Anomalous high ozone in the Pearl River Delta, China in 2019: A cause attribution analysis

ATMOSPHERIC ENVIRONMENT(2024)

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摘要
Surface ozone (O-3) pollution in the Pearl River Delta (PRD), especially persistent O-3 pollution episodes lasting three days or longer (OPE3), showed an anomalous peak during the autumn of 2019 (bulge-2019). Our research reveals a robust connection between the occurrence of OPE3 events and days with low cloud cover. Specifically, low-cloud days comprised 55% of the total number of OPE3 events in the PRD during the autumn season spanning from 2015 to 2021. Notably, as much as 74% of the bulge-2019 is related to low-cloud days. However, establishing a direct causal link remains challenging. In autumn 2019, an anomalous cyclonic center developed over the Philippine Sea, with anomalous northerly winds on its northwestern flank, leading to anomalous downdraft over South China. This pattern was characterized by stagnant, low relative humidity, intensified solar radiation, and low cloud cover conditions that were conducive to the photochemical formation and accumulation of O-3. Further analysis of the relationship between low-cloud days and El Nino-Southern Oscillation shows that eastern Pacific El Ni & ntilde;o events are associated with fewer low-cloud days in the PRD in autumn, whereas central Pacific (CP) El Ni & ntilde;o events result in more low-cloud days. In addition, the anomalous cyclonic center over the Philippine Sea serves as the crucial atmospheric circulation system connecting CP El Ni & ntilde;o and high O-3 level over southern China. Therefore, we propose that the increased downdraft of PRD brought by northward tropical cyclones (TCs) during more low-cloud days in 2019 is highly likely the primary cause of the occurrence of the bulge-2019.
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关键词
Persistent ozone pollution episodes,ElNino-Southern oscillation,Central Pacific ElNino,Downdrafts
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