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 NOX and O3, 34S tended to enrich the lighter sulfur isotopes in the presence of NH3. Furthermore, in consideration of the potential competitive effects of NOX, O3, and NH3 in the heterogeneous oxidation processes, the contributions of each gas were evaluated via Rayleigh distillation model. Notably, NOX 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 δ34Ssulfate approached the δ34Semission as oxidation of SO2 progressed, suggesting that NOX played a major rather than a sole role in the different heterogeneous oxidation processes of SO2.
Effect of NO<sub>X</sub>, O<sub>3</sub> and NH<sub>3</sub> on sulfur isotope composition during
heterogeneous oxidation of SO<sub>2</sub>: a laboratory investigation
