Severe high ozone (O<sub>3</sub>) episodes usually have close relations to synoptic systems. A regional continuous O<sub>3</sub> pollution episode is detected over the Yangtze River Delta (YRD) region in China during August 7&ndash;12, 2013, in which the O<sub>3</sub> concentrations in more than half of the cities exceeding the national air quality standard. The maximum hourly concentration of O<sub>3</sub> reaches 167.1&thinsp;ppb. By means of the observational analysis and the WRF/CMAQ numerical simulation, the characteristics and the essential impact factors of the typical regional O<sub>3</sub> pollution is integratedly investigated. The observational analysis shows that the atmospheric subsidence dominated by Western Pacific subtropical high plays a crucial role in the formation of high-level O<sub>3</sub>. The favorable weather conditions, such as extremely high temperature, low relative humidity and weak wind speed, caused by the abnormal strong subtropical high are responsible for the trapping and the chemical production of O<sub>3</sub> in the boundary layer. In addition, when the YRD cities at the front of Typhoon Utor, the periphery circulation of typhoon system can enhance the downward airflows and cause worse air pollution. But when the typhoon system weakens the subtropical high, the prevailing southeasterly surface wind leads to the mitigation of the O<sub>3</sub> pollution. The Integrated Process Rate (IPR) analysis incorporated in CMAQ is applied to further illustrate the combined influence of subtropical high and typhoon system in this O<sub>3</sub> episode. The results show that the vertical diffusion (VDIF) and the gas-phase chemistry (CHEM) are two major contributors to O<sub>3</sub> formation. During the episode, the contributions of VDIF and CHEM to O<sub>3</sub> maintain the high values over 10&thinsp;ppb/h in Shanghai, Hangzhou, and Nanjing. On August 10&ndash;11, the cities close to the sea are apparently affected by the typhoon system, with the contribution of VDIF increasing to 28.45&thinsp;ppb/h in Shanghai and 19.76&thinsp;ppb/h in Hangzhou. When the YRD region is under the control of the typhoon system, the contribution values of all individual processes decrease to a low level in all cities. These results provide an insight for the O<sub>3</sub> pollution synthetically impacted by the Western Pacific subtropical high and the tropical cyclone system.
Shu, L., Xie, M., Wang, T., Gao, D., Chen, P., Han, Y., … Li, M. (2016). Integrated studies of a regional ozone pollution synthetically affected by subtropical high and typhoon system in the Yangtze River Delta region, China. Atmospheric Chemistry and Physics, 16(24), 15801–15819. https://doi.org/10.5194/acp-16-15801-2016