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

=== 6.5.2 Impact of Climate Change on Particulate Matter ===

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Changes in concentration and chemistry of particulate matter (PM) in a changing climate depend in a complex manner on the response of the multiple interactions of changes in emissions, chemical processes, deposition and other factors (e.g., temperature, precipitation, circulation patterns). These changes are difficult to assess and, at the time of AR5, no confidence level was attached to the overall impact of climate change on PM <sub>2.5</sub> ( [[#Kirtman--2013|Kirtman et al., 2013]] ). Possible changes induced by climate change may concern both atmospheric concentration levels and chemical composition.

Higher temperatures increase the reaction rate of gaseous SO <sub>2</sub> to particulate sulphate conversion but also favour evaporation of particulate ammonium nitrate ( [[#Megaritis--2013|Megaritis et al., 2013]] ). Also, higher temperatures are expected to affect BVOC emissions (e.g., [[#Pacifico--2012|Pacifico et al., 2012]] ) that would influence SOA concentrations, although this effect has been questioned by more recent evidence ( [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|B. Wang et al., 2018]] ; Z. [[#Zhao--2019|]] [[#Zhao--2019|]] [[#Zhao--2019|Zhao et al., 2019]] ). More generally, climate change will also affect dust concentration levels in the atmosphere (Section 6.2.2.4) and the occurrence of forest fires, both very large sources of aerosols to the global troposphere (Section 6.2.2.6).

Wet deposition constitutes the main sink for atmospheric PM ( [[#Allen--2016|]] [[#Allen--2016|Allen et al., 2016]] , 2019; [[#Xu--2018|Xu and Lamarque, 2018]] ). In particular, precipitation frequency has a higher effect on PM wet deposition than precipitation intensity ( [[#Hou--2018|Hou et al., 2018]] ). PM is also sensitive to wind speed and atmospheric stability conditions emphasizing the importance of stagnation episodes and low planetary boundary layer heights for increasing PM atmospheric concentrations ( [[#Porter--2015|Porter et al., 2015]] ).

At the global scale, depending on its magnitude, the warming leads either to a small increase in global mean PM concentration levels (about 0.21 µg m <sup>–3</sup> in 2100 for RCP8.5), mainly controlled by sulphate and organic aerosols or a small decrease (–0.06 µg m <sup>–3</sup> for RCP2.6, [[#Westervelt--2016|Westervelt et al. (2016)]] and [[#Xu--2018|Xu and Lamarque (2018)]] ). On the other hand, [[#Xu--2018|Xu and Lamarque (2018)]] and Allen et al. (2016, 2019) found an increase of aerosol burden and PM surface concentration throughout the 21st Century, attributed to a decrease in wet-removal flux despite the overall projected increase in global precipitation, on the ground of an expected shift of future precipitation towards more frequent heavy events. Based only on three models, the CMIP6 ensemble shows that for most land areas, there is low agreement between models on the sign of the effect of climate change on annual mean PM <sub>2.5</sub> (Supplementary Material Figure 6.SM.2).

Due to the typical atmospheric lifetime of PM in the atmosphere, of the order of a few days, most studies dealing with the future PM concentration levels have a regional character and concern mainly Europe (Megaritis et al. , 2013; Lacressonnière et al. , 2016, 2017; Lemaire et al. , 2016; Cholakian et al. , 2019), the USA (Penrod et al. , 2014; Fiore et al. , 2015; Gonzalez-Abraham et al. , 2015; Shen et al. , 2017; He et al. , 2018; Nolte et al. , 2018), Southern and Eastern Asia ( [[#Jiang--2013|Jiang et al., 2013]] ; [[#Nguyen--2019|Nguyen et al., 2019]] ) and India ( [[#Pommier--2018|Pommier et al., 2018]] ). No studies are available for other areas of the world.

Changes in the chemical composition of PM as a result of future climate change can also be an important issue for the effects of PM on human health and the environment, but only sparse data are available in the literature on this and the results are, as yet, inconclusive (Im et al. , 2012; Jiang et al. , 2013; Megaritis et al. , 2013; Gonzalez-Abraham et al. , 2015; Gao et al. , 2018; He et al. , 2018; Cholakian et al. , 2019).

Overall, there is ''medium confidence'' ( ''medium evidence'' , ''high agreement'' ) in a small effect, positive or negative, on PM global burden due to climate change.

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