Fig. 4: SO2 oxidation mechanism.
From: Fast sulfate formation from oxidation of SO2 by NO2 and HONO observed in Beijing haze
a Illustration of the mechanism for SO2 oxidation to sulfate by NO2 and HONO in fog and cloud. Here S(IV) ≡ SO2∙H20 + HSO3− +  SO32− denotes the different forms of dissolved SO2, S(VI) ≡ H2SO4(aq) + HSO4− + SO42− denotes the different forms of sulfate, and N(III) ≡ HNO2(aq) + NO2− denotes the different forms of dissolved HONO. S(IV) is oxidized in the aqueous phase by dissolved NO2 (reaction (R2)), producing N(III) which may oxidize additional S(IV) and produce N2O by reaction (R1). b pH-dependent sulfate and N2O production from the (R2) + (R1) mechanism. The plot shows accumulated concentrations of sulfate and N2O after 5 h in a nighttime fog air parcel simulation initialized with [SO2] = 20 ppb, [NO2] = 80 ppb, and [HONO] = 5 ppb, with a fog LWC = 0.15 g m−3 and T = 271 K. Contributions from aqueous-phase SO2 oxidation by HONO (reaction (R1)) and NO2 (reaction (R2)) are shown separately. A sulfate production of 20 ppb implies complete oxidation of SO2 over the 5 h.
