Stone, D; (2006) Kinetics of key peroxy radical reactions involved in atmospheric oxidation processes. Doctoral thesis , UCL (University College London).

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Abstract

The aims of this work were to investigate the kinetics of key peroxy radical reactions involved in atmospheric oxidation processes. The principal peroxy radicals in the Earth's atmosphere are the hydroperoxy radical (H02) and the methylperoxy radical (CH3O2), and both play an important role in determining the oxidising capacity of the atmosphere - the ability of the atmosphere to remove pollutants. Both HO2 and CH3O2 radicals self-react: H02 + H02 (+M) -> H202 + 02 (+M) CH3O2 + CH3O2 -> 2 CH3O + 02 -> CH3OH + HCHO + 02 in addition to reacting with other species in the atmosphere, including each other: H02 + CH3O2 -> CH3OOH + 02 Despite the importance of these reactions in the troposphere, few investigations of their reaction kinetics have been conducted under tropospheric conditions. In addition, comparisons between modelled and observed concentrations of HO2, H2O2 and CH3OOH have indicated a need for further laboratory studies of the kinetics of HO2 and CH3O2 radicals. Experiments have been conducted in this work on the HO2 and CH3O2 self-reactions, and on the HO2 + CH3O2 cross-reaction, under a wide range of experimental conditions pertinent to the troposphere. This work employed conventional and laser flash photolysis with broadband ultraviolet absorption spectroscopy, incorporating charge coupled device (CCD) detection to facilitate data acquisition over a broad wavelength range, and on rapid timescales. Classical or numerical models were used to simulate and fit to experimental data by optimising the kinetic parameters in the models. Results show the rate coefficient for the HO2 self-reaction to be more strongly dependent on temperature and water vapour than previously thought, while the rate coefficient for the CH3O2 self-reaction was found to be only very weakly dependent on temperature under the conditions employed in this work. Results for the HO2-CH3O2 cross-reaction were found to be in good agreement with previous work. The improvements in both accuracy and precision associated with CCD detection, and the use of experimental conditions directly relevant to the troposphere, have led to a considerable improvement in our understanding of the roles of HO2 and CH3O2 radicals in the Earth's atmosphere, which is also discussed in this thesis.

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