biostudies-literatureLi KEuropean Research CouncilFoundation for Innovative Research Groups of the National Natural Science Foundation of Chinae2220228120https://www.ebi.ac.uk/biostudies/studies/S-EPMC10104570biostudies-literatureUnknown120(15)Hydroxyl radical (OH) is a key oxidant that triggers atmospheric oxidation chemistry in both gas and aqueous phases. The current understanding of its aqueous sources is mainly based on known bulk (photo)chemical processes, uptake from gaseous OH, or related to interfacial O₃ and NO₃ radical-driven chemistry. Here, we present experimental evidence that OH radicals are spontaneously produced at the air-water interface of aqueous droplets in the dark and the absence of known precursors, possibly due to the strong electric field that forms at such interfaces. The measured OH production rates in atmospherically relevant droplets are comparable to or significantly higher than those from known aqueous bulk sources, especially in the dark. As aqueous droplets are ubiquitous in the troposphere, this interfacial source of OH radicals should significantly impact atmospheric multiphase oxidation chemistry, with substantial implications on air quality, climate, and health.Proceedings of the National Academy of Sciences of the United States of AmericaSpontaneous dark formation of OH radicals at the interface of aqueous atmospheric droplets.PMC1010457010105260142020104001 and 42007192Rudich YGuo YNizkorodov SAAngelaki MWang XPerrier SGeorge CLi KAn TfalseSpontaneous dark formation of OH radicals at the interface of aqueous atmospheric droplets.Hydroxyl radical (OH) is a key oxidant that triggers atmospheric oxidation chemistry in both gas and aqueous phases. The current understanding of its aqueous sources is mainly based on known bulk (photo)chemical processes, uptake from gaseous OH, or related to interfacial O₃ and NO₃ radical-driven chemistry. Here, we present experimental evidence that OH radicals are spontaneously produced at the air-water interface of aqueous droplets in the dark and the absence of known precursors, possibly due to the strong electric field that forms at such interfaces. The measured OH production rates in atmospherically relevant droplets are comparable to or significantly higher than those from known aqueous bulk sources, especially in the dark. As aqueous droplets are ubiquitous in the troposphere, this interfacial source of OH radicals should significantly impact atmospheric multiphase oxidation chemistry, with substantial implications on air quality, climate, and health.2023-01-01T00:00:00Z2023 Apr2025-07-09T03:04:58.529Z2025-04-06T01:32:22.329ZS-EPMC101045703701118710.1073/pnas.2220228120