Role of Carbonaceous Aerosols in Catalyzing Sulfate Formation

The persistent and fast formation of sulfate is a primary factor driving the explosive growth of fine particles and exacerbating China’s severe haze development. However, the underlying mechanism for the persistent production of sulfate remains highly uncertain. Here, we demonstrate that soot is not only a major component of the particulate matter but also a natural carbocatalyst to activate molecular O2 and catalyze the oxidation of SO2 to sulfate under ambient conditions. Moreover, high relative humidity, typically occurring in severe haze events, can greatly accelerate the catalytic cycle by reducing the reaction barriers, leading to faster sulfate production. The formation pathway of sulfate catalyzed by carbonaceous soot aerosols uses the ubiquitous O2 as the ultimate oxidant and can proceed at night when photochemistry is reduced. The high relative humidity during haze episodes can further promote the soot-catalyzed sulfate-producing process. Therefore, this study reveals a missing and widespread source for the persistent sulfate haze formation in the open atmosphere, particularly under highly polluted conditions characterized by high concentrations of both SO2 and particulate carbon, and is helpful to the development of more efficient policies to mitigate and control haze pollution