A Compact Low-Power Direct Absorption Spectrometer for Trace Gas Measurements

Global greenhouse gas (GHG) emissions from natural and agricultural ecosystems are continuously increasing, but the accurate quantification of their sources and sinks to guide mitigation strategies still remain challenging. There is a need for measurement technologies capable of tracking and identification of GHG sources with high temporal and spatial resolution that will inform sustainable agricultural management practices. To this end, a prototype of a compact low-power mid-IR tunable diode laser spectrometer platform has been developed for high-precision fast-response trace gas measurements. The instrument leverages on a proven field rugged closed path design and on the advancements of stable low-noise interband cascade lasers. It incorporates a novel miniature single pass sample cell with and power efficient laser driving technique. The platform includes integrated pump and thermoelectric modules for automatic pressure, flow and temperature control enabling fully autonomous unattended operation in diverse environmental conditions. Under laboratory conditions the device achieves precision (Alan variance) of 1.5 ppb N2O and 7 ppb CH4 with 100 ms averaging.  A sub-ppb performance is possible at longer averaging times allowing for atmospheric methane and nitrous oxide concentration and flux monitoring. Field deployment in remote areas using solar energy is possible due to the low power consumption. Laboratory and field tests were performed to quantify the effects of pressure, temperature and humidity on the operation of the instrument. We describe the design rational of an inertial particle separator acting as a non-barrier filter to prevent contamination of the optics and the use of sulfonated tetrafluoroethylene ionomer intake tube acting as water vapor permeable membrane to dry the air sample and minimize the effects of humidity on the concentration measurements. The results of a field deployment over fertilized corn field will be presented.