<p>Atmospheric oxidation of sulfur dioxide (SO<sub>2</sub>) to sulfate has been widely investigated by means of gas phase and in-cloud chemistry studies. Recent field measurements have shown significant sulfate formation in cloud-free environments with high aerosol loadings. As an important fraction of biomass burning aerosol components, particulate phenolic and non-phenolic aromatic carbonyls may initiate photosensitized aerosol multiphase oxidation of SO<sub>2</sub>, of which our knowledge however is still in its nascent stage. In this study, on the basis of single-particle aerosol mass spectrometry (SPAMS) measurements, we find evident sulfate formation in the biomass burning-derived photosensitizer particles under UV and SO<sub>2</sub> exposure, attributable to photosensitized oxidation of S(IV), while almost no sulfate was observed under dark and existence of SO<sub>2</sub>. The efficiency of sulfate production under UV irradiation, represented by the number percentage of sulfate-containing particles (99–43 %) and sulfate relative peak area (RPA) (0.67–0.12) in single particle spectra, in descending order, were 3,4-dimethoxybenzaldehyde (DMB), vanillin (VL) and syringaldehyde (SyrAld). Internal mixtures of VL and potassium nitrate gave a slightly lower number percentage and RPA of sulfate to VL particles alone. In externally mixed potassium nitrate and VL particles, sulfate was predominantly formed on the latter, confirming that sulfate formation via photosensitization prevails over that via nitrate photolysis. Our results suggest that photosensitized oxidation of S(IV) could make an important contribution to aerosol sulfate formation, especially in areas influenced by biomass burning.</p>