Spontaneous dark formation of OH radicals at the interface of aqueous atmospheric droplets

Hydroxyl radical (OH) is a key oxidant that triggers atmospheric oxidation chem-istry 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.

Automated summary

Hydroxyl radicals (OH) play a crucial role as oxidants in atmospheric chemistry, and their aqueous sources have been mainly attributed to known bulk processes or interfacial chemistry driven by O3 and NO3 radicals. This study provides experimental evidence that OH radicals can be spontaneously produced at the air-water interface of aqueous droplets, even in the absence of known precursors, possibly due to the strong electric field formed at such interfaces. The measured OH production rates in atmospherically relevant droplets are comparable to or higher than those from known bulk sources, particularly in the dark. As aqueous droplets are widespread in the troposphere, this interfacial source of OH radicals is expected to have significant impacts on atmospheric multiphase oxidation chemistry, with implications for air quality, climate, and health.