A pH dependent sulfate formation mechanism caused by hypochlorous acid in the marine atmosphere

Secondary sulfate plays a crucial role in forming marine aerosol, which in turn is an important source of natural aerosol at a global level. Recent experimental studies suggest that oxidation of S(IV) compounds, in practice dissolved sulfur dioxide, to sulfate (S(VI)) by hypochloric acid could be one of the most significant pathways for sulfate formation in marine areas. However, the exact mechanism responsible for this process remains unknown. Using high-level quantum chemical calculations, we studied the reaction between dissolved sulfur dioxide and hypochloric acid. We account for the dominant protonation states of reactants in the pH range 3.0-9.0. We also consider possible catalytic effects of species such as H₂O. Our results show that sulfate formation in HOCl+HOSO₂^- and HOCl+SO₃^2- reactions relevant to acidic and nearly neutral conditions can occur either through previously proposed Cl⁺ transfer or through a novel HO⁺ transfer mechanism. In alkaline conditions, where the dominant reactants are OCl^- and SO₃^2-, an O atom transfer mechanism proposed in previous experimental studies may be more important than Cl⁺ transfer. Catalysis by common cloud-water species is found to lower barriers of Cl⁺ transfer mechanisms substantially. Nevertheless, we find that the dominant S(IV) + HOCl reaction mechanism for the full studied pH range is HO⁺ transfer from HOCl to SO₃^2-, which leads directly to sulfate formation without ClSO₃^- intermediates. The rate-limiting barrier of this reaction is low, leading to an essentially diffusion-controlled reaction rate. S(IV) lifetimes due to this reaction decrease with increasing pH due to the increasing fractional population of SO₃^2-. Especially in neutral and alkaline conditions, depletion of HOCl by the reaction is so rapid that S(IV) oxidation will be controlled mainly by mass transfer of gas-phase HOCl to the liquid phase. The mechanism proposed here may help to explain marine sulfate sources missing from current atmospheric models.