Wet Deposition Chemistry at a High Elevation Site in the Khumbu Valley (Nepal Himalaya). Implication for the N Biogeochemical Cycle

Among the alpine environments of the world, the region of Mount Everest is a unique ecosystem with a degree of biodiversity among the highest existing, but characterized by a recognized fragility and low resilience. There is a growing concern about the potential effects of global warming and other large scale phenomena (e.g. atmospheric brown cloud) on aquatic undisturbed systems in the Himalayan region. 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 since June 2012 until now, within the GAW programme. The main hydro-chemical species and the stable isotopes of oxygen ( 18O) and deuterium ( 2H) were determined on precipitation samples. The obtained results revealed a very low ionic content, also compared to precipitation sampled in remote sites from the Tibetan Plateau. The temporal variations of NO3 and SO4 exhibited maximum concentrations at the beginning and ending phases of monsoon. A similar trend is observed for the carbonaceous and ionic aerosols, at the same site. Also the isotopic composition is strongly related to the amount of precipitation, withdepleted values characterizing the central monsoon period. Results suggest that during monsoon the air masses reaching the Pyramid site were almost completely depleted of potential contaminants originated from the lower urbanized Asian areas. The low nitrogen concentrations associated with very modest amounts of precipitation have produced nitrogen loads considerably lower than those measured in high-altitude environments in Europe and North America. Higher N loads were reported for other different ecosystems in the most remote regions of the world, as the Northern Africa dry savannah and for the Central Amazonia tropical rain forest. These results will be discussed further considering the nitrogen level in surface waters collected in the high Khumbu valley catchment.