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

Abstract

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 Br₂ in the presence of excess ozone. Both BrO and O₃ 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 + O₂ (1a); BrO + BrO → Br₂ + O₂ (1b), hence the calculation of the overall rate of reaction (1) and the branching ratio, α: k_1a/cm³ molecule⁻¹ s⁻¹ = (1.92 ± 1.54) × 10⁻¹² exp[(126 ± 214)/T], k_1b/cm³ molecule⁻¹ s⁻¹ = (3.4 ± 0.8) × 10⁻¹³ exp[(181 ± 70)/T], k₁/cm³ molecule⁻¹ s⁻¹ = (2.3 ± 1.5) × 10⁻¹² exp(134 ± 185 /T) and α = k_1a/k₁ = (0.84 ± 0.09) exp[(−7 ± 32)/T]. Errors are 1σ, 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 α 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).