The chemistry of precipitation and its relation to aerosol in Beijing

Fifty-three rain events throughout the entire year of 2003 were collected in Beijing and the concentration of 15 major ions with the pH of the rainwater were analyzed. The close correlations of the acidity in rainwater with the concentrations of aerosol and gases in the air were observed and elaborated. The scavenging of the pollutants from the air directly affects the pH and composition of the rainwater. The major ions in rainwater in Beijing were SO42−, NO3−, Cl−, NH4+, Ca2+, Mg2+, K+, and F−. The alkaline cations, NH4+ and Ca2+, acted as acid neutralizers and buffered the acidity of rain. The temporal variation of those major pollution ions in rainwater was similar to that of the air pollution. The concentrations of SO42−, NO3−, and NH4+ have increased for the last two decades and likely indicated that the air pollution has become more and more serious in Beijing since the 1980s. The ratio of the concentration of SO42− to NO3− observed in 2003 was much lower than that in 1981, indicating that the air pollution resulting from the traffic emission, NOx, has been more significant. A remarkably positive correlation between pH and Ca2+ concentration illustrates that Ca2+ generally from mineral aerosols plays an important role in buffering rainwater acidity in Beijing and in northern China at all times. The concentrations of SO42− and NO3−, the predominant acid anions in rain, was much higher in Beijing than other cities, suggesting that the anthropogenic air pollution in Beijing has been a much more serious problem than other places.