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

Severe ozone (O) pollution in China has magnified global concerns over the past years. Exploring the synoptic impacts and quantifying the source contributions are important for mitigating O pollution. This study focuses on the summertime O 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 O source attribution based on ozone source apportionment technology (OSAT) in a regional transport model. Surface O pollution is found to be sensitive to the predominant SWPs, including four O-polluted types (northeasterly, northerly and southwesterly and anticyclone) and two O-clean types (cyclone and the Meiyu front). The integrated influences of local chemistry and regional transport are important in O pollution under different SWPs. Daily maximum 8-h average (MDA8) O holds strong negative and positive correlations with relative humidity and sunshine duration, respectively. Among the six SWPs, the concentration and exceedance of MDA8 O reach their maxima under the northerly type on a regional average. Higher O 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 O sources under the anticyclone type. Under the southwesterly type, favourable weather conditions superimposed on regional transport lead to severe O 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 O formation. This study illustrates the imperative implementation of joint emission control over eastern China to reduce O pollution.