Revealing oxidative chemistry at the water-air interface using a levitated water droplet

The water-air interface is ubiquitous in environmental systems and provides an overlooked pathway for the chemical transformation of volatile compounds. Identifying such interfacial chemistry remains challenging because products generated at the interface are rapidly diluted in bulk water or escape into the atmosphere. Here we demonstrate that oxidative chemistry initiated at the water-air interface can be revealed when bulk dilution is sufficiently constrained. Using a single millimeter-sized water droplet to isolate a stable interface, we show that gas-phase dimethyl sulfide undergoes oxidation at the interface, producing water-soluble products that gradually accumulate in the aqueous phase. Ion chromatography confirms the formation of formate and sulfate, while field-induced droplet ionization mass spectrometry directly detects these products at the droplet surface. Isotopic labeling further identifies water as the oxygen source in the oxidation process. The accumulation of reaction products depends strongly on droplet geometry, reactant supply, and ionic environment, indicating that the observability of this chemistry is governed primarily by interfacial production and bulk dilution rather than bulk-phase reaction kinetics. These results demonstrate that the water-air interface can sustain oxidative chemistry whose contributions may have been underestimated in macroscopic aqueous systems.