Keywords:Atmospheric nitrate, the Everest, Isotope, Aerosol, Oxidation pathways
Studies on the sources and formation pathways of nitrate is crucial to better understand nitrogen cycle and atmospheric oxidation capacity. Tibetan Plateau (TP) and surroundings, referred to as the Third Pole and “Asia water tower”, contain the largest number of glaciers outside the polar regions and are largely experiencing shrinkage. A complete understanding on the source and formation mechanisms of atmospheric nitrate in the TP is important to indicate the impact of human activities on the environment and also crucial to investigate nitrogen cycle in glacier-hydrology systems of this region. Besides, it can supply information on interpreting ice core records to understand the past atmosphere chemistry. Here we collected aerosol samples from the Everest through pre-monsoon (April-May) and monsoon season (June-August) in 2018, and measured for the first time the isotopic compositions (δ15N, δ18O and Δ17O) of nitrate in this region, allowing insights into their sources and potential formation mechanism. The results show that the seasonal variations in both δ15N and Δ17O values have a similar trends with maximum during pre-monsoon season and minimum during monsoon season. The relatively high values of δ15N in pre-monsoon seasons, maybe indicate the enhanced contribution from fossil fuel combustions in South Asia which can be transported to our sampling sites across the Himalayas by the mountain/valley wind system. The Δ17O (NO3-) value, which is dependent on both the Δ17O (NO2) value and the oxidation pathways (O3 and OH ) of NO2 to NO3-, show that the O3 oxidation pathway is dominant during pre-monsoon season, resulting in higher values of both δ18O and Δ17O. In contrast, during monsoon season, formation involving HO2/RO2 radicals becomes important, producing lower values of δ18O and Δ17O. To better understand the oxidation process of past atmosphere in the Everest region, observations of oxygen-17 excess of nitrate and sulfate in ice cores collected from this region in the future are recommended.