Ozone affected by a succession of four landfall typhoons in the Yangtze River Delta, China: major processes and health impacts

Landfall typhoons can significantly affect O-3 in the Yangtze River Delta (YRD) region. In this study, we investigate a unique case characterized by two multiday regional O-3 pollution episodes related to four successive landfall typhoons in the summer of 2018 in the YRD. The results show that O-3 pollution episodes mainly occurred during the period from the end of a typhoon to the arrival of the next typhoon. The time when a typhoon reached the 24 h warning line and the time when the typhoon dies away in mainland China can be roughly regarded as time nodes. Meanwhile, the variations of O-3 were related to the track, duration and landing intensity of the typhoons. The impact of typhoons on O-3 was like a wave superimposed on the background of high O-3 concentration in the YRD in summer. When a typhoon was near the 24 h warning line before it landed on the coastline of the YRD, the prevailing wind originally from the ocean changed to be from inland, and it transported lots of precursors from the polluted areas to the YRD. Under influences of the typhoon, the low temperature, strong upward airflows, more precipitation and wild wind hindered occurrences of high O-3 episodes. After the passing of the typhoon, the air below the 700 hPa atmospheric layer was warm and dry, and the downward airflows resumed. The low troposphere was filed with high concentration of O-3 due to O-3-rich air trans- ported from the low stratosphere and strong photochemical reactions. It is noteworthy that O-3 was mainly generated in the middle of the boundary layer (similar to 1000 m) instead of at the surface. High O-3 levels remained in the residual layer at night, and would be transported to the surface by downward airflows or turbulence by the second day. Moreover, O-3 can be accumulated and trapped on the ground due to the poor diffusion conditions because the vertical diffusion and horizontal diffusion were suppressed by downward air-flows and light wind, respectively. The premature deaths attributed to O-3 exposure in the YRD during the study period were 194.0, more than the casualties caused directly by the typhoons. This work has enhanced our understanding of how landfall typhoons affect O-3 in the YRD and thus can be useful in forecasting O-3 pollution in regions strongly influenced by typhoon activities.