Determination of the high-pressure limiting rate coefficient and the enthalpy of reaction for OH+SO2

The kinetics of the association reaction OH + SO2 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] much less than [SO2]. The removal kinetics of OH(v = 1) + SO2 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) + SO2 is a good approximation for the high-pressure rate coefficient of the OH(v = 0) + SO2 + M reaction, giving k(l)(infinity)(T) = (2.04 +/- 0.10) x 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) + SO2 + 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 + SO2 and HOSO2. This represents the first direct observation of equilibration, and an analysis of the data, using a Third-Law method, with Delta(r)S(298)(0) = -142 +/- 10 J mol(-1) K-1 gives a reaction enthalpy of DeltarH(298)(0) = -113.3 +/- 6 kJ mol(-1), and Delta(f)H(298)(0)(HOSO2) = -373 +/- 6 kJ mol(-1). These numerical values are significantly lower than literature values. k(l)(infinity)(T) has been used to generate a consistent set of parameters to describe k(l)([M], T) for OH + SO2 for use in atmospheric modeling, and Delta(r)H(298)(0) has been used to assess the role of HOSO2 in the oxidation of SO2 at elevated temperatures.