Laser flash photolysis studies of radical-radical reaction kinetics:: The HO2+IO reaction

Laser flash photolysis of Cl-2/CH3OH/O-2/I-2/NO2/SF6/N-2 mixtures at 308 nm has been coupled with simultaneous time-resolved detection of HO2 (by infrared tunable diode laser absorption spectroscopy) and IO (by visible absorption spectroscopy) to investigate the kinetics of the atmospherically important reaction HO2 + IO --> products over the temperature range 274-373 K in N-2 buffer gas at pressures of 12 and 25 Torr. All experiments were performed under near pseudo-first-order conditions with HO2 in excess over IO. At 298 K, the rate coefficient was determined to be (9.7 +/- 2.9) x 10(-11) cm(3) molecule(-1) s(-1), with the primary sourer of uncertainty being knowledge of the infrared line strength(s) required to convert measured HO2 absorbances to absolute concentrations. The temperature dependence of the HO2 + IO rate coefficient was found to be adequately described by the Arrhenius expression k = 9.3 x 10(-12) exp(680/T) cm(3) molecule(-1) s(-1). The results reported in this study are compared with other recent studies of HO2 + IO kinetics, and the potential roles of this reaction in atmospheric chemistry are discussed.