NO2-promoted heterogeneous photochemical oxidation of SO2 to sulfates on brown carbon

The role of NO 2 in the heterogeneous photochemical formation of sulfates by SO 2 uptake on typical brown carbon (BrC) species (Fluoranthene, FL; 4-(benzoyl) benzoic acid , 4–BBA; 3,7-dihydroxy-2-naphthoic acid, 3,7–DHNA) was systemically explored in a flow tube reactor with the infrared spectroscopy , ion chromatography as well as theoretical calculation. NO 2 enhanced SO 2 uptake and sulfate generation on BrC under irradiation, as shown by larger steady-state uptake coefficients of SO 2 and sulfate formation rates. NO 2 − can be produced through the reduction of NO 2 by the photogenerated electrons on BrC, and it was proven to be the trigger that promoted the oxidation of SO 2 to sulfates. NO 2 − was more easily formed on FL, followed by 4−BBA and 3,7−DHNA, which can be evidenced by the energy barrier for NO 2 − production: FL (0.01 kJ mol −1 ) < 4−BBA (0.12 kJ mol −1 ) < 3,7−DHNA (1.33 kJ mol −1 ). SO 2 lifetimes and sulfate formation rates during the reaction of SO 2 /NO 2 with BrC were estimated to be 10.60–31.75 days and 0.02–0.04 μg m −3 h −1 , respectively.