Radical Cascades on Seawater Microdroplets Drive Atmospheric Mercury Oxidation

The air-water interface of aqueous microdroplets enables the spontaneous generation of hydroxyl radicals (·OH). However, the environmental fate and effect of these primary oxidants in complex, ion-rich media such as seawater remains uncharted. Here, we discover that the interface of seawater microdroplets sustains a radical cascade, which markedly amplifies their oxidative power beyond that of pure water microdroplets. Spontaneously produced ·OH at the interface quickly engages in synergistic reactions with abundant bromide and bicarbonate ions, initiating a network of secondary radicals, including reactive bromine (·Br) and carbonate (·CO₃^-) species. The coexistence and interconversion within this self-sustaining interfacial cycle are demonstrated through radical trapping and spectroscopic methods. To assess the environmental implications of this process, we have quantified its role in the dark oxidation of gaseous elemental mercury. Our findings advance the understanding from mere interfacial oxidant generation to the radical networks by specific ions, offering a novel framework for predicting chemical reactivity in environmental aqueous microdroplets.