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

==== 6.6.3.1 Climate Response to Past AQ Policies ====

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Air-quality policies emerged several decades ago focusing on emissions mitigation, first driven by local- then by regional-scale air-quality and ecosystem-damage concerns, that is, health impacts, acidification and eutrophication. They have made it possible to reduce or limit pollution exposure in many megacities or highly populated regions, for example, in Los Angeles, Mexico City and Houston in North America ( [[#Parrish--2011|Parrish et al., 2011]] ), Santiago in Chile ( [[#Gallardo--2018|Gallardo et al., 2018]] ), São Paulo in Brazil ( [[#Andrade--2017|Andrade et al., 2017]] ), Europe ( [[#Reis--2012|Reis et al., 2012]] ; [[#Crippa--2016|Crippa et al., 2016]] ; [[#Serrano--2019|Serrano et al., 2019]] ), and over Eastern Asia during the last decade ( [[#Silver--2018|Silver et al., 2018]] ; [[#Zheng--2018b|Zheng et al., 2018b]] ). However, very few studies have quantified the impact of these policies on climate. The AR5 concluded that air-quality control will have consequences on climate including strong regional variability, however, no estimates of impacts of specific air-quality policy were available. Since AR5, few studies have provided estimates of climate-relevant indicators affected by significant air pollutant burden changes due to air-quality policy in selected regions. [[#Turnock--2016|Turnock et al. (2016)]] estimated that the strong decrease in NO <sub>x</sub> , SO <sub>2</sub> and PM <sub>2.5</sub> emissions in Europe, induced by air-quality policies resulting in implementation of abatement measures since the 1970s, have caused a surface warming of +0.45°C ± 0.11°C and increase of precipitation +13 ± 0.8 mm yr <sup>–1</sup> over Europe, compared to the scenario without such policies. While the temperature increase is likely overestimated since the impact of the increase in ammonium nitrate was not considered in this study, the simulated European all-sky TOA radiative effect of the European air pollutant mitigation over the period 1970–2009 is 2.5 times the change in global mean CO <sub>2</sub> radiative forcing over the same period ( [[#Myhre--2013|Myhre et al., 2013]] ). Other studies found that the recent measures to reduce pollution over China have induced a decrease of aerosols and increase of ozone over east China (K. [[#Li--2019|]] [[#Li--2019|Li et al., 2019]] , 2020), resulting in an overall warming effect mainly due to the dominant effect of sulphate reductions in the period 2012–2017 ( [[#Dang--2019|Dang and Liao, 2019]] ).

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