Dual Quantum Cascade Laser System For Simultaneous Measurements of 13CH4 and CH3D Methan Isotopolgues

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 quantum cascade laser technology allow continuous wave operation near room temperature without cryogenic cooling. 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 6 per mil and 0.3 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 I results demonstrated the feasibility of the Phase II project with quantitative detection of CH3D and showed that the selected spectral region is free from interference. We also developed a preliminary design for the Phase II instrument and studied other isotopic variants of methane. We will complete the detailed design and construction of the prototype instrument during year one. In the second year, we will demonstrate the utility of the instrument in laboratory studies at MIT with pre-concentrated methane samples and with ambient samples in field studies of gas emissions from the Marcellus shale with the Wofsy group at Harvard. 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 though improved understanding and mitigation of global warming and global climate change.