The effect of atmospheric aerosol on the acidity of precipitation

Abstract

Acid rain is one of the most widely publicized and emotional environmental issues of the world today. Although profound literature on the acid rain problem is available, the precise mechanism of the formation of acid rain is still not well understood. It is believed that acid rain is caused by the increasing release of SO2 and NOx to the atmosphere and thir subsequent transformation to sulfate and nitrate, but the actual causes have never been well defined. Acid rain was first observed in Europe by Swedish scientists in early 1950s. Since then it has been observed in many places in the world, e.q. Europe, U.S.A. Canada and Japan. It was reported that the area affected by acid rain and the acidity of rain in the US have been increasing since 1950s, as the release of SO2 and NOx to the atmosphere has been increasing. However,the validity of this conclusion has always been questioned. The critics pointed out that the data used to make the conclusion were collected at different sites for different time periods, and some times with different techniques.[ I] Careful reanalysis of the data shows that if only the data collected at common sites for different time periods were used, no secular trend could be deduced. Observations of a nationwide acid rain observation network of China shows no secular trend for the acidity and acid rain affected area, although the release of SO2 and NO× to the atmosphere in China has been increasing quickly. It was suggested that the occurence of acid rain in the US may be largely due to the reduce of basic particulate matter from coal burning by the shift of energy resources from coal to oil in 1950s. Again, this conclusion has been questioned and has no direct experimental evidence. We report here the results of recent studies on the chemical composition of aerosol and precipitation in Beijing and Chongqing that do show great importance of atmospheric aerosol in controling the acidity of precipitation.