Process analysis of regional ozone formation over the Yangtze River Delta, China using the Community Multi-scale Air Quality modeling system
A high O-3 episode was detected in urban Shanghai, a typical city in the Yangtze River Delta (YRD) region in August 2010. The CMAQ integrated process rate method is applied to account for the contribution of different atmospheric processes during the high pollution episode. The analysis shows that the maximum concentration of ozone occurs due to transport phenomena, including vertical diffusion and horizontal advective transport. Gas-phase chemistry producing O-3 mainly occurs at the height of 300-1500 m, causing a strong vertical O-3 transport from upper levels to the surface layer. The gas-phase chemistry is an important sink for O-3 in the surface layer, coupled with dry deposition. Cloud processes may contribute slightly to the increase of O-3 due to convective clouds or to the decrease of O-3 due to scavenging. The horizontal diffusion and heterogeneous chemistry contributions are negligible during the whole episode. Modeling results show that the O-3 pollution characteristics among the different cities in the YRD region have both similarities and differences. During the buildup period, the O-3 starts to appear in the city regions of the YRD and is then transported to the surrounding areas under the prevailing wind conditions. The O-3 production from photochemical reaction in Shanghai and the surrounding area is most significant, due to the high emission intensity in the large city; this ozone is then transported out to sea by the westerly wind flow, and later diffuses to rural areas like Chongming island, Wuxi and even to Nanjing. The O-3 concentrations start to decrease in the cities after sunset, due to titration of the NO emissions, but ozone can still be transported and maintain a significant concentration in rural areas and even regions outside the YRD region, where the NO emissions are very small.