The water-soluble ionic composition of PM2.5 in Shanghai and Beijing, China

A year-long field study to characterize the ionic species in PM2.5 was carried out in Shanghai and Beijing, China, in 1999-2000. Weekly samples of PM2.5 were collected using a special low flow rate (0.41 min(-1)) sampler. In Shanghai, SO42- NO3- and NH4+ were the dominant ionic species, which accounted for 46%, 18% and 17% of the total mass of ions respectively. Local SO2 emissions were an important source SO42- in PM2.5 because the SO42- concentration was correlated with the SO2 concentration (r = 0.66,). The relatively stable SO42-/SO2 mass ratio over a large range of temperatures suggests that gas-phase oxidation of SO2 played a minor role in the formation of SO42-. The sum of SO42- and NO3- was highly correlated with NH4+ (r = 0.96), but insufficient ammonium was present to totally neutralize the aerosol. In Beijing, SO42-, NO3- and NH4+ were also the dominant ionic species, constituting 44%, 25% and 16% of the of total mass of water-soluble ions, respectively. Local SO2 emissions were an important source of SO42- in the winter since SO42- was correlated with SO2 (r = 0.83). The low-mass SO42-/SO2 ratio (0.27) during winter, which had low humidity, suggests that gas-phase oxidation of SO2 was a major route of sulfate formation. In the summer, however, much higher mass ratios of SO42-/SO2 (5.6) were observed and were ascribed to in-cloud sulfate formation. The annual average ratio of NO3-/SO42- was 0.4 and 0.6 in Shanghai and in Beijing, respectively, suggesting that stationary emissions were still a dominant source in these two cities. (C) 2002 Elsevier Science Ltd. All rights reserved.