Unexpected SO₂ Photooxidation on Quantum-Sized Carbon

Surface photochemistry of carbon quantum dots (CQD) under atmospheric conditions remains poorly understood. Here, we systematically investigate the photoreactivity of CQD in the presence of SO₂ and reveal that CQD photoageing and its interaction with SO₂ occur independently of O₂. Through a combined experimental and theoretical approach, we demonstrate that these unexpected surface reactions are driven by unique energy transfer and photocatalytic processes induced by photoexcited CQD. Key reactive intermediates─photogenerated hole, hydroxyl radical, sulfite radical, and carbon-centered radical─are identified as critical drivers of CQD surface photooxidation. This discovery provides new insights into the unresolved atmospheric photochemistry of flame-formed fresh soot, advances our understanding of photocatalytic processes on synthetic nanosized carbon materials, and offers clues to the origins of sulfated carbonaceous chondrites in interstellar media.