Environmental applications of cavity enhanced spectroscopy in the mid infra-red region

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Ultrasensitive and quantitative absorption spectroscopy techniques will be developed in the mid infra-red region for determination of mixing ratios and isotopic abundances of trace atmospheric constituents. Technological developments based on recent work in the Bristol group will be exploited in a new generation of analytical spectrometers based on state-of-the-art laser technology in the infra-red at wavelengths beyond 3 microns. In this region, the strong fundamental vibrational transitions of a variety of molecules can be accessed spectroscopically, presenting considerable potential advantages of improved selectivity and detection limits over current instruments designed around telecoms diode lasers operating in the near infra-red. The project builds on: (a) our recent advances in cavity enhanced spectroscopy at 3.25 microns using efficient difference frequency generation (DFG) laser sources based on compact, low-cost diode lasers; and (b) an on-going project to couple a 7.8 micron quantum cascade laser (QCL) with an optical cavity using feedback of light to the laser to enhance the coupling efficiency. The PG student will focus on analytical and environmental applications. He/she will: (i) test the performance of an optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) spectrometer with the 7.8 micron wavelength cw QCL laser (an apparatus being designed and built by a current PDRA, using existing equipment) by studying 13CH4 and 12CH4 absorption lines in close spectral proximity, and optimizing the precision and accuracy of determination of carbon isotope delta-13C values; (ii) couple the 3.2 micron wavelength DFG mid-IR spectrometer with a previously developed and automated pre-concentration apparatus (proven to be highly specific to ethene through use of a molecular sieve of appropriate pore size) to explore the feasibility of delta-13C determinations on C2H4 (present at less than a few ppbv in ambient air) as a proof-of-principle study; (iii) with the expertise gained from (i) and (ii), evaluate the potential for CEAS methods, coupled to pre-concentration stages if required, for isotopologue ratio measurements for a variety of further small compounds in air (e.g. N2O, C2H2); (iv) establish a collaboration with Prof R.P. Evershed (Chemistry, Bristol) and Dr E. Hornibrook (Earth Sciences, Bristol) to determine CH4 delta-13C values in studies of the effects of soil type and the role of methanotrophic bacteria on atmospheric methane; and (v) to explore the feasibility of deployment of a QCL-based spectrometer for in situ measurements of methane fluxes and their isotopic composition. Such delta-13C value determinations are a key means of establishing atmospheric sources and sinks, and a portable analytical spectrometer has considerable advantages of size, cost and ease of deployment over use of isotope ratio mass spectrometry if competitive accuracy and precision can be demonstrated. In addition to the regular academic, research and transferable skills training delivered through the Bristol Graduate School of Chemistry, the PG student will receive expert training in: lasers and molecular spectroscopy; optics and optical cavities; atmospheric and environmental chemistry; analytical spectroscopy; analysis of data sets; preparation of data for publication, etc. Within Bristol, the PG student will gain a broad perspective of the research area through interaction (e.g. joint group meetings and seminars) with the Atmospheric Chemistry Research Group headed by Prof D.E. Shallcross and Dr S. O'Doherty, the Organic Geochemistry Unit (led by Prof R.P. Evershed and Dr R.D. Pancost) and the cross-faculty Biogeochemistry Research Centre (encompassing groups from Chemistry, Biology, Earth Sciences, and Geography). The student will attend the European Research Course on Atmospheres (ERCA, held annually in Grenoble) and a planned workshop on Cavity Enhanced Spectroscopy (scheduled for 2011 in Canada).