Reaction of SO₂ with OH in the atmosphere

The OH + SO₂ reaction plays a critical role in understanding the oxidation of SO₂ in the atmosphere, and its rate constant is critical for clarifying the fate of SO₂ in the atmosphere. The rate constant of the OH + SO₂ reaction is calculated here by using beyond-CCSDT correlation energy calculations for a benchmark, validated density functional methods for direct dynamics, canonical variational transition state theory with anharmonicity and multidimensional tunneling for the high-pressure rate constant, and system-specific quantum RRK theory for pressure effects; the combination of these methods can compete in accuracy with experiments. There has been a long-term debate in the literature about whether the OH + SO₂ reaction is barrierless, but our calculations indicate a positive barrier with an transition structure that has an enthalpy of activation of 0.27 kcal mol^-1 at 0 K. Our results show that the high-pressure limiting rate constant of the OH + SO₂ reaction has a positive temperature dependence, but the rate constant at low pressures has a negative temperature dependence. The computed high-pressure limiting rate constant at 298 K is 1.25 × 10^-12 cm³ molecule^-1 s^-1, which agrees excellently with the value (1.3 × 10^-12 cm³ molecule^-1 s^-1) recommended in the most recent comprehensive evaluation for atmospheric chemistry. We show that the atmospheric lifetime of SO₂ with respect to oxidation by OH depends strongly on altitude (in the range 0-50 km) due to the falloff effect. We introduce a new interpolation procedure for fitting the combined temperature and pressure dependence of the rate constant, and it fits the calculated rate constants over the whole range with a mean unsigned error of only 7%. The present results provide reliable kinetics data for this specific reaction, and also they demonstrate convenient theoretical methods that can be reliable for predicting rate constants of other gas-phase reactions.