Wet deposition at the base of Mt Everest (5050 m asl, Nepal Himalaya): seasonal evolution of the chemistry and isotopic composition.

Perceived as one of the most uncontaminated places of the Earth, the Himalayas host a unique ecosystem with a high degree of biodiversity, but characterized by a recognized fragility that makes this region most sensitive to any environmental change. Recent studies in the southern slopes of Mt. Everest revealed a significant temperature increase in post monsoon period and a substantial weakening of rainfall during monsoon season, with important consequences on glacier shrinkage. In this context, the chemistry of wet deposition was investigated at the Pyramid International Laboratory in the northernmost portion of the Khumbu Valley, at 5050 m asl, during 2007-2008, and 2012-2013, within the GAW programme. The main hydro-chemical species and the stable isotopes of oxygen and deuterium were determined on monsoon and snow samples. Precipitation contents for all ions were the lowest ones among those measured in high elevation sites around the world suggesting that these values can be considered as the regional background concentrations in the depositions. During the monsoon periods the depositions were not substantially influenced by anthropogenic inputs. However, in the late monsoon phase the ionic content in precipitation increased; possibly due to a change in the moisture source. The analysis of snow samples supports this hypothesis, revealing SO42- and NO3- concentrations 15 and 5 folds those measured in rain samples. The low nitrogen concentrations associated with very small amounts of precipitation have produced nitrogen loads considerably lower than those measured in the most remote regions of the world (e.g. European and North American mountain environments, Northern Africa dry savanna, Central Amazonia tropical rain forest). These results will be discussed further considering the nitrogen level in the surface water collected in the high Khumbu valley catchment.