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 (0.4lmin^-1) 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=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=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=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. Copyright © 2002 Elsevier Science Ltd.