Quantifying SO2 oxidation pathways to atmospheric sulfate using stable sulfur and oxygen isotopes: laboratory simulation and field observation

The formation of secondary sulfate in the atmosphere remains controversial, and it is an urgent need to seek a new method to quantify different sulfate formation pathways. Thus, SO2 and PM2.5 samples were collected from 4 to 22 December 2019 in the Nanjing region. Sulfur and oxygen isotopic compositions were synchronously measured to study the contribution of SO2 homogeneous and heterogeneous oxidation to sulfate. Meanwhile, the correlation of delta O-18 values between H2O and sulfate from SO2 oxidation by H2O2 and Fe3+ O-2 was simulatively investigated in the laboratory. Based on isotope mass equilibrium equations, the ratios of different SO2 oxidation pathways were quantified. The results showed that secondary sulfate constituted higher than 80 % of total sulfate in PM2.5 during the sampling period. Laboratory simulation experiments indicated that the delta O-18 value of sulfate was linearly dependent on the delta O-18 value of water, and the slopes of linear curves for SO2 oxidation by H2O2 and Fe3+ O-2 were 0.43 and 0.65, respectively. The secondary sulfate in PM2.5 was mainly ascribed to SO2 homogeneous oxidation by OH radicals and heterogeneous oxidation by H2O2 and Fe3+ O-2. SO2 heterogeneous oxidation was generally dominant during sulfate formation, and SO2 oxidation by H2O2 predominated in SO2 heterogeneous oxidation reactions, with an average ratio around 54.6 %. This study provided an insight into precisely evaluating sulfate formation by combining stable sulfur and oxygen isotopes.