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

• Main Authors: C. Zhan, M. Xie, C. Huang, J. Liu, T. Wang, M. Xu, C. Ma, J. Yu, Y. Jiao, M. Li, S. Li, B. Zhuang, M. Zhao, D. Nie
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-11-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/20/13781/2020/acp-20-13781-2020.pdf

<p>Landfall typhoons can significantly affect O<span class="inline-formula">₃</span> in the Yangtze River Delta (YRD) region. In this study, we investigate a unique case characterized by two multiday regional O<span class="inline-formula">₃</span> pollution episodes related to four successive landfall typhoons in the summer of 2018 in the YRD. The results show that O<span class="inline-formula">₃</span> 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&thinsp;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<span class="inline-formula">₃</span> were related to the track, duration and landing intensity of the typhoons. The impact of typhoons on O<span class="inline-formula">₃</span> was like a wave superimposed on the background of high O<span class="inline-formula">₃</span> concentration in the YRD in summer. When a typhoon was near the 24&thinsp;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<span class="inline-formula">₃</span> episodes. After the passing of the typhoon, the air below the 700&thinsp;hPa atmospheric layer was warm and dry, and the downward airflows resumed. The low troposphere was filed with high concentration of O<span class="inline-formula">₃</span> due to O<span class="inline-formula">₃</span>-rich air transported from the low stratosphere and strong photochemical reactions. It is noteworthy that O<span class="inline-formula">₃</span> was mainly generated in the middle of the boundary layer (<span class="inline-formula">∼</span>&thinsp;1000 m) instead of at the surface. High O<span class="inline-formula">₃</span> 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<span class="inline-formula">₃</span> 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 O<span class="inline-formula">₃</span> 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<span class="inline-formula">₃</span> in the YRD and thus can be useful in forecasting O<span class="inline-formula">₃</span> pollution in regions strongly influenced by typhoon activities.</p>