Br2-sensitised decomposition of ozone: kinetics of the reaction BrO + O3→ products

Abstract

The Br₂-sensitised decomposition of O₃ has been studied over the temperature range 298–343 K with N₂ and O₂ as bath gas using time-resolved UV absorption spectroscopy to investigate steady-state kinetics. High O₃ concentrations were employed to explore the occurrence of the reaction: BrO + O₃→ products [reaction (2)]. At 298 K the quantum yield for ozone-photosensitised decomposition was found to be in good agreement with previous studies yielding values of ϕ_–o3= 12.1 ± 1.0 and 13.9 ± 0.7 with O₂ and N₂ as the bath gas, respectively. At elevated temperatures a thermal reaction was observed which, after an induction period, led to enhanced BrO concentrations, substantial ozone loss and formation of OBrO as an intermediate. The overall rate constant for reaction (2) was ca. 10^–17 cm³ molecule^–1 s^–1 over the temperature range 318–343 K. A mechanism for the thermal reaction is proposed involving chain branching in which the rate-determining step is BrO + O₃→ OBrO + O₂[reaction (2b)]; analysis of the dark reaction enabled values for the Arrhenius parameters of reaction (2b) to be determined over the temperature range 318–343 K with k= 7 × 10^–14 exp(– 3100/T) cm³ molecule^–1 s^–1. This reaction and the parallel channel forming BrO₂+ O₂[reaction (2a)] are too slow to have any significance for ozone loss in the lower stratosphere.