LAUSR

Abstract The concentrations of O 3 and CO at a coastal site of eastern China were analyzed using hourly data from February 2015 to January 2016. Conditional probability function, bivariate correlation, trajectory clustering and potential source contribution function were applied to determine the potential regions contributing to surface O 3 at the study site. The O 3 and CO concentrations reached their maxima in autumn and winter, respectively. Relatively high exceeding rate (8.8%) over the national 8-h O 3 standard of China (75 ppb) was observed, which indicated significant health risks of O 3 exposure to people. The results of all the analyses suggested that the major potential regions contributing to O 3 were located in the coastal and marine areas of eastern China. The highest O 3 levels occurred with relatively high wind speeds (5 m/s), revealing significant contributions of external sources. Based on the cluster analyses of backward trajectories, the air masses associated with the coastal and marine clusters contributed to 32.2% and 37.4% of surface O 3 at the study site, respectively. The domain of major potential region varied with seasons, with larger areas appearing in summer and autumn, and smaller area in winter, which suggested more contributions to the O 3 pollution from distant transport during warm seasons. As to the influence mechanisms on surface O 3 , the spatial position of the study site, mesoscale process and long-range transport of pollutants from northern China might play important roles. Besides, favorable climate conditions (e.g. high temperature) could also significantly contribute to the O 3 pollution during the study period.