Isotopic constraints on the formation pathways and sources of atmospheric nitrate in the Mt. Everest region

Inorganic particulate nitrate (p-NO3-), gaseous nitric acid (HNO3(g)) and nitrogen oxides (NOx = NO + NO2), as main atmospheric pollutants, have detrimental effects on human health and aquatic/terrestrial ecosystems. Referred to as the 'Third Pole' and the 'Water Tower of Asia', the Tibetan Plateau (TP) has attracted wide attention on its environmental changes. Here, we evaluated the oxidation processes of atmospheric nitrate as well as traced its potential sources by analyzing the isotopic compositions of nitrate (delta N-15, delta O-18, and Delta O-17) in the aerosols collected from the Mt. Everest region during April to September 2018. Over the entire sampling campaigns, the average of delta N-15(NO3-), delta O-18(NO3-), and Delta O-17(NO3-) was -5.1 +/- 2.3 parts per thousand, 66.7 +/- 10.2 parts per thousand, and 24.1 +/- 3.9 parts per thousand, respectively. The seasonal variation in Delta O-17(NO3-) indicates the relative importance of O-3 and HO2/RO2/OH in NOx oxidation processes among different seasons. A significant correlation between NO3- and Ca2+ and frequent dust storms in the Mt. Everest region indicate that initially, the atmospheric nitrate in this region might have undergone a process of settling; subsequently, it got re-suspended in the dust. Compared with the Delta O-17(NO3-) values in the northern TP, our observed significantly higher values suggest that spatial variations in atmospheric Delta O-17(NO3-) exist within the TP, and this might result from the spatial variations of the atmospheric O-3 levels, especially the stratospheric O-3, over the TP. The observed delta N-15(NO3-) values predicted remarkably low delta N-15 values in the NOx of the sources and the N isotopic fractionation plays a crucial role in the seasonal changes of delta N-15(NO3-). Combined with the results from the backward trajectory analysis of air mass, we suggest that the vehicle exhausts and agricultural activities in South Asia play a dominant role in determining the nitrate levels in the Mt. Everest region. (c) 2020 Elsevier Ltd. All rights reserved.