Long-range transport of ozone across the eastern China seas: A case study in coastal cities in southeastern China.

Tropospheric ozone (O) can be transported influenced by large-scale circulation. In this study, an ozone pollution episode in 6 cities of southeastern coastal area of China (SCA) in autumn 2017 was investigated. Compared with the typical local ozone pollution, there was no significant diurnal variations in this pollution episode, the O concentrations maintained a stable level of about 47 ppb continuously. The WRF-CMAQ model as well as the coupled process analysis (PA) and source apportionment modules were used to simulate the formation and transport and quantify the contributions to O. Besides, the HYSPLIT model was used to calculate the backward trajectories arriving in the cities. We find that this pollution was mainly caused by O transport from the eastern China seas (ECS). Under the movement of the Mongolian high-pressure, the O precursors emitted from Beijing-Tianjin-Hebei (BTH), Northeast China (NEC) and Japan-Korea (JK) were transported to ECS then generated O through photochemical reactions. Due to the weak nitrogen oxide titration and the extremely weak ozone deposition on the water surface, O concentrations maintained high during the movement of air masses over ECS and finally affected SCA after long-range transport. The contributions of horizontal advections were significant basically all the day with hourly contribution about 10 ppb hr and extended from surface to 500 m above the ground level. JK contributed the most, with multi-days averaged contribution about 5 ppb and peak up to 30 ppb. The contributions of BTH and NEC were comparable, with average about 2 ppb and hourly peak of 19 and 10 ppb, respectively. For the first time, this study clearly shows that the O precursors emitted from northern China and Japan-Korea contribute to the O pollution in SCA under certain weather conditions, which will help to better understand and predict the O pollution in that area.