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

Abstract. The formation of secondary sulfate in the atmosphere remains controversial, and it is urgent to seek for a new method to quantify different sulfate formation pathways. Thus, SO₂ and PM_2.5 samples were collected from 4 to 22 Dec. 2019 in Nanjing. Sulfur and oxygen isotope compositions were synchronously measured to study the contribution of SO₂ homogeneous and heterogeneous oxidation to sulfate. Meanwhile, the correlation of δ¹⁸O values between H₂O and sulfate from SO₂ oxidation by H₂O₂ and Fe³⁺/O₂ were investigated in the lab. Based on isotope mass equilibrium equations, the ratios of different SO₂ oxidation pathways were calculated. The results showed that secondary sulfate constituted higher than 80 % of total sulfate in PM_2.5 during the sampling period. Laboratory simulation experiments indicated that δ¹⁸O of sulfate was linearly dependent on δ¹⁸O of water, and the slopes of linear curves for SO₂ oxidation by H₂O₂ and Fe³⁺/O₂ were 0.43 and 0.65, respectively. The secondary sulfate in PM_2.5 was mainly ascribed to SO₂ homogeneous oxidation by OH radicals and heterogeneous oxidation by H₂O₂ and Fe³⁺/O₂. SO₂ heterogeneous oxidation was generally dominant during sulfate formation, and the contribution of SO₂ heterogeneous oxidation was about 52 %. Especially, SO₂ oxidation by H₂O₂ predominated in SO₂ heterogeneous oxidation reactions with an average ratio around 55 %. This study provided an insight into precisely evaluating sulfate formation pathways by combining stable sulfur and oxygen isotopes.