The Oxidation of Nitric Oxide on Water Microdroplets and its Atmospheric Implications

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The oxidation of atmospheric nitric oxide (NO) significantly impacts global air quality, human health, and climate change. This study reports a highly efficient pathway for NO oxidation at the air–water interface of aqueous microdroplets. At this interface, NO is oxidized by hydroxyl radicals (•OH), yielding mainly nitrous acid (HONO) and trace amounts of nitrogen dioxide (NO2), nitrate radicals (•NO3), and nitric acid (HNO3). The structures of both ionic and neutral products were unambiguously confirmed through isotope labeling, collision-induced dissociation, and chemical ionization experiments. More importantly, this study reveals that the •OH on microdroplets is from water, but not ambient O2. Given the ubiquity of environmental microdroplets, the interfacial pathways may contribute substantially to the overall atmospheric oxidation capacity, serving as a crucial platform for the formation of key atmospheric species.