Summertime ozone pollution in the Yangtze River Delta of eastern China during 2013-2017: Synoptic impacts and source...

Severe ozone (O₃) pollution in China has magnified global concerns over the past years. Exploring the synoptic impacts and quantifying the source contributions are important for mitigating O₃ pollution. This study focuses on the summertime O₃ pollution over the Yangtze River Delta (YRD) in eastern China. We identify six predominant synoptic weather patterns (SWPs) over the YRD during 2013-2017 using the self-organising map (SOM) approach. We conduct O₃ source attribution based on ozone source apportionment technology (OSAT) in a regional transport model. Surface O₃ pollution is found to be sensitive to the predominant SWPs, including four O₃-polluted types (northeasterly, northerly and southwesterly and anticyclone) and two O₃-clean types (cyclone and the Meiyu front). The integrated influences of local chemistry and regional transport are important in O₃ pollution under different SWPs. Daily maximum 8-h average (MDA8) O₃ holds strong negative and positive correlations with relative humidity and sunshine duration, respectively. Among the six SWPs, the concentration and exceedance of MDA8 O₃ reach their maxima under the northerly type on a regional average. Higher O₃ levels, particularly under the northerly and northeasterly types, are primarily related to regional transport on various spatial scales. Local production accounts for the largest proportion of O₃ sources under the anticyclone type. Under the southwesterly type, favourable weather conditions superimposed on regional transport lead to severe O₃ pollution in coastal cities. The impact of super-regional transport is most pronounced along the eastern coast under the cyclone type. Source category analysis shows that transportation and industrial emissions play a prime role in surface O₃ formation. This study illustrates the imperative implementation of joint emission control over eastern China to reduce O₃ pollution.