Insight into the Mechanism and Kinetics of the Heterogeneous Reaction between SO and NO on Diesel Black Carbon under Light Irradiation.

The heterogeneous oxidation of SO by NO has been extensively proposed as an important pathway of sulfate production during haze events in China. However, the kinetics and mechanism of oxidation of SO by NO on the surface of complex particles remain poorly understood. Here, we systematically explore the mechanism and kinetics of the reaction between SO and NO on diesel black carbon (DBC) under light irradiation. The experimental results prove that DBC photochemistry can not only significantly promote the heterogeneous reduction of NO to produce HONO via transferring photoinduced electrons but also indirectly promote OH radical formation. These reduction products of NO as well as NO itself greatly promote the heterogeneous oxidation of SO on DBC. NO oxidation, HONO oxidation, and the surface photo-oxidation process are proven to be three major surface oxidation pathways of SO. The kinetics results indicate that the surface photooxidation pathway accounts for the majority of the total SO uptake (∼63%), followed by the HONO oxidation pathway (∼27%) and direct oxidation by NO (∼10%). This work highlights the significant synergistic roles of DBC, NO, and light irradiation in enhancing the atmospheric oxidation capacity and promoting the heterogeneous formation of sulfate.