Hydrolysis
of Sulfur Dioxide in Small Clusters of
Sulfuric Acid: Mechanistic and Kinetic Study
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Abstract
The
deposition and hydrolysis reaction of SO<sub>2</sub> + H<sub>2</sub>O in small clusters of sulfuric acid and water are studied
by theoretical calculations of the molecular clusters SO<sub>2</sub>–(H<sub>2</sub>SO<sub>4</sub>)<sub><i>n</i></sub>–(H<sub>2</sub>O)<sub><i>m</i></sub> (<i>m</i> = 1,2; <i>n</i> = 1,2). Sulfuric acid exhibits a dramatic
catalytic effect on the hydrolysis reaction of SO<sub>2</sub> as it
lowers the energy barrier by over 20 kcal/mol. The reaction with monohydrated
sulfuric acid (SO<sub>2</sub> + H<sub>2</sub>O + H<sub>2</sub>SO<sub>4</sub> – H<sub>2</sub>O) has the lowest energy barrier of
3.83 kcal/mol, in which the cluster H<sub>2</sub>SO<sub>4</sub>–(H<sub>2</sub>O)<sub>2</sub> forms initially at the entrance channel. The
energy barriers for the three hydrolysis reactions are in the order
SO<sub>2</sub> + (H<sub>2</sub>SO<sub>4</sub>)–H<sub>2</sub>O > SO<sub>2</sub> + (H<sub>2</sub>SO<sub>4</sub>)<sub>2</sub>–H<sub>2</sub>O > SO<sub>2</sub> + H<sub>2</sub>SO<sub>4</sub>–H<sub>2</sub>O. Furthermore, sulfurous acid is more
strongly bonded to
the hydrated sulfuric acid (or dimer) clusters than the corresponding
reactant (monohydrated SO<sub>2</sub>). Consequently, sulfuric acid
promotes the hydrolysis of SO<sub>2</sub> both kinetically and thermodynamically.
Kinetics simulations have been performed to study the importance of
these reactions in the reduction of atmospheric SO<sub>2</sub>. The
results will give a new insight on how the pre-existing aerosols catalyze
the hydrolysis of SO<sub>2</sub>, leading to the formation and growth
of new particles