The Editor,

Journal of Glaciology

Sir,

The physical and chemical analyses of ice cores recovered from glaciers in the Himalaya provide some of the best records of past climate change in the region (e.g. Reference DaheQin and others, 2000; Reference Thompson, Yao, Mosley-Thompson, Davis, Henderson and LinThompson and others, 2000; Reference Shichang, Dahe, Mayewski, Wake and JiawenKang and others, 2001, Reference Kang2002). In order to better understand the climatic and environmental records preserved in snow and ice, studies have investigated the precipitation chemistry in the high Himalaya, notably that of the northern slopes of the central Himalaya (Reference Mayewski, Lyons, Spencer and ClaytonMayewski and others, 1986; Jenkins and others, 1987) and the southern slopes of the central Himalaya (Reference Shrestha, Wake and DibbShrestha and others, 1997; Reference Marinoni, Polesello, Smiraglia and ValsecchiMarinoni and others, 2001). Short-term aerosol/fresh-snow chemistry sampling during monsoon and post-monsoon seasons has shown low pollutant concentrations, suggesting these Himalayan sites are representative of the remote troposphere (Reference Wake, Dibb, Mayewski, Li and XieWake and others, 1994; Reference Shrestha, Wake and DibbShrestha and others, 1997; Reference Marinoni, Polesello, Smiraglia and ValsecchiMarinoni and others, 2001). Here we present chemistry data of fresh snow sampled from the northern slope of Xixabangma peak (28°33′ N, 85°44′ E), central Himalaya, during the 1997 summer. The main purpose is to expand the fresh-snow chemistry database for the high mountain regions in the Himalaya.

During August and September 1997, fresh-snow samples were collected within 12 hours of deposition at two camps (5400 and 5800 m a.s.l.) and along the climbing route (5400–7000 m a.s.l.) in the Dasuopu glacier region on the northern slope of Xixabangma peak. Methods of sampling were the same as those used by Reference Mayewski, Lyons, Spencer and ClaytonMayewski and others (1986) and Reference Wake, Dibb, Mayewski, Li and XieWake and others (1994). Care was taken during sample collection and transfer to avoid contamination (Reference Shichang, Wake, Dahe, Mayewski and TandongKang and others, 2000). Oxygen isotope, pH and conductivity analysis were performed using a Finnigan MAT-252 spectrometer (accuracy of 0.5‰), pH meter (model PHS-2) and conductivity meter (model DDS-11A), respectively, at the Laboratory of Ice Core and Cold Regions Environment, Chinese Academy of Sciences (CAS). Ion concentrations were measured at the Climate Change Research Center, University of New Hampshire, U.S.A., using a Dionex ion chromatograph model 2010.

The mean, maximum and minimum values of ion concentrations are shown in Table 1. The mean Na⁺/Cl⁻ ratio of 0.85 in snow is close to that of 0.86 found in sea water, suggesting that Cl⁻ and Na⁺ mainly come from sea salt during the summer monsoon season. From Table 1, the ion balance ΔC (sum of cations − sum of anions) equals 3.55 μeq L⁻¹, indicating that there is an excess of cations. Studies from the Tien Shan and Qomolangma (Mount Everest; about 100 km away from Xixabangma) suggest that the ΔC term is made up primarily of CO₃ ²⁻/HCO₃ ⁻ (Reference Williams, Tonnessen, Melack and DaqingWilliams and others, 1992; Reference Wake, Dibb, Mayewski, Li and XieWake and others, 1994). Generally the excess ΔC may indicate that the atmosphere in this region is alkaline as has been noted previously (Reference Galloway, Zhao, Xiong and LikensGalloway and others, 1987; Reference Wake, Dibb, Mayewski, Li and XieWake and others, 1994).

Table 1. Chemical composition of fresh snow from Xixabangma during the summer monsoon season

Chemistry data from fresh-snow samples collected in high-altitude regions in the Himalaya and other remote regions are presented in Table 2. Ion concentrations in summer fresh snow from our work are similar to those from the southern slopes of Qomolangma (Reference Marinoni, Polesello, Smiraglia and ValsecchiMarinoni and others, 2001), indicating that the atmospheric environment is consistent on both slopes of the central Himalaya during the summer monsoon season. Ion concentrations (e.g. Ca²⁺, S0₄ ²⁻) in fresh summer snow are lower than those in fresh spring snow from the same region. This observation is consistent with results from firn-core samples (Reference Shichang, Wake, Dahe, Mayewski and TandongKang and others, 2000), suggesting snow chemistry over the central Himalaya is influenced by Asia dust storms during spring (Reference Wake, Dibb, Mayewski, Li and XieWake and others, 1994). Compared to other remote regions in the Qinghai−Tibetan Plateau (from the Himalaya to the north), ion concentrations in fresh snow are also much lower on Xixabangma. Higher ion concentrations in summer snow over the central and northern plateau may be related to the local arid conditions (more dust aerosols) (Reference Longyuan, Yanxia and LiYang and others, 1991). Na⁺ and Cl⁻ concentrations in fresh snow on Xixabangma are two orders of magnitude lower than those of remote marine regions, such as Amsterdam Island in the Indian Ocean (Reference Galloway, Likens, Keene and MillerGalloway and others, 1982), indicating that the depletion of sea-salt aerosol is very effective during the transport of marine air masses from the Indian Ocean to the Himalayan range. The mean NH₄ ⁺ concentration is similar to those from other remote regions, but S0₄ ²⁻, NO₃ ⁻ and crustal ion concentrations are lower than background values reported from other remote regions of the world (Reference Galloway, Likens, Keene and MillerGalloway and others, 1982). This indicates that the Xixabangma region is minimally influenced by anthropogenic emissions during the summer monsoon season.

Table 2. Comparison of chemical composition of fresh snow from Xixabangma with those from other remote regions of the world

Conductivity of the snow ranged from 1.2 to 19.1 μS cm⁻¹, with a mean value of 3.6 μS cm⁻¹ (Table 1; Fig. 1). The consistent chemistry of precipitation during the summer monsoon season is indicated by the narrow conductivity range (0.7–4.5 μS cm⁻¹) of 80% of our data and by the low variability (standard deviation of 3.0). Using data from remote regions of the world, Reference Galloway, Likens, Keene and MillerGalloway and others (1982) estimated that the low limit of the natural mean pH of precipitation is 5.0. The mean pH of 6.0 and very low frequency of pH < 5.0 (Fig. 1) in our samples also suggest that the atmosphere around Xixabangma peak is minimally influenced by anthropogenic-acids emissions (sulfate, nitrate and others) during the summer monsoon season. The polynomial regression curve of pH vs conductivity shows a strong correlation between the two variables (Fig. 2). The minimum conductivity occurs when pH = 6.0, and the negative and positive correlation between the two parameters occurs when pH < 6.0 and pH > 6.0, respectively. This indicates that the dominant chemical species of precipitation are changing between acid anions and crustal cations during the summer monsoon season in the Xixabangma peak region (Reference Galloway, Zhao, Xiong and LikensGalloway and others, 1987). Specifically, acid anions are dominant when pH < 6.0, and crustal cations are dominant when pH > 6.0.

Fig. 1. Frequencies of pH (upper) and conductivities (lower) of fresh snow during the summer monsoon season from Xixabangma.

Fig. 2. The polynomial regression between PH and conductivity of fresh snow during the summer monsoon season from Xixabangma.