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

==== 6.3.3.5 Sulphur Dioxide (SO <sub>2</sub> ) ====

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The AR5 did not assess trends in SO <sub>2</sub> concentrations. Trends in SO <sub>2</sub> abundances are consistent with the overall anthropogenic emissions changes as presented in Section 6.2 and Figure 6.18. Long-term surface-based in situ observations in North America and Europe show reductions of more than 80% since the measurements began around 1980 (Table 6.5). Europe had the largest reductions in the first part of the period while the highest reduction came later in North America. Observed trends are qualitatively reproduced by global and regional models over North America and Europe during the period 1990–2015 for which emissions changes are well quantified (Table 6.5; [[#Aas--2019|Aas et al., 2019]] ).

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'''Table 6.5 |''' '''Summary of changes or trends in atmospheric abundance of sulphur dioxide (SO''' <sub>2</sub> ''') and sulphate (SO''' <sub>4</sub> <sup>2–</sup> ''') aerosols based on in situ and satellite observations.'''

{| class="wikitable"
|-
| '''Analysis Period'''

| '''Trends in SO''' <sub>2</sub>

| '''Trends in Particulate SO''' <sub>4</sub> <sup>2–</sup>

| '''Reference'''

|-
| colspan="4"| Global Models/Assimilated Models

|-
| 1990–2000

| –8.54 ± 1.40% yr <sup>–1</sup> (EU, 43 sites)

–2.63 ± 0.30% yr <sup>–1</sup> (NA, 53 sites)

| –5.23 ± 1.17% yr <sup>–1</sup> (EU, 41 sites)

–1.94 ± 0.43% yr <sup>—</sup> (NA 101 sites)

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

|-
| 2000–2015

| –0.41 ± 0.92% yr <sup>–1</sup> (EA, 19 sites)

–4.86 ± 1.31% yr <sup>–1</sup> (EU, 47 sites)

–4.40 ± 0.93% yr <sup>–1</sup> (NA, 77 sites)

| 0.02 ± 0.91% yr <sup>–1</sup> (EA, 13 sites)

–3.26 ± 0.85% yr <sup>–1</sup> (EU, 36 sites)

–3.18 ± 0.66% yr <sup>–1</sup> (NA, 218 sites)

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

|-
| colspan="4"| Ground-based In Situ Observations

|-
| 1980–1990

| –5.03 ± 2.04% yr <sup>–1</sup> (EU, 20 sites)

–2.5% yr <sup>–1</sup> (US)

| –2.56 ± 3.10% yr <sup>–1</sup> (EU, 16 sites)

–1.80 ± 4.09% yr <sup>–1</sup> (US SO <sub>4</sub> <sup>2–</sup> in precipitation, 78 sites)

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

[https://www.epa.gov/air-trends/sulfur-dioxide-trends US EPA] <sup>a</sup>

|-
| 1990–2000

| –7.56 ± 1.81% yr <sup>–1</sup> (EU, 43 sites)

–3.27 ± 1.69% yr <sup>–1</sup> (NA, 53 sites)

| –5.16 ± 2.11% yr <sup>–1</sup> (EU, 41 sites)

–2.08 ± 1.44% yr <sup>–1</sup> (NA, 101 sites)

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

|-
| 2000–2015

| –0.14 ± 5.32% yr <sup>–1</sup> (EA, 19 sites)

–3.89 ± 2.16% yr <sup>–1</sup> (EU, 47 sites)

–4.69 ± 1.35% yr <sup>–1</sup> (NA, 77 sites)

| 2.68 ± 9.41% yr <sup>–1</sup> (EA, 13 sites)

–2.67 ± 2.03% yr <sup>–1</sup> (EU, 36 sites)

–3.15 ± 1.30% yr <sup>–1</sup> (NA, 218 sites)

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

|-
| colspan="4"| Change Based on Satellite Observations

|-
| 2005–2015

| ca –80% (Eastern US)

|
| [[#Krotkov--2016|Krotkov et al. (2016)]]

|-
| 2005–2015

| ca –60% (Eastern EU)

|
| [[#Krotkov--2016|Krotkov et al. (2016)]]

|-
| 2005–2015

| 200 ± 50% (India)

|
| [[#Krotkov--2016|Krotkov et al. (2016)]]

|-
| 2005 (and 2012) –2015

| ca –50% (The North China Plain)

|
| [[#Krotkov--2016|Krotkov et al. (2016)]]

|}

<sup>a</sup> https://www.epa.gov/air-trends/sulfur-dioxide-trends

In situ observations over other parts of the world are scattered. However, the limited in situ observations in Eastern Asia indicate an increase in atmospheric SO <sub>2</sub> up to around 2005 and then a decline ( [[#Aas--2019|Aas et al., 2019]] ). This is confirmed by satellite observations ( [[#Krotkov--2016|Krotkov et al., 2016]] ), which further reveal a rapid decline in SO <sub>2</sub> since around 2012 or 2013 ( [[#Krotkov--2016|Krotkov et al., 2016]] ; [[#Zheng--2018b|Zheng et al., 2018b]] ). In India, on the other hand, SO <sub>2</sub> levels have doubled between 2005 and 2015 ( [[#Krotkov--2016|Krotkov et al., 2016]] ).

In summary, surface and satellite observations indicate strong regional variations in trends of atmospheric SO <sub>2</sub> abundance. The SO <sub>2</sub> concentrations in North America and Europe have declined over 1980–2015 with slightly stronger reductions in North America (70 ± 20%) than in Europe (58 ± 32%) over 2000–2015, though Europe had larger reductions than the US in the prior decade (1990–2000). In Asia, the SO <sub>2</sub> 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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