Modeling tropospheric O₃ evolution during the 2016 Group of Twenty summit in Hangzhou, China

Abstract. To elucidate the factors governing the urban O₃ pollution during the campaign period of 2016 Group of Twenty (G20) summit in China, the Weather Research Forecast with Chemistry (WRF-Chem) model was used to simulate the spatial and temporal O₃ evolution in the Yangtze River Delta (YRD) region from August 24 to September 06, 2016. A unique mechanism was found to modulate the high ozone episodic event. Before the tropical cyclone, a prevailing north wind component brought in emission sources which are favorable for ozone formation. With the invasion of tropical cycle, subsidence air and stagnant weather were induced. Together with local urban heat island effect, there factors intensify ozone pollution in the YRD region. Different atmospheric processes were further analyzed to investigate the control factors of ozone formation through the integrated process rate method. It was found that both the vertical diffusion and the enhancing process of local chemical generation accounted for the growth of surface O3 concentration in Hangzhou. Besides, dynamical circulations of O₃ advection associated with urban heat island effect were observed during the high O₃ episode (August 24–25, 2016), and low O₃ episode on September 5–6, 2016 was mainly resulting from the local chemical consumption. This provides insight into urban O₃ formation and dispersion in East China during the tropical cyclone events.