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

The role of NO2 in the heterogeneous photochemical formation of sulfates by SO2 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. NO2 enhanced SO2 uptake and sulfate generation on BrC under irradiation, as shown by larger steady-state uptake coefficients of SO2 and sulfate formation rates. NO2- can be produced through the reduction of NO2 by the photogenerated electrons on BrC, and it was proven to be the trigger that promoted the oxidation of SO2 to sulfates. NO2- was more easily formed on FL, followed by 4-BBA and 3,7-DHNA, which can be evidenced by the energy barrier for NO2- production: FL (0.01 kJ mol-1) < 4-BBA (0.12 kJ mol-1) < 3,7-DHNA (1.33 kJ mol-1). SO2 lifetimes and sulfate formation rates during the reaction of SO2/NO2 with BrC were estimated to be 10.60-31.75 days and 0.02-0.04 μg m-3 h-1, respectively.