Comprehensive analysis of prevailing weather patterns and high-impact typhoon tracks to reveal where and how tropical cyclone affects regional ozone pollution in the Yangtze River Delta region, China

The Yangtze River Delta (YRD) region is a typical economically developed and densely populated coastal city cluster. During warm seasons, the YRD generally suffers from extreme weather when tropical cyclones (TCs) make landfall. However, the YRD is also affected by severe ozone (O₃) pollution when TCs are not far away. It has been reported that the peripheral flow of TCs is related to the regional O₃ pollution over the YRD. Nevertheless, the tracks and locations of TCs, as well as the mechanisms of their effects, are still unclear. This study tries to reveal how and where TCs cause regional O₃ pollution in the YRD based on a comprehensive analysis of prevailing synoptic weather patterns (SWPs) and high-impact TC tracks. For weather patterns, the NCEP reanalysis data and the Principal Component Analysis in T-mode (PCT) analysis method were adopted to objectively classify the 850 hPa geopotential height field in the YRD from 2018 to 2021. By integrating the air pollution observation data and the meteorological data in the YRD of the same period, a systematic analysis of the O₃ pollution characteristics and their formation mechanism under different SWPs was conducted. Meanwhile, trajectory clustering analysis was conducted on TC track data to investigate the mechanisms of TCs with different paths on O₃ pollution in the YRD. The results indicate that O₃ concentration in the YRD generally peaks in June and September, and regional O₃ pollution usually occurs before TCs land and after TCs dissipate. The SWPs in the YRD can be categorized into seven categories. Among them, SWP3 and SWP4 are SWPs that will be affected by TCs, while SWP6 and SWP7, which have similar circulation situations, are not affected by TCs. TCs are very important weather systems in SWP3 and SWP4. Furthermore, based on O₃ pollution classification under different SWPs and TC track clustering analysis, how TC trajectories and central locations affect regional O₃ pollution over the YRD is revealed. Under SWP3, TCs usually have the northwest-turning track, and cause the highest O₃ pollution levels in the YRD when their centers are located in the area of (130°–135°E, 20°–30°N). Downdrafts caused by western Pacific subtropical high (WPSH) and the peripheral downward airflows of TCs make the air more stagnant over the YRD. The accumulation of air pollutants, as well as more intensive chemical reactions due to higher temperature, are the main cause of regional O₃ pollution. For SWP4, however, TCs generally have the westward track, and regional O₃ pollution in the YRD occurs when TCs are located over the area of (125°–130°E, 10°–16°N) with stronger intensities. More O₃ is transported from inland polluted areas to the YRD due to the peripheral advection airflows of the strong TCs under SWP4. There are fewer downward airflows caused by TCs when the YRD is under SWP4 than under SWP3. These findings provide a new perspective on the intricate mechanisms underlying the interactions between TCs and coastal region O₃ pollution. They also provide an important scientific basis for the prediction, early warning, and targeted prevention and control of severe O₃ pollution in coastal cities, while also offering crucial guidance for optimizing regional air quality management strategies and formulating more precise pollution prevention and control measures.