> emissions has seen some progress since AR5. More specifically, many global climate models now have a more complete description of chemical reactions such that oxidant levels (including ozone) are better described, include a pH-dependence of SO <sub>2</sub> oxidation (e.g., [[#Kirkevåg--2018|Kirkevåg et al., 2018]] ; [[#Bauer--2020|Bauer et al., 2020]] ) , and implement explicit descriptions of ammonium and nitrate aerosol components, which may influence the partitioning of sulphate (Bian et al. , 2017; Lund et al. , 2018a) . The pH influences the heterogeneous chemistry as well as the physical properties of the aerosols, and this topic has been a subject of growing interest since AR5 ( [[#Cheng--2016|Cheng et al., 2016]] ; [[#Freedman--2019|Freedman et al., 2019]] ; [[#Nenes--2020|Nenes et al., 2020]] ). Increases in cloudwater pH have been shown to significantly increase the radiative forcing due to sulphate aerosols ( [[#Turnock--2019|Turnock et al., 2019]] ).

Sulphate is removed from the atmosphere by dry deposition and wet scavenging, and these processes depend on the characteristics of the Earth’s surface, and the intensity, frequency and amount of precipitation ( [[#Boucher--2013|Boucher et al., 2013]] ). Even though there have been some improvements since AR5, representation of atmospheric transport and of wet scavenging and related cloud processes remains a key source of uncertainty in the simulated aerosol distribution and lifetime, with further consequences for the sulphate forcing estimates ( [[#Kristiansen--2016|Kristiansen et al., 2016]] ; [[#Lund--2018a|Lund et al., 2018a]] ). There are also still relatively large uncertainties in the emission height used in models affecting the simulated aerosol distribution (Yang et al. , 2019a) .

Based on long-term surface-based in situ observations, AR5 reported a strong decline in sulphate aerosols in Europe and the USA over 1990–2009, with the largest decreases occurring before 2000 in Europe and post-2000 in the USA. Since AR5, atmospheric measurements in conjunction with model results have provided insights into the spatial and temporal distribution of sulphate and sulphur deposition ( [[#Vet--2014|Vet et al., 2014]] ; [[#Tan--2018|Tan et al., 2018]] ; [[#Aas--2019|Aas et al., 2019]] ). The in situ observations in North America and Europe reveal substantial reduction since the measurements started around 1980, though the trends have not been linear through this period (Table 6.5). Several regional studies agree with these trend estimates for Europe (Banzhaf et al. , 2015; Theobald et al. , 2019) and North America ( [[#Sickles%20II--2015|Sickles II and Shadwick, 2015]] ; Paulot et al. , 2016) . Further, the concentrations of primary emitted SO <sub>2</sub> (Section 6.3.3.5) show greater decreases than secondary sulphate aerosols over these regions due to a combination of higher oxidation rate (hence more SO <sub>2</sub> converted to SO <sub>4</sub> <sup>2–</sup> ) and increased dry deposition rate of SO <sub>2</sub> (Fowler et al. , 2009; Banzhaf et al. , 2015) . In situ observations over other parts of the world are scattered (Figure 6.7), and the lack of observations makes it too uncertain to quantify regional representative trends ( [[#Hammer--2018|Hammer et al., 2018]] ). However, limited in situ observations in Eastern Asia indicate an increase in atmospheric sulphate up to around 2005 and then a decline ( [[#Aas--2019|Aas et al., 2019]] ), which is confirmed by satellite observations of SO <sub>2</sub> (Section 6.3.3.5). In India, on the other hand, satellite observations indicate a rapid increase in the SO <sub>2</sub> levels ( [[#Krotkov--2016|Krotkov et al., 2016]] ), and long-term measurements of sulphate in precipitation in India further provide evidence of an increasing trend from 1980–2010 ( [[#Bhaskar--2017|Bhaskar and Rao, 2017]] ; [[#Aas--2019|Aas et al., 2019]] ). Further improvements in global trend assessments are expected with new integrated reanalysis products from the Earth-system data assimilation projects ( [[#Randles--2017|Randles et al., 2017]] ; [[#Inness--2019|Inness et al., 2019]] ).

Indirect evidence of decadal trends in the atmospheric loading of sulphur are provided by Alpine ice cores, mainly influenced by European sources ( [[#Engardt--2017|Engardt et al., 2017]] ), and ice cores from Svalbard ( [[#Samyn--2012|Samyn et al., 2012]] ) and Greenland ( [[#Patris--2002|Patris et al., 2002]] ; [[#Iizuka--2018|Iizuka et al., 2018]] ) influenced by sources in Europe and North America. These show similar patterns with a weak increase from the end of the 19th century up to around 1950, followed by a steep increase up to around 1980, and then a significant decrease over the next two decades (see also [[IPCC:Wg1:Chapter:Chapter-2#2.2.6|Section 2.2.6]] ). This general trend is consistent with the emissions of SO <sub>2</sub> in North America and Europe (Figures 6.18 and 6.19; Hoesly et al. , 2018) .

Global and regional models qualitatively reproduce observed trends over North America and Europe for the period 1990–2015 for which emissions changes are generally well quantified ( [[#Aas--2019|Aas et al., 2019]] ; [[#Mortier--2020|Mortier et al., 2020]] ), building confidence in the relationship between emissions, concentration, deposition and radiative forcing derived from these models. However, the models seem to systematically underestimate sulphate ( [[#Bian--2017|Bian et al., 2017]] ; [[#Lund--2018a|Lund et al., 2018a]] ) and AOD ( [[#Lund--2018a|Lund et al., 2018a]] ; [[#Gliß--2021|Gliß et al., 2021]] ), and there are quite large differences in the models’ distribution of the concentration fields of sulphate driven by differences in the representation of photochemical production and sinks of aerosols. One global model study also highlighted biases in simulated sulphate trends over the 2001–2015 period over eastern China due to uncertainties in the CEDS anthropogenic SO <sub>2</sub> emissions trends (Paulot et al. , 2018a) .

In summary, there is ''high confidence'' that the global tropospheric sulphate burden increased from 1850 to around 2005, but there are large regional differences in the magnitude. Sulphate aerosol concentrations in North America and Europe have declined over 1980–2015 with slightly stronger reductions in North America (47 ± 20%) than in Europe (40 ± 30%) over 2000–2015, though Europe had larger reductions in the prior decade (1990–2000; 52 ± 21% and 21 ± 14% respectively for Europe and North America). In Asia, the trends are more scattered, though there is ''medium confidence'' that there was a strong increase up to around 2005, followed by a steep decline in China, while over India, the concentrations are increasing steadily.

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