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 (0.4 lmin ) sampler. In Shanghai, SO4 2 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 of SO4 2 in PM2.5 because the SO4 2 concentration was correlated with the SO2 concentration (r 1⁄4 0:66). The relatively stable SO4 2 /SO2 mass ratio over a large range of temperatures suggests that gas-phase oxidation of SO2 played a minor role in the formation of SO4 2 . The sum of SO4 2 and NO3 was highly correlated with NH4 + (r 1⁄4 0:96), but insufficient ammonium was present to totally neutralize the aerosol. In Beijing, SO4 2 , NO3 and NH4 + were also the dominant ionic species, constituting 44%, 25% and 16% of the total mass of water-soluble ions, respectively. Local SO2 emissions were an important source of SO4 2 in the winter since SO4 2 was correlated with SO2 (r 1⁄4 0:83). The low-mass SO4 2 /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 SO4 2 /SO2 (5.6) were observed and were ascribed to in-cloud sulfate formation. The annual average ratio of NO3 /SO4 2 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. r 2002 Elsevier Science Ltd. All rights reserved.