Microheterogeneous singlet oxygen generation at air-water interfaces

Singlet oxygen (¹O₂) is a short-lived, highly reactive oxidant driving natural element cycles, yet its spatial distribution in complex multiphase systems remains poorly understood. Here, we show that irradiation of organic carbon (OC)-containing aqueous microdroplets leads to pronounced interfacial enrichment of ¹O₂, driven by the surface accumulation of photosensitizing OC. Combining fluorescence imaging with a reaction-diffusion kinetic model, we resolve steep ¹O₂ gradients across the air-water boundary: In a 1 µm-radius droplet, ¹O₂ levels drop by 90% within 10 µm from the interface into the gaseous phase, and within 230 nm from the interface into the aqueous phase. Tailored peptide probes reveal that molecules residing at the interface undergo substantially faster ¹O₂-mediated transformation than their bulk counterparts. These findings identify the air-water interface as a privileged site for ¹O₂ photochemistry that strongly accelerates redox processes in aerosols, sea spray, and the ocean-atmosphere boundary.