Insight into the Mechanism and Kinetics of the Heterogeneous Reaction between SO 2 and NO 2 on Diesel Black Carbon under Light Irradiation.

The heterogeneous oxidation of SO 2 by NO 2 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 2 by NO 2 on the surface of complex particles remain poorly understood. Here, we systematically explore the mechanism and kinetics of the reaction between SO 2 and NO 2 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 2 to produce HONO via transferring photoinduced electrons but also indirectly promote OH radical formation. These reduction products of NO 2 as well as NO 2 itself greatly promote the heterogeneous oxidation of SO 2 on DBC. NO 2 oxidation, HONO oxidation, and the surface photo-oxidation process are proven to be three major surface oxidation pathways of SO 2 . The kinetics results indicate that the surface photooxidation pathway accounts for the majority of the total SO 2 uptake (∼63%), followed by the HONO oxidation pathway (∼27%) and direct oxidation by NO 2 (∼10%). This work highlights the significant synergistic roles of DBC, NO 2 , and light irradiation in enhancing the atmospheric oxidation capacity and promoting the heterogeneous formation of sulfate.