Anthropogenic effects on the concentration of hydroxyl radicals in the troposphere : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry at Massey University, Albany, New Zealand

Abstract

This thesis investigated the use of a TiO₂ catalyst to increase atmospheric ·OH concentrations in order to reduce atmospheric CH₄ concentrations, an important greenhouse gas that contributes to climate change effects. Firstly, a reference library of important atmospheric molecules was created using the Matrix-Isolation technique coupled with Fourier-Transform Infrared Spectroscopy (MI-FTIR). The spectra collected for these molecules were assigned using a combination of literature data and data calculated by Gaussian-16 software. This reference library was then used to create correction functions that could be applied to spectra collected of other molecules. Detection of ·OH was then attempted using the MI-FTIR technique, combined with Tesla-coil discharge applied to gas-phase water mixtures to form the radicals. This technique was ultimately deemed unsuitable due to difficulties in detection. A spectrofluorophotometer was then used as the excitation source, in combination with a built in-house cell that housed a TiO₂-coated slide in the presence of water and hydrocarbons, detected by a residual gas analyser (RGA). The results of these experiments were also inconclusive due to detection difficulties. The photocatalytic activity of TiO₂ slides under aqueous conditions was tested and confirmed using probe molecules of coumarin and terephthalic acid to detect for ·OH oxidation products using emission fluorescence spectroscopy, from which a quantum yield for ·OH formation under aqueous conditions was established. Finally, a rough atmospheric model was established to give insight to the effect of changing atmospheric concentrations of molecules on other concentrations. A perturbation rate for a specific particle density of TiO₂ was added to this model to establish the effect on the concentrations, and it was found that CH₄ and O₃ experienced significant decreases. Overall, TiO₂ was predicted to be an appropriate atmospheric catalyst for removal of methane, but further research is required for implementation.