Abstract. Landfall typhoons can significantly affect O3 in the Yangtze River Delta (YRD) region. In this study, we investigate a unique case characterized by two multiday regional O3 pollution episodes related to four successive landfall typhoons in the summer of 2018 in the YRD. The results show that O3 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 O3 were related to the track, duration and landing intensity of the typhoons. The impact of typhoons on O3 was like a wave superimposed on the background of high O3 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 O3 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 O3 due to O3-rich air transported from the low stratosphere and strong photochemical reactions. It is noteworthy that O3 was mainly generated in the middle of the boundary layer (∼ 1000 m) instead of at the surface. High O3 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, O3 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 airflows and light wind, respectively. The premature deaths attributed to O3 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 O3 in the YRD and thus can be useful in forecasting O3 pollution in regions strongly influenced by typhoon activities.

Abstract. Landfall typhoon can significantly affect O3 in the Yangtze River Delta (YRD) region. In this study, we investigate a unique case characterized by two multiday regional O3 pollution episodes related to four successive landfall typhoons in the summer of 2018 in the YRD. The results show that O3 pollution episodes mainly occurred during the period from the end of typhoon and the arrival of the next typhoon. The moment that typhoon reached the 24-h warning line and the last moment of typhoon activity in the mainland China can be roughly regarded as time nodes. Meanwhile, the variations of O3 was related to the track, duration and landing intensity of the typhoons. The impact of typhoon on O3 was like a wave superimposed on the background of high O3 concentration in the YRD in summer. When typhoon was near the 24-h warning line before it landed the coast line of the YRD, the prevailing wind originally from the ocean changed to from the inland, and transported lots of precursors from the polluted areas to the YRD. With typhoon, the low temperature, strong upward airflows, more precipitation and wild wind prevented high O3 episodes. After typhoon, the air below the 700 hPa atmospheric layer was warm and dry, which was conductive to the formation of O3 from the abundance of precursors. It is note-worthy that O3 is mainly generated in the middle of boundary layer (~ 1000 m), and then transported to the surface by downward airflows or turbulences. Moreover, O3 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 airflows and light wind, respectively. The premature mortalities attributed to O3 exposure in the YRD during the study period is 194.0, more than the casualties caused directly by the typhoons. This work enhances our understanding of how landfall typhoons affect O3 in the YRD, which can be helpful to forecast the O3 pollution synthetically impacted by the subtropical high and typhoon.