Interfacial Electric Fields Drive Fast Hydroxyl Radical Production in Black-Carbon-Bearing Microdroplets

While black carbon (BC) is generally recognized as a substrate for various heterogeneous atmospheric-relevant reactions of significance, its intrinsic oxidative potential─specifically the production of hydroxyl radicals (^•OH)─has received minimal attention. Here, we demonstrate that the gas-liquid-solid interface of BC-bearing microdroplet aerosols functions as a highly active catalytic microreactor in the atmosphere. The interfacial environment leads to ultrafast ^•OH production (even up to ∼12 μmol s^-1) via a mechanism driven by a strong electric field. This localized force induces Field-Induced Carrier Separation (FICS), which overcomes exciton binding energy and suppresses electron-hole recombination. Furthermore, we identify a synergy between this electric field and photoaging that promotes surface defect proliferation, establishing a positive feedback loop that sustains rapid redox cycling. These findings reveal BC aerosols not merely as a radiative forcing agent, but as potent, overlooked natural "photocatalysts" that significantly amplify atmospheric oxidative capacity and accelerate secondary aerosol formation.