Transport of atmospheric impurities over the Qinghai–Xizang (Tibetan) Plateau as shown by snow chemistry

Abstract

Chemical records were recovered from samples collected from snow pits over two transects (S–N and SE–NW) across the Qinghai–Xizang (Tibetan) Plateau. The records show that the flux of major ions in the regions to the north of Mt Tanggula is, on the average, 6–30 times higher than that of the regions to the south, and 0.6–5 times higher than regions to the southeast. It appears that Mt Tanggula provides a barrier to continental dust transported across the plateau from the north and to marine aerosols transported from the southeast. The spatial patterns of snow chemistry as well as the total suspended particle (TSP) distribution along the northern Qinghai–Xizang Highway demonstrate that continental dust originates mainly from the deserts to the north, while monsoon sources of moisture exert greater influence on snow chemistry on the southern part of the plateau.Seasonal variations of the major ions were also examined in a firn core drilled at a height of 7000m on the Dasuopu Glacier and in a snow pit from the Meikuang Glacier (eastern Kunlun). The choice of a high altitude site at Dasuopu, and the sampling season (before snowmelt) at Meikuang, avoided post-depositional modification processes (mainly the elution effect of meltwater), thus guaranteeing the reliable recovery of the distribution of impurities in different seasons. The peak of the impurities appear at Dasuopu during the spring and at Meikuang in the winter/spring, coinciding with the Asian dust season. However, the amplitude of the seasonal variations at Meikuang is much higher than at Dasuopu.pH values at the Dongkemadi Glacier are greater than 6, but at the Dasuopu Glacier (Himalayas) they range between 4 and 7.5. The liquid conductivity (LC) from Dongkemadi is positively correlated with the pH, while at Dasuopu, LC is positively correlated with pH when pH is below 6 and negatively correlated with pH when it is greater than 6. This implies that during August to September the Himalayan range is a transition zone between the transportation of continental dust and marine aerosols.