Kinetics of OH + SO₂ + M: temperature-dependent rate coefficients in the fall-off regime and the influence of water vapour

Abstract

The OH-initiated oxidation of SO2 is the dominant, first step in the transformation of this atmospherically important trace gas to particulate sulfate, and accurate rate coefficients for the title reaction under all atmospheric conditions (pressures, temperatures, and humidity) are required to assess its role in, for example, new particle formation. Prior to this study, no temperature-dependent data were available in the fall-off regime for atmospherically relevant bath gases. We thus address an important omission in the kinetic database for this reaction and highlight significant discrepancies in recommended parameterizations. In this work, generation of OH via pulsed laser photolysis at 248 and 351 nm was coupled to its detection by laser-induced fluorescence to obtain rate coefficients (k1) for the title reaction at pressures of 14–742 Torr (1 Torr = 1.333 hPa) and temperatures of 220–333 K in N2 bath gas. In situ SO2 concentrations, central to accurate kinetic measurements under pseudo-first-order conditions, were measured by optical absorption. Under the conditions of the present study, the termolecular reaction between OH and SO2 is in the fall-off regime, and we parameterized the rate coefficients in N2 in terms of low- (k1,0) and high-pressure (k1,∞) limiting rate coefficients and a broadening factor (FC) to obtain k1,0N2=3.03×10-31 (T/300 K)−4.10 cm6 molecule−2 s−1, k1,∞=2.00×10-12 cm3 molecule−1 s−1, and FC=0.58. The effects of water vapour on the title reaction were explored through measurements in N2–H2O mixtures at 273, 298, and 333 K using the same experimental methods. The rate coefficients are significantly enhanced by the presence of water vapour with k1,0H2O=1.65×10-30 (T/300 K)−4.90 cm6 molecule−2 s−1, which indicates that H2O is a factor >5 more efficient in quenching the HOSO2* association complex than N2. A model-based comparison of our rate coefficients and parameterization with previous literature measurements and recommendations of evaluation panels are presented and discussed. The use of the new parameterization instead of the IUPAC or NASA evaluations, particularly after including H2O as a third-body quencher, leads to a significant (10 %–30 %) reduction in the lifetime of SO2 in some parts of the atmosphere and increases the H2SO4/SO2 ratio concomitantly.