Intense secondary aerosol formation due to strong atmospheric photochemical reactions in summer: observations at a rural site in eastern Yangtze River Delta of China

High pollution episodes of PM25 and O-3 were frequently observed at a rural site (N31.0935 degrees, E120.978 degrees) in eastern Yangtze River Delta (YRD) in summer. To study the impacts of photochemical reactions on secondary aerosol formation in this region, we performed real-time measurements of the mass concentration and composition of PM2.5, particle size distribution (13.6 similar to 736.5 nm), concentrations of gas pollutants including O-3, SO2, NO2, CO, non-methane hydrocarbons (NMHC)), and nitrate radical in 2013. During the sampling period; the average concentration of PM2.5 was 76.1 (+/- 16.5) mu g/m(3), in which secondary aerosol species including sulfate, nitrate, ammonium, and secondary organic aerosol (SOA) accounted for similar to 62%. Gas-phase oxidation of SO2 was mainly responsible for a fast increase of sulfate (at 1.70 mu g/m(3)/h) in the morning. Photochemical production of nitric acid was intense during daytime, but particulate nitrate concentration was low in the afternoon due to high temperature. At night, nitrate Was mainly formed through the hydrolysis of NO3 and/or N2O5. The correlations among NMHC, O-x (= O-3 + NO2), and SOA suggested that a combination of high emission of hydrocarbons and active photochemical reactions led to the rapid formation of SOA. In addition, several new particle formation and fast growth events were observed despite high ambient aerosol loading. Since the onset of new particle events was accompanied by a rapid increase of H2SO4 and SOA, enhanced formation of sulfate and SOA driven by photochemical oxidation likely promoted the formation and growth of new particles. Together, our results demonstrated that strong atmospheric photochemical reactions enhanced secondary aerosols formation and led to the synchronous occurrence of high concentrations of PM2.5 and O-3 in a regional scale. These findings are important for better understanding the air pollution in summer in YRD. (C) 2016 Elsevier B.V. All rights reserved.