[Ionic chemistry in snowpits from Yamzhog Yumco Basion]

During August and September, 2006, a total of 50 samples had been collected from three different snowpits at the Yamzhog Yumco Basion in the south of the Tibetan Plateau. All samples were analyzed for major cations (Na+, NH4+, K+, Ca2+ and Mg2+), anions (Cl-, SO4(2-) and NO3-) and stable oxygen isotope ratio. The results of analyses show that the three snowpits represent accordant chemical characteristics, with NO3- (16.1-187.2 microg x L(-1), averaging at 93.7 microg x L(-1)) and Ca2+ (19.0-236.7 microg x L(-1), averaging at 81.0 microg x L(-1)) being the highest concentration of anions and cations respectively. Compared with data from other representative sites, major ion concentrations in the Yamzhog Yumco Basion accord with those in the south of the plateau, but they differ much from those in the north of th eplateau. Remarkable variabilities of major ion concentrations from monsoon period to non-monsoon period are demonstrated. Ion concentrations of NO3-, NH4+ increase 30%-40% in monsoon period due to the influences of vegetation, live-stock, anthropogenic activity and thunderstorm, whilst the concentrations of crustal source ions, such as Ca2+, Mg2+ reduce 80% due to decrease of dust and strong wind from the north of the plateau and crustal aerosols being washed out of the atmosphere by heavy precipitation during the monsoon period. Variation of ion concentrations are also impacted by elevation and post-deposition process, with Ca2+, Mg2+ increasing with a decrease in elevation and SO4(2-), NO3- decrease with an increase in elevation and the influence of post-deposition.