Probing the dynamics and bottleneck of the key atmospheric SO₂ oxidation reaction by the hydroxyl radical

SO₂ (Sulfur dioxide) 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 recent 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 are poorly understood to date. Herein, we use photoelectron spectroscopy of cryogenically cooled HOSO₂ anion to access the neutral HOSO₂ radical near the transition state of the HO + SO₂ reaction. Spectroscopic and dynamic calculations are conducted on the first ab initio-based full-dimensional potential energy surface to interpret the photoelectron spectra of HOSO₂ and to probe the dynamics of the HO + SO₂ reaction. In addition to the finding of a unique pre-reaction complex (HO⋯SO₂) directly connected to the transition state, dynamic calculations reveal 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. This study underlines the importance of understanding the full multidimensional potential energy surface to elucidate the dynamics of complex bimolecular reactions involving polyatomic reactants.