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

=== 6.5.3 Impact of Climate Change on Extreme Pollution ===

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Extreme air pollution is identified as the concentration of an air pollutant that is above a given threshhold value (high concentration or a high percentile) as the sensitivity of peak values to meteorological conditions can be different from sensitivity of the median or mean ( [[#Porter--2015|Porter et al., 2015]] ). The AR5 assessed with ''medium confidence'' that uniformly higher temperatures in polluted environments will trigger regional feedbacks in chemistry and local emissions that will increase peak ozone and PM pollution, but assessed ''low confidence'' in projecting changes in meteorological blocking associated with these extreme episodes.

Meteorological conditions, such as heatwaves, temperature inversions and atmospheric stagnation episodes favour air quality extremes and are influenced by changing climate ( [[#Fiore--2015|Fiore et al., 2015]] ). The body of literature on the connection between climate change and extreme anthropogenic pollution episodes is essentially based on correlation and regression applied to observation reanalysis but the metrics and methodologies differ making quantitative comparisons difficult. Many emission processes in the natural systems are sensitive to temperature, and bursts of emissions as a reponse to extreme weather, as in the case of wildfires in dry conditions ( [[#Bondur--2020|Bondur et al., 2020]] ; [[#Xie--2020|Xie et al., 2020]] ) can occur, which would then add to the risk of extreme air pollution but are not sufficiently constrained to be quantitatively assessed.

Since AR5, published studies provide augmented evidence for the connections between extreme ozone and PM pollution events and high temperatures, especially long-lasting heatwaves, whose frequency is increasing due to a warming climate ( [[#Lelieveld--2014|Lelieveld et al., 2014]] ; [[#Porter--2015|Porter et al., 2015]] ; [[#Hou--2016|Hou and Wu, 2016]] ; [[#Jing--2017|Jing et al., 2017]] ; [[#Schnell--2017|Schnell and Prather, 2017]] ; [[#Sun--2017|Sun et al., 2017]] ; H. [[#Zhang--2017|]] [[#Zhang--2017|Zhang et al., 2017]] ). However, the relationship between air pollution and individual meteorological parameters is exaggerated because of covariation on synoptic time scales ( [[#Fiore--2015|Fiore et al., 2015]] ). For example, heatwaves are often associated with clear skies and stagnation, making clear attribution to specific meteorological variables complicated. In Asia, future changes in winter conditions have also been shown to favour more particulate pollution ( [[#Cai--2017|Cai et al., 2017]] ; [[#Zou--2017|Zou et al., 2017]] ). The relationship between the occurrence of stagnation episodes and high concentrations of ozone and PM <sub>2.5</sub> has been shown to be regionally and metric dependant ( [[#Oswald--2015|Oswald et al., 2015]] ; [[#Sun--2017|Sun et al., 2017]] ; [[#Kerr--2018|Kerr and Waugh, 2018]] ; [[#Schnell--2018|Schnell et al., 2018]] ; [[#Garrido-Perez--2019|Garrido-Perez et al., 2019]] ).

The increase of frequency, duration and intensity of heatwaves is extremely likely on all continents for different future warming levels ( [[IPCC:Wg1:Chapter:Chapter-11#11.3.5|Section 11.3.5]] , Table 11.2). However, there is low confidence in projected changes in storm tracks, jets and blocking, and thus their influence on extreme temperatures in the mid-latitudes ( [[IPCC:Wg1:Chapter:Chapter-11#11.3.1|Section 11.3.1]] ).

In conclusion, there is still ''medium confidence'' that climate-driven changes in meterorological conditions, such as heatwaves or stagnations, will favour extreme air pollution episodes over highly polluted areas, however, the relationship between these meteorological conditions and high concentrations of ozone and PM <sub>2.5</sub> have been shown to be regionally and metric dependant.

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