Determination of the High-Pressure Limiting Rate Coefficient and the Enthalpy of Reaction for OH + SO₂

The kinetics of the association reaction OH + SO 2 have been studied using laser flash photolysis at 248 nm to generate OH radicals and laser-induced fluorescence to monitor their decay under pseudo-first-order conditions, [OH] ≪ [SO 2 ]. The removal kinetics of OH( v = 1) + SO 2 have been measured over the temperature range of 295 to 673 K. Master equation calculations were performed to demonstrate that, provided intramolecular vibrational redistribution is fast, OH( v = 1) + SO 2 is a good approximation for the high-pressure rate coefficient of the OH( v = 0) + SO 2 + M reaction, giving k 1 ∞ ( T ) = (2.04 ± 0.10) × 10 -12 ( T /300 K) -0.27 ± 0.11 cm 3 molecule -1 s -1 . This temperature dependence of the rate coefficient suggests that the reaction occurs on a barrierless surface. The kinetics of the reaction OH( v = 0) + SO 2 + M, k 1, were also studied. At room temperature, the kinetic data were in good agreement with literature values. At elevated temperatures, 523 to 603 K, equilibrium behavior was observed between OH + SO 2 and HOSO 2 . This represents the first direct observation of equilibration, and an analysis of the data, using a Third-Law method, with Δ r S 0 298 = −142 ± 10 J mol -1 K -1 gives a reaction enthalpy of Δ r H 0 298 = −113.3 ± 6 kJ mol -1, and Δ f H 0 298 (HOSO 2 ) = −373 ± 6 kJ mol -1 . These numerical values are significantly lower than literature values. k 1 ∞ ( T ) has been used to generate a consistent set of parameters to describe k 1 ([M], T ) for OH + SO 2 for use in atmospheric modeling, and Δ r H 0 298 has been used to assess the role of HOSO 2 in the oxidation of SO 2 at elevated temperatures.