Precipitation chemistry of Lhasa and other remote towns, Tibet

Precipitation event samples during 1987-1988 field expedition periods and 1997, 1998, 1999 and 2000 have been collected at Lhasa, Dingri, Dangxiong and Amdo, Tibet. The sampling and analysis were based on WMO recommendations for a background network with some modifications according to local conditions and environmental characteristics, which include the following precipitation constituents and related parameters: pH, conductivity, CO2 partial pressure, total suspended particles, and the content of K, Na, Ca, Mg, Fe, Mn, NH, Cl, NO2, NO3, SO4 Br, HCO3 and HPO4. Some atmospheric dusts samples have also been collected. Over 300 hundred precipitation events have been measured for pH and conductivity. Among these, 60 have been analysed for their chemical components. The results show that Lhasa’s precipitation events are constant alkaline with weighted averages of pH 8.36 in the 1987-1988 period, and 7.5 for 1997 to 1999. Only one event is weakly acidic during 1997-1999. Although CO2 partial pressure, a major producer of acidity in natural water on the Plateau, reduces with rising elevation, the lowest measure CO2 pressure can only raise pH value by 0.1 units in the sampling areas. Chemical analysis indicates that the major contributor to alkaline precipitation is the continental dust, which is rich in calcium. The analysis also shows that Tibet is still one of the cleanest areas in the world with little air pollution. However, the decline of pH value from the 80s to 90s, which was reflected by an increase of NO3 and SO4 in precipitation, alert us the urgency of environmental protection in this fragile paradise.