Kinetics of Sulfur Trioxide Reaction with Water Vapor to Form Atmospheric Sulfuric Acid

Although experimental methods can be used to obtain the quantitative kinetics of atmospheric reactions, experimental data are often limited to a narrow temperature range. The reaction of SO₃ with water vapor is important for elucidating the formation of sulfuric acid in the atmosphere; however, the kinetics is uncertain at low temperatures. Here, we calculate rate constants for reactions of sulfur trioxide with two water molecules. We consider two mechanisms: the SO₃···H₂O + H₂O reaction and the SO₃ + (H₂O)₂ reaction. We find that beyond-CCSD(T) contributions to the barrier heights are very large, and multidimensional tunneling, unusually large anharmonicity of high-frequency modes, and torsional anharmonicity are important for obtaining quantitative kinetics. We find that at lower temperatures, the formation of the termolecular precursor complexes, which is often neglected, is rate-limiting compared to passage through the tight transition states. Our calculations show that the SO₃···H₂O + H₂O mechanism is more important than the SO₃ + (H₂O)₂ mechanism at 5-50 km altitudes. We find that the rate ratio between SO₃···H₂O + H₂O and SO₃ + (H₂O)₂ is greater than 20 at altitudes between 10 and 35 km, where the concentration of SO₃ is very high.