The reactions of sulfur dioxide (SO ₂ ) with surface-bound compounds on atmospheric aerosols lead to the formation of organic sulfur (OS) compounds, thereby affecting the air quality and climate. Here, we show that the heterogeneous reaction of SO ₂ with authentic urban grime under near-ultraviolet sunlight irradiation leads to a large suite of various organic compounds including OS released in the gas phase. Calculations indicate that at the core area of Guangzhou, building surface uptake of SO ₂ is 15 times larger than uptake of SO ₂ on aerosol surfaces, yielding ~20 ng m ⁻³ of OS that represents an important fraction of the observed OS compounds (60 to 200 ng m ⁻³ ) in ambient aerosols of Chinese megacities. This chemical pathway occurring during daytime can contribute to the observed fraction of OS compounds in aerosols and improve the understanding of haze formation and urban air pollution.