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Kinetics of OH + SO<sub>2</sub> + 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.
