Laser Flash Photolysis Studies of Radical−Radical Reaction Kinetics:  The HO2 + BrO Reaction

Laser flash photolysis of Cl2/CH3OH/O2/Br2/O3/N2 mixtures at 308 nm has been coupled with simultaneous time-resolved detection of HO2 (by infrared tunable diode laser absorption spectroscopy) and BrO (by ultraviolet absorption spectroscopy) to investigate the kinetics of the important stratospheric reaction HO2 + BrO → products at 296 ± 3 K in N2 buffer gas at pressures of 12 and 25 Torr. All experiments were performed under near pseudo-first-order conditions with HO2 in excess over BrO. The HO2 + BrO rate coefficient is found to be k 1 = (2.0 ± 0.6) × 10-11 cm3 molecule-1 s-1, with the primary source of uncertainty being knowledge of the infrared line strength(s) required to convert measured HO2 absorbances to absolute concentrations. The rate coefficient for the reaction HO2 + HO2 → H2O2 + O2 derived based on infrared absorption measurements of the HO2 concentration is consistent with the currently accepted value. The results reported in this study are compared with other recent studies of HO2 + BrO kinetics, and their implications for our understanding of stratospheric chemistry are discussed.