Mechanism of Aqueous-Phase Ozonation of S(IV)

The ozonation of dissolved sulfur dioxide is an important route for sulfate formation, especially in fog and cloud droplets of high pH. However, little is known about the detailed chemical mechanism of this process. We have mapped out the fate of aqueous SO₂ in the presence of ozone by use of density functional theory (DFT) calculations in solution (via the polarized continuum model, PCM), including up to two explicit water molecules. The calculations predict that the hydrolysis of SO₂·H₂O, although possessing a barrier, is still more energetically favorable than its ozonation. The ozonation of HOSO₂⁻ and SO₃²⁻ proceeds without barriers and gives S(VI) products that are more stable than the reagents by 77.1 and 88.6 kcal/mol, respectively. By comparing our calculated pH dependence of the ozonation kinetics to those determined experimentally, we conclude that, despite a high calculated energy barrier to the ozonation of sulfonate (HSO₃⁻), it is the dominant form of S(IV) in solutions of neutral pH and is the species through which ozonation occurs.