Diesel soot photooxidation enhances the heterogeneous formation of H 2 SO 4 .

Both field observation and experimental simulation have implied that black carbon or soot plays a remarkable role in the catalytic oxidation of SO 2 for the formation of atmospheric sulfate. However, the catalytic mechanism remains ambiguous, especially that under light irradiation. Here we systematically investigate the heterogeneous conversion of SO 2 on diesel soot or black carbon (DBC) under light irradiation. The experimental results show that the presence of DBC under light irradiation can significantly promote the heterogeneous conversion of SO 2 to H 2 SO 4 , mainly through the heterogeneous reaction between SO 2 and photo-induced OH radicals. The detected photo-chemical behaviors on DBC suggest that OH radical formation is closely related to the abstraction and transfer of electrons in DBC and the formation of reactive superoxide radical (•O 2 - ) as an intermediate. Our results extend the known sources of atmospheric H 2 SO 4 and provide insight into the internal photochemical oxidation mechanism of SO 2 on DBC.