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 l 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 of 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 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.