<p>Atmospheric aerosols affect climate and air quality through chemical reactions that are not yet fully understood. Here, we revealed that black carbon (BC) particles can promote the heterogeneous oxidation of isoprene, a major biogenic volatile organic compound, under simulated sunlight. Using a flow reactor coupled with proton-transfer-reaction mass spectrometry and in situ infrared spectroscopy, we observed that this surface-driven process formed small carbonyl-containing oligomers derived from C₁–C₄ fragments, rather than the C₅ products commonly associated with gas-phase isoprene oxidation. Electron spin resonance measurements further confirmed that BC can act as a photosensitized catalyst via the formation of surface reactive oxygen species (ROS). Light-activated BC generated singlet oxygen (¹O₂), which reacted with adsorbed water to produce surface hydroxyl radicals (OH). Additional surface bidentate sites supported continued reactant uptake and product formation. These results identified a previously underrecognized pathway for particle growth during haze episodes and suggest that aerosols can actively drive oxidant chemistry rather than simply remove oxidants from the atmosphere. Including BC-catalyzed heterogeneous reactions in atmospheric models may improve predictions of secondary organic aerosol formation in polluted environments.</p>