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.