Photoelectron Spectroscopy of Cryogenically-cooled SO3– and HOSO2– Anions and the Dynamics of the Key Atmospheric SO2 Oxidation Reaction by the Hydroxyl Radical

This talk will present two recent photoelectron spectroscopic studies on the title anions. The study on SO3– led to the first experimental measurement of the electron affinity of SO3 and the dissociation energy of SO3–  SO2 + O– [1]. The second experiment was a joint experimental investigation on the dynamics of the SO2 oxidation by the OH radical [2]. SO2 is the major precursor to the production of sulfuric acid (H2SO4), contributing to acid rain and atmospheric aerosols. Sulfuric acid formed from SO2 generates light-reflecting sulfate aerosol particles in the atmosphere. This property has prompted geoengineering proposals to inject sulfuric acid or its precursors into the Earth’s atmosphere to increase the planetary albedo to counteract global warming. SO2 oxidation in the atmosphere by the hydroxyl radical HO to form HOSO2 is a key rate-limiting step in the mechanism for forming acid rain. However, the dynamics of the HO + SO2  HOSO2 reaction and its slow rate in the atmosphere were poorly understood. We have used photoelectron spectroscopy of cryogenically-cooled HOSO2– anion to access the neutral HOSO2 radical near the transition state of the HO + SO2 reaction. Theoretical collaboration was carried out to interpret the photoelectron spectra of HOSO2– and to probe the dynamics of the HO + SO2 reaction [2]. In addition to the discovery of a new pre-reaction complex (HO…SO2) directly connected to the transition state, dynamic calculations revealed that the accessible phase space for the HO + SO2  HOSO2 reaction is extremely narrow, forming a key reaction bottleneck and slowing the reaction rate in the atmosphere, despite the low reaction barrier.
[1] D. F. Yuan, T. Trabelsi, Y. R. Zhang, J. S. Francisco, and L. S. Wang, J. Am. Chem. Soc. 144, 13740-13747 (2022).
[2] D. F. Yuan, Yang Liu, T. Trabelsi, Y. R. Zhang, J. Li, J. S. Francisco, H. Guo, and L. S. Wang, Proc. Natl. Acad. Sci. (USA) 121, e2314819121 (2024).