Effect of NO_X, O₃ and NH₃ on sulfur isotope composition during heterogeneous oxidation of SO₂: a laboratory investigation

Abstract. Sulfate aerosol is a major fraction of haze, playing an important role in aerosol formation and aging processes. In order to understand the mechanism of sulfate formations, the characteristics of sulfur isotope composition were determined during different heterogeneous oxidation reactions of sulfur dioxide. Although NH₃ was more beneficial to the formation of sulfate, compared with NO_X and O₃, ³⁴S tended to enrich the lighter sulfur isotopes in the presence of NH₃. Furthermore, in consideration of the potential competitive effects of NO_X, O₃, and NH₃ in the heterogeneous oxidation processes, the contributions of each gas were evaluated via Rayleigh distillation model. Notably, NO_X oxidation contributed 67.5±10 % of the whole sulfate production, which is higher than O₃ (13.3±10 %), and NH₃ oxidation (19.2±10 %) on the basic of the average fractionation factor. The observed δ³⁴S values of sulfate aerosols were negatively correlated with sulfur oxidation ratios, owing to the sulfur isotopic fractionations during the sulfate formation processes. Given the isotope mass balance, the overall δ³⁴S_sulfate approached the δ³⁴S_emission as oxidation of SO₂ progressed, suggesting that NO_X played a major rather than a sole role in the different heterogeneous oxidation processes of SO₂.