Process analysis of regional ozone formation over the Yangtze River Delta, China using the Community Multi-scale Air Quality modeling system

Abstract

Abstract. High ozone concentration has become an important issue in summer in most economically developed cities in Eastern China. In this paper, observations at an urban site within the Shanghai city are used to examine the typical high ozone episodes in August 2010, and the MM5-CMAQ modeling system is then applied to reproduce the typical high ozone episodes. In order to account for the contribution of different atmospheric processes during the high pollution episodes, the CMAQ integrated process rate (IPR) is used to assess the different atmospheric dynamics in rural and urban sites of Shanghai, Nanjing and Hangzhou, which are typical cities of the Yangtze River Delta (YRD) region. In order to study the contributions of the main atmospheric processes leading to ozone formation, vertical process analysis in layer 1 (0–40 m), layer 7 (350–500 m), layer 8 (500–900 m) and layer 10 (1400–2000 m) has been considered. The observations compare well with the results of the numerical model. IPR analysis shows that the maximum concentration of ozone occurs due to transport phenomena, including vertical diffusion and horizontal advective transport. The gas-phase chemistry producing O3 mainly occurs in the height of 300–1500 m, causing a strong vertical O3 transport from upper levels to the surface layer. The gas-phase chemistry is an important sink for O3 in the surface layer, coupled with dry deposition. The cloud processes, horizontal diffusion and heterogeneous chemistry contributions are negligible during the whole episode. In the urban Shanghai area, the average O3 production rates contributed by vertical diffusion and horizontal transport are 24.7 ppb h−1, 3.6 ppb h−1, accounting for 27.6% and 6.6% of net surface O3 change, respectively. The average contributions of chemistry, dry deposition and vertical advective transport to O3 production are −21.9, −4.3 and −2.1 ppb h−1, accounting for −25.3%, −5.0% and −3.7% of net O3 change, respectively. In the suburban and industrial areas of Shanghai, net transport accounts for 26.3% and chemical reaction for −11.1% of net surface O3 change. At the Nanjing site, the net transport accounts for 9% and chemical reaction for −32%. However, at the heights of 350–500 m and 500–900 m, during the time period of 10:00–15:00 LST, photochemistry plays the most important role in net O3 production, with the highest positive contributions from gas-phase chemistry to net O3 production reaching 87.3% and 68.6%, respectively, and making a strong vertical O3 transport from upper levels to the surface layer. At the Hangzhou site, the net transport accounts for 9% and chemical reaction for −9% of the net O3 change. Modeling results show that the O3 pollution characteristics among the different cities in the YRD region have both similarities and differences. During the buildup period (usually from 08:00 in the morning after sunrise), the O3 starts to appear in the city regions like Shanghai, Hangzhou, Ningbo and Nanjing and is then transported to the surrounding areas under the prevailing wind conditions. The O3 production from photochemical reaction in Shanghai and the surrounding area are 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 O3 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.