摘要:

The heterogeneous oxidation of sulfur dioxide (SO₂) on a-Al₂O₃ particles was investigated using a flow reactor coupled with a transmission-Fourier transform infrared (T-FTIR) spectrometer at different relative humidities (RH) in the absence or presence of hydrogen peroxide (H₂O₂), with an emphasis on the saturation coverage of SO₂ and the timescale on which the reaction reaches saturation. It is found that the saturation coverage of SO₂ in the absence of H₂O₂ increases with rising RH due to the hydrolysis of SO₂ by surface adsorbed water. However, the reaction ultimately reaches saturation since the produced sulfite/bisulfite cannot be further converted to sulfate/bisulfate in the absence of oxidants. In addition, the presence of H₂O₂ can significantly increase the saturation coverage of SO₂ by efficiently oxidizing sulfite/bisulfite to sulfate/bisulfate. Under humid conditions, adsorbed water facilitates the hydrolysis of SO₂ and mitigates the increase of surface acidity, which can inhibit the hydrolysis of SO₂. Hence, in the presence of H₂O₂, the saturation coverage of SO₂ as well as the time of reaction reaching saturation increases with rising RH and the surface is not saturated on the timescale of the experiments (40 h) at 60% RH. Furthermore, the increase of saturation coverage of SO₂ in the presence of H₂O₂ was observed on chemically inactive SiO₂ particles, indicating that the hydrolysis of SO₂ and subsequent oxidation by H₂O₂ likely occurs on other types of particles. Our findings are of importance for understanding the role of water vapor and trace gases (e.g., H₂O₂₎ in the heterogeneous reaction of SO₂ in the atmosphere.