Dual Quantum Cascade Laser System for Simultaneous Measurements of 13CH4 and CH3D Methane Isotopologues

Methane is the second most important atmospheric greenhouse gas after CO2 yet its global sources and sinks are still inadequately characterized. Monitoring the isotopic composition of atmospheric methane is one of the most promising approaches to closing the methane budget. Detailed process studies of isotopic fractionation associated with methane sinks and sources are also crucial. For both purposes, a real time instrument that can measure multiple isotopologues of methane is required. There are no existing field deployable instruments for the sensitive, real time measurement of CH3D. There are also no instruments available with sufficient precision to monitor ambient atmospheric 13CH4 (excluding the Aerodyne Research, Inc. predecessor to the proposed instrument). Hence, there is a need for sensitive, measurements of both isotopologues ideally within one instrument. Recent advances in mid infrared laser technology allow us to detect methane in its strongest absorption band at 3 microns. We propose to couple these new lasers with advanced infrared detectors and new optical designs (for longer absorption path length in a compact instrument). The resulting instrument will simultaneously measure the isotopic ratios of CH3D and 13CH4 with excellent precision 3 per mil and 0.1 per mil, respectively, for samples with methane mixing ratios near the ambient value of 1.8 ppm. For process studies with elevated methane mixing ratios (such as chamber studies of peat bog emissions), the precision of the isotopic ratios will be even better (by perhaps a factor of five). The resulting instrument will be compact, portable and autonomous and will be sufficiently sensitive to deploy to remote field sites or even from light aircraft to assess sources and sinks of methane throughout the world. Our Phase II project reached its stated precision goals and demonstrated the feasibility of this approach using 8 micron lasers. We also successfully deployed multiple instruments in various settings. In collaboration with MIT we quantified the isotopic abundance of the primary clumped isotope of methane thus demonstrating laser based isotopic thermometry. We will obtain and utilize new 3 micron ICL lasers to significantly exceed the precision targets of the original proposal. We will evaluate and optimize several of our instruments as they are employed in research settings. We will extend our initial clumped isotope work focusing on improved accuracy and the detection of multiple species (13CH3D and CH2D2). Commercial Applications and Other Benefits: This instrument is eagerly anticipated by leading climate change researchers around the world since it will provide scientifically meaningful isotopic ratio measurements in real time, without pre-concentration and without cryogenic cooling of either laser or detector. Other applications of this technology include air pollution monitoring, human breath analysis, geochemical prospecting, and industrial process monitoring. This technology will provide a significant societal benefit through improved understanding and mitigation of global warming and global climate change.