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

Inorganic particulate nitrate (p-NO ₃ ^- ), gaseous nitric acid (HNO _3(g) ) and nitrogen oxides (NO _x  = NO + NO ₂ ), 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 (δ ¹⁵ N, δ ¹⁸ O, and Δ ¹⁷ O) in the aerosols collected from the Mt. Everest region during April to September 2018. Over the entire sampling campaigns, the average of δ ¹⁵ N(NO ₃ ^- ), δ ¹⁸ O(NO ₃ ^- ), and Δ ¹⁷ O(NO ₃ ^- ) was -5.1 ± 2.3‰, 66.7 ± 10.2‰, and 24.1 ± 3.9‰, respectively. The seasonal variation in Δ ¹⁷ O(NO ₃ ^- ) indicates the relative importance of O ₃ and HO ₂ /RO ₂ /OH in NO _x oxidation processes among different seasons. A significant correlation between NO ₃ ^- and Ca ²⁺ 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 Δ ¹⁷ O(NO ₃ ^- ) values in the northern TP, our observed significantly higher values suggest that spatial variations in atmospheric Δ ¹⁷ O(NO ₃ ^- ) 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 δ ¹⁵ N(NO ₃ ^- ) values predicted remarkably low δ ¹⁵ N values in the NO _x of the sources and the N isotopic fractionation plays a crucial role in the seasonal changes of δ ¹⁵ N(NO ₃ ^- ). 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.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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