Compact High Precision Field Instrument for all Major Greenhouse Gases

To quantify emissions of greenhouse gases in field settings, high precision and fast response trace gas measurements are needed. High precision and fast response allow measurements of emissions by means of correlation with wind fluctuations (eddy correlation method). While such instruments exist for single or even multiple trace gases, there is not yet available a single compact instrument that will measure all four major greenhouse gases [carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and water vapor (H2O)] with the necessary precision. Simultaneous measurement of all the major greenhouse gases allows researchers to gain a comprehensive picture of emission processes. We propose to develop a highly compact field instrument for high precision fast response measurement of all four major greenhouse gases: CO2, CH4, N2O and H2O. The instrument will be based on quantitative laser spectroscopy with a pair of mid-infrared lasers. The proposed effort will draw upon and extend trace gas measurement technology previously developed at Aerodyne Research, Inc (ARI) that has already yielded the most precise field-ready trace gas instruments presently available. Our instruments have defined the current state of the art in trace gas measurements. Based on our results to date we expect the new instrument to have a precision of one part in 10,000 (0.1 per mil) with 1 second averaging, while performing simultaneous measurement of the three most significant anthropogenic greenhouse gases, CO2, CH4, and N2O. By measuring water vapor simultaneously in the same sample volume, we will be able to report mixing ratios on a dry-gas basis without needing to actually dry the air sample. An exciting new opportunity for a two laser instrument is to employ a quantum cascade laser (QCL) operating at 5 m to measure CO2, N2O and H2O, and a newly available antimonide semiconductor diode laser operating at 3 m to measure CH4. These new 3 m diode lasers will allow access to the strongest absorption lines of methane and other gases with C-H stretch transitions, and will allow use of more sensitive, linear and stable infrared detectors. Beyond the goal for extreme precision, we plan to produce a highly compact optical design that is comparable in size to our current rack-mount size single laser instrument, and this will be the main technical challenge. There is currently no instrument that is available with such high precision for simultaneous greenhouse gas measurements that can fit in a single 19-inch rack mounted box. The path to reducing the instrument size is primarily by reducing the size of the optical module, especially in the optics for collecting and combining light from the two lasers. We will develop a design for collecting and focusing laser light that employs (proprietary) miniature optical elements that can be contained within the hermetically sealed laser housing. This will maintain good optical quality in a much more compact and stable package than currently available. Commercial applications and other benefits: The availability of a field-ready compact high precision instrument for the simultaneous measurement of all the major greenhouse gases (or the isotopologues of two major greenhouse gases) will find wide use in research on the carbon cycle and fluxes of greenhouse gases in the natural environment. The capability to make a flexible selection of lasers will make this a highly flexible instrument that can be set up to measure other sets of gases for other research and monitoring purposes. The approaches to be explored in this project will yield optical designs that are generally applicable to a wide variety of measurement problems that require simultaneous measurement of multiple gases. If configured with two lasers at another set of wavelengths the same instrument platform will measure the three major isotopologues of CO2 (12C16O2, 13C16O2, 12C18O16O) and two major isotopologues of H2O (H216O and H2 (0.1 per-mil) in 1 second.