The temperature dependence of the bimolecular channels of the ClO + ClO reaction over the range T = 298–323 K

Abstract

The bimolecular channels of the ClO self-reaction, although negligible under stratospheric conditions, become significant above ambient temperature. The kinetics of two of the three bimolecular channels of the ClO self-reaction, ClO + ClO → Cl 2 + O 2 (1b) and ClO + ClO → OClO + Cl (1d), were studied at T = 298-323 K and at ambient pressure ( p atm ≈ 760 ± 10 Torr). Radicals were generated via laser photolysis and monitored using UV absorption spectroscopy. The inclusion of charge-coupled device (CCD) detection allowed broadband monitoring of the radicals of interest along with the temporal resolution of their concentrations. Accurate and unequivocal quantification of the structured absorbers (ClO and OClO) was obtained via differential fitting procedures. The Arrhenius expressions obtained are k 1b = 2.9 The temperature dependences obtained in this work for both channels monitored are considerably less pronounced than those reported by Nickolaisen et al. C

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

“…A suitable reference spectrum of ClO is then analogously filtered and the filtered spectrum fitted to the experimental spectrum, minimising the sum of squares of residuals to determine the ClO species concentration using the Beer Lambert law (ii), and also as described in detail previously. 22 , 23 In this way, the ClO concentration as a function of time was unequivocally extracted from the time resolved spectra recorded on the CCD, despite the presence of many other absorbing species in the spectral window studied.…”

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

“…As discussed previously, 22 uncertainty in s ClO was defined at the 2s level from three values of the ClO cross-sections measured at T = 298 K. This gave rise to a relative error in s ClO of 0.23 at T = 298 K, of 0.26 at T = 314 K and of 0.15 at T = 246 K. This temperature dependence of the relative errors reflected the uncertainties in the high-resolution differential ClO crosssections derived by Boakes et al and employed in the current work to obtain s ClO at the resolution of the current study. The errors in the ClO cross-section were propagated through the analytical procedure and the values of k 1 and k 1c resulting from perturbed cross-sections were within AE12% and AE28% of those obtained using unperturbed cross-sections at T = 314 K. Similarly, at T = 246 K, values of k 1 and k 1c were within AE8% and AE13% of those obtained using unperturbed cross-sections.…”

“…This was consistent with FACSIMILE simulations, showing that (as expected) OClO production via one of the bimolecular channels of the ClO self-reaction would be too slow to be appreciable over the timescales used. 22 It was therefore concluded that the mechanism forming OClO in the precursor mixture, although unidentified, did not significantly affect the [OClO] temporal trace.…”

“…The ClO cross-section used to fit in eqn (ii) was obtained by degrading the higher resolution (0.8 nm FWHM) ClO differential cross-section, s ClO diff , calibrated at the (12, 0) peak (l = 275.2 nm) of the A 2 P ' X 2 P vibronic transition, measured by Boakes et al 21 to the lower resolution (1.1 nm FWHM) used in the present study. This procedure has been described in detail previously 22 hence a brief summary is given here. Back-to-back experiments consisting of the photolysis of Cl 2 O-Cl 2 -N 2 mixtures were performed, alternating between high (0.8 nm FWHM) and low (1.1 nm FWHM) resolution spectral settings but otherwise under identical conditions.…”

“…However, a baseline shift would not affect the spectral fit carried out in the current work as differential fitting procedures are adopted. As described previously, 22 the differential cross-section of the a(15) peak of OClO at l = 322.78 nm was however perturbed by AE4 Â 10 À19 cm 2 molecule À1 and the whole spectrum was scaled to match the altered differential cross-section at all experimental temperatures. Perturbed OClO cross-sections were then employed in the quantification of [OClO] via the Beer-Lambert Law and the traces thus generated were fitted with the FACSI-MILE model.…”