Hydrolysis of Sulfur Dioxide in Small Clusters of Sulfuric Acid: Mechanistic and Kinetic Study

The deposition and hydrolysis reaction of SO₂ + H₂O in small clusters of sulfuric acid and water are studied by theoretical calculations of the molecular clusters SO₂–(H₂SO₄)_<i>n</i>–(H₂O)_<i>m</i> (<i>m</i> = 1,2; <i>n</i> = 1,2). Sulfuric acid exhibits a dramatic catalytic effect on the hydrolysis reaction of SO₂ as it lowers the energy barrier by over 20 kcal/mol. The reaction with monohydrated sulfuric acid (SO₂ + H₂O + H₂SO₄ – H₂O) has the lowest energy barrier of 3.83 kcal/mol, in which the cluster H₂SO₄–(H₂O)₂ forms initially at the entrance channel. The energy barriers for the three hydrolysis reactions are in the order SO₂ + (H₂SO₄)–H₂O > SO₂ + (H₂SO₄)₂–H₂O > SO₂ + H₂SO₄–H₂O. Furthermore, sulfurous acid is more strongly bonded to the hydrated sulfuric acid (or dimer) clusters than the corresponding reactant (monohydrated SO₂). Consequently, sulfuric acid promotes the hydrolysis of SO₂ both kinetically and thermodynamically. Kinetics simulations have been performed to study the importance of these reactions in the reduction of atmospheric SO₂. The results will give a new insight on how the pre-existing aerosols catalyze the hydrolysis of SO₂, leading to the formation and growth of new particles