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

Abstract

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] ≪ [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 k1∞(T) = (2.04 ± 0.10) × 10-12 (T/300 K)-0.27 ± 0.11 cm3 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, k1, 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 ΔrS0298 = −142 ± 10 J mol-1 K-1 gives a reaction enthalpy of ΔrH0298 = −113.3 ± 6 kJ mol-1, and ΔfH0298(HOSO2) = −373 ± 6 kJ mol-1. These numerical values are significantly lower than literature values. k1∞(T) has been used to generate a consistent set of parameters to describe k1([M], T) for OH + SO2 for use in atmospheric modeling, and ΔrH0298 has been used to assess the role of HOSO2 in the oxidation of SO2 at elevated temperatures.