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/cm(3) molecule(-1) s(-1) = (2.5 ± 2.2) × 10(-12) exp[(630 ± 240)/T] and k(1c)/cm(3) 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 with a positive temperature dependence, in disagreement with previous studies. 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.

Kinetics of the ClO + CH3O2 reaction over the temperature range T = 250–298 K

“…The apparatus for this radical generation and monitoring has been described in detail previously and its adoption here was identical to that employed in our recent studies of the ClO + HO 2 and ClO + BrO reactions. 5,20,21 Briefly, gas mixtures were prepared in a continuous flow of oxygen or synthetic air in a mixing line using calibrated mass flow controllers or PTFE needle valves. These gases (used as supplied, 499.9% purity) were flowed into a temperature controlled reaction cell and excimer laser photolysis was achieved by coupling laser output through cylindrical expansion lenses and dichroic reflectors longitudinally through the 1 metre long reaction cell.…”

Kinetics of the BrO + HO2 reaction over the temperature range T = 246–314 K

“…34,34,35 The experimental conditions chosen for the present study were similar, the only difference being the use of Cl 2 in the present work as an extra source of Br atoms via the reaction of Cl with Br 2 (8). Therefore the results of the current work are compared to the more comprehensive and extensive study of Ferracci et al 33 Figure 7(a) shows that the overall rate constant at T = 298 K obtained for reaction (3) However, the overall effect on the values of k 1 arising from these differences in approach appears to be small as indicated by their general agreement and the sensitivity parameters discussed above.…”

“…This enabled unequivocal determination of BrO concentrations at each time point, despite the presence of other absorbers, as has been demonstrated in our previous work. 33,34,35 The BrO reference spectra were taken from the work of Wilmouth et al 36 These crosssections are reported at higher spectral resolution (0.4 nm FWHM) than spectra recorded in the present study (1.1 nm FWHM), so a smoothing function using a…”

“…Signal (photocharge) was transferred row by row down the array, out of the illuminated region, to record sequential wavelength and time resolved transmission spectra of the photolysed mixture on kinetic timescales, as shown, specifically for BrO, in our previous studies. 33,34,35 Spectra were recorded over the wavelength range λ = 274 -338 nm, at a spectral resolution of 1.1. nm full width half maximum. Typically, 1000…”