Kinetic studies on the temperature dependence of the BrO plus BrO reaction using laser flash photolysis

The BrO self-reaction, BrO + BrO -> products (1), has been studied using laser flash photolysis coupled with UV absorption spectroscopy over the temperature range T = 266.5-321.6 K, under atmospheric pressure. BrO radicals were generated via laser photolysis of Br(2) in the presence of excess ozone. Both BrO and O(3) were monitored via UV absorption spectroscopy using charge-coupled device (CCD) detection. Simultaneous fitting to both temporal concentration traces allowed determination of the rate constant of the two channels of reaction (1), BrO + BrO -> 2Br + O(2) (1a); BrO + BrO -> Br(2) + O(2) (1b), hence the calculation of the overall rate of reaction (1) and the branching ratio, a: k(1a)/cm(3) molecule(-1) s(-1) = (1.92 +/- 1.54) x 10(-12) exp[(126 +/- 214)/T], k(1b)/cm(3) molecule(-1) s(-1) = (3.4 +/- 0.8) x 10(-13) exp[(181 +/- 70)/T], k(1)/cm(3) molecule(-1) s(-1) = (2.3 +/- 1.5) x 10(-12) exp(134 +/- 185 /T) and alpha = k(1a)/k(1) = (0.84 +/- 0.09) exp[(-7 +/- 32)/T]. Errors are 1 sigma, statistical only. Results from this work show a weaker temperature dependence of the branching ratio for channel (1a) than that found in previous work, leading to values of alpha at temperatures typical of the Polar Boundary Layer higher than those reported by previous studies. This implies a shift of the partitioning between the two channels of the BrO self-reaction towards the bromine atom and hence directly ozone-depleting channel (1a).