Efficient Conversion of NO to NO2 on SO2-Aged MgO under Atmospheric Conditions

The NO–NO 2 cycle determines the formation of O 3 and hence plays a critical role in the oxidizing capacity of troposphere. Traditional view concluded that the heterogeneous oxidation of NO to NO 2 was negligible due to the weak reactivity of NO on aerosols, compared to the homogeneous oxidation process. However, the results here reported for the first time that SO 2 can greatly promote the heterogeneous transformation of NO into NO 2 and HONO on MgO particles under ambient conditions. The uptake coefficients of NO were increased by 2–3 orders of magnitudes on SO 2 -aged MgO, compared to the fresh sample. Based on spectroscopic characterization and density functional theory (DFT) calculations, the active sites for the adsorption and oxidation of NO were determined to be sulfates, where an intermediate [SO 4 –NO] complex was formed during the adsorption. The decomposition of this species led to the formation of NO 2 and the change of sulfate configuration. The formed NO 2 could further react with surface sulfite to form HONO and sulfate. The conversion of NO to NO 2 and HONO on the SO 2 -aged MgO surface under ambient conditions contributes a new formation pathway of NO 2 and HONO and could be quite helpful for understanding the source of atmospheric oxidizing capacity as well as the formation of air pollution complexes in polluted regions such as the northern China.