Kinetic studies of the BrO + ClO cross-reaction over the range T = 246–314 K

Abstract

The kinetics of the atmospherically important gas phase radical reaction between BrO and ClO have been studied over the temperature range T = 246–314 K by means of laser flash photolysis coupled with UV absorption spectroscopy. Charge-coupled-device (CCD) detection allowed simultaneous monitoring of both free radicals and the OClO product using ‘differential’ spectroscopy, which minimised interference from underlying UV absorbing species. In this way, the total rate coefficient for BrO + ClO → products (1) was measured, along with that for the OClO producing channel of this process BrO + ClO → OClO + Br (1c). These reaction rate coefficients are described by the Arrhenius expressions: k₁/cm³ molecule⁻¹ s⁻¹ = (2.5 ± 2.2) × 10⁻¹² exp[(630 ± 240)/T] and k_1c/cm³ molecule⁻¹ s⁻¹ = (4.6 ± 3.0) × 10⁻¹² exp[(280 ± 180)/T], where errors are 2σ, statistical only. An extensive sensitivity analysis was performed to quantify the potential additional systematic uncertainties in this work arising from uncertainties in secondary chemistry, absorption cross-sections and precursor concentrations. This analysis identified the reactions of initial and secondarily generated bromine atoms (specifically Br + O₃ and Br + Cl₂O) as particularly important, along with the reversible combination of ClO with OClO forming Cl₂O₃. Potential uncertainty in this latter process was used to define the lowest temperature of the present study. Results from this work indicate larger absolute values for k₁ and k_1c than those reported in previous studies, but a weaker negative temperature dependence for k_1c than previously observed, resulting in a branching ratio for channel (1c) with a positive temperature dependence, in disagreement with previous studies. Reaction (1c) is the principal source of OClO in the polar stratosphere and is commonly used in atmospheric models as an indicator of stratospheric bromine chemistry. Thus these measurements might lead to a reinterpretation of modelled stratospheric OClO, which has also been suggested by previous comparisons of observations with atmospheric model studies.