Analysis on Chemical Composition of Rainfall on the Tibet Plateau

Abstract

Some rainfall and dustfall samples were collected during 1987-1988 field expedition periods and in 1997,1998,1999 and 2000 at Lhasa,Dingri,Dangxiong and Amdo,Tibet.The parameters of rainfall,such as the pH values,conductivity,CO2 partial pressure,content of total suspended particles,and concentrations of K+,Na+,Ca2+,Mg2+,Fe,Mn,NH4+,Cl-,NO2-,NO3-,SO42-,Br-,HCO3-and HPO42-,were measured.Some atmospheric dust samples were also collected.The pH values and conductivity of more than 300 rainfall events were measured,in which the chemical composition of 60 rainfall events were analyzed.The results show that rainwater in Lhasa was constantly alkaline with weighted average pH value of 8.36 during the period of 1987-1988 and 7.5 during the period of 1997-1999.Only one rainfall event was weakly acidic during 1997-1999.Although CO2 partial pressure,a major producer of acidity in natural water on the Plateau,falls with increasing elevation, the lowest measured CO2 partial pressure can only raise pH value by 0.1 in the sampling areas.The analysis on chemical composition reveals that the major contributor to alkaline rainfall is 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 decrease of pH value of rainwater from the 1980s to 1990s,which was reflected by an increase of NO3-and SO42- in rainwater,awakes us the urgency of environmental protection in this fragile paradise.

1987-1988年及1997-2000年間,在西藏拉薩、定日、當雄、安多采集降雨樣品及部分大氣浮塵樣品,分析測定了pH、電導率、CO2分壓、總懸浮顆粒含量及10多種離子濃度。共測量了300場次降雨的pH值、電導率和其中60場次降雨的化學成分含量。結果顯示:青藏高原降雨常呈弱堿性,1987-1988年間加權平均pH值為8.36,1997-1999年間為7.5。造成高原降雨呈弱堿性的主要原因是來自地面富含鈣的粉塵。西藏是世界上空氣污染最輕的地方,但是從20世紀80~90年代的降雨中,NO3-與SO42-含量增加,pH值降低,表明保護這片脆弱環境的緊迫性。