Ferracci, V; (2012) Kinetic studies of gaseous halogen oxide radical reactions implicated in ozone depletion. Doctoral thesis , UCL (University College London).

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Abstract

Increased atmospheric emissions of photolabile halogen-containing species as a result of human activity over the Twentieth Century have had a dramatic impact on atmospheric ozone. This has generated significant scientific interest in understanding the reactivity of halogens in the gas phase. In particular, the self- and cross-reactions of halogen monoxide radicals, XO (where X = F, Cl, Br, I), which are the first-formed intermediates in the reaction of photolytically released halogen atoms with ozone, have been identified as key processes in ozone-depleting events as they initiate reaction cycles that catalytically destroy ozone. Understanding the kinetics of these reactions is therefore crucial to establishing their potential for ozone destruction. These reactions are, however, complex multichannel processes, with both bimolecular and termolecular components contributing to the overall reaction. As some product channels do not contribute to the catalytic destruction of ozone, an accurate determination of the product branching is also of utmost importance for a comprehensive understanding of the atmospheric ozone budget. In this thesis, results from studies of the kinetics of the ClO and BrO self-reactions and of the BrO + ClO cross-reaction, carried out under appropriate atmospheric conditions, are presented. These reactions were studied using laser flash photolysis coupled with UV absorption spectroscopy. This technique adopted the rapid generation of the XO radicals of interest following laser photolysis and monitoring their temporal behaviour via UV absorption spectroscopy facilitated by charge-coupled device (CCD) detection. The use of a CCD detector allowed broadband time-resolved acquisition of spectra, leading to the unequivocal identification of multiple species and to the accurate quantification of their concentrations via the Beer-Lambert law. The desired kinetic information was then obtained from fitting classical or numerical integration simulations to the experimental concentration profiles. Extensive sensitivity analyses of the results obtained were performed to identify the principal sources of uncertainty in these measurements. The results from the present work are compared to those obtained in previous studies and their implications for ozone in the Earth’s atmosphere are discussed.

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