Greenhouse gas detection using cavity enhanced absorption spectroscopy and cavity ring-down spectroscopy : trace detection of CH₄, CO₂ and N₂0 in ambient air, standard gas samples and in the headspace of soils

• Main Author: Chase, Tanya
• Published: University of Bristol 2015
• Subjects: 363.738
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.686246

The work presented in this thesis is predicated upon the environmental applications of cavity enhanced absorption spectroscopy and cavity ring-down spectroscopy. These are related techniques that are highly useful for sensitive gas detection which is important in terms of anthropologically induced climate change and the detection of the changing levels of greenhouse gases. Sensitive gas detection techniques, specifically isotope ratio analysis, are useful for determining the sources and sinks of greenhouse gases and for distinguishing whether sources and sinks are natural or anthropogenic. The research involved gas detection using commercial near-infrared cavity ring-down spectrometers, made by Picarro, and highlights how well and to what environmental uses these instruments can be applied. Various gas mixtures containing methane and carbon dioxide were analysed by the CRDS instruments to try to determine the detection limits, and the effect that varying the concentrations would have upon the precision and accuracy of the measurements made. Headspace soil measurements of CH4 and C02 were also demonstrated to be made easily without processing of the gas stream.The main work described in this Thesis involved the implementation of a home-built optical feedback cavity enhanced absorption spectroscopy /cavity ring-down spectroscopy experiment which made use of a V-shaped optical cavity and a 7.8 υm quantum cascade laser for the detection of greenhouse gases in the mid-infrared. This comprised of the detection and analyses of spectral lines of methane and nitrous oxide isotopologues. Measurement in the mid-IR took advantage of the excitation of the stronger fundamental vibrational transitions occurring in this region and increased optical path lengths from the optical cavity and signal amplification from optical feedback are features that gave high signal to noise measurements. These techniques have the potential to be further developed for field usage by overcoming many of the limitations of alternative greenhouse gas detection techniques, such as instrument sensitivity and portability.