Increasing surface ozone and enhanced secondary organic carbon formation at a city junction site: An epitome of the Yangtze River Delta, China (2014-2017).

This study aims to understand the characteristics of surface ozone (O), search for factors affecting the variations in its concentration, and estimate its impacts on the secondary organic carbon (SOC) levels and atmospheric oxidation capacities in the Yangtze River Delta (YRD). Four years of continuous observations (2014-2017) of the surface O, organic carbon, elemental carbon, nitrogen oxides, PM and meteorological factors along with three years of measurements (2015-2017) of the concentrations of 56 volatile organic compounds were conducted at a rural site. Our measurements showed that the total number of O pollution days more than doubled over the four-year period, from 28 days in 2014 to 76 days in 2017. The annual mean of the maximum daily 8-h average O concentration during the months with the strongest solar radiation (July-September) showed a 6.8% growth rate, from 124.5 (2014) to 149.8 μg m (2017). Regional transport was shown to be the dominant contributor to the high level of O based on a process analysis of the O variation using the Weather Research and Forecasting-Community Multiscale Air Quality model for this site. The simulation results indicated that the city junction site served well as an epitome of the regional background of the YRD. We also found that the level of SOC, which is a major component of PM that results from atmospheric oxidizing processes, gradually increased with the increase in the surface O level, even though the overall PM concentration significantly decreased each year. There was an increasingly strong correlation between SOC and O (O + nitrogen dioxide) during both the daytime and night-time from 2014 to 2017 when the highest annual O concentration was observed. These findings imply that the atmospheric oxidation capacity increased and likely contributed to the SOC production in the YRD during 2014-2017.