Adaptation and performance assessment of a quantum and interband cascade laser spectrometer for simultaneous airborne in situ observation of CH 4 , C 2 H 6 , CO 2 , CO and N 2 O

Abstract

Abstract. Tunable laser direct absorption spectroscopy is a widely used technique for\nthe in situ sensing of atmospheric composition. Aircraft deployment poses a\nchallenging operating environment for instruments measuring\nclimatologically relevant gases in the Earth s atmosphere. Here, we\ndemonstrate the successful adaption of a commercially available continuous\nwave quantum cascade laser (QCL) and interband cascade laser (ICL) based\nspectrometer for airborne in situ trace gas measurements with a local to\nregional focus. The instrument measures methane, ethane, carbon dioxide,\ncarbon monoxide, nitrous oxide and water vapor simultaneously, with high\n1\u2009 s – 1σ precision (740\u2009 ppt , 205\u2009 ppt , 460\u2009 ppb , 2.2\u2009 ppb , 137\u2009 ppt and 16\u2009 ppm ,\nrespectively) and high frequency (2\u2009 Hz ). We estimate a\ntotal 1\u2009 s – 1σ uncertainty of 1.85\u2009 ppb , 1.6\u2009 ppb , 1.0\u2009 ppm , 7.0\u2009 ppb \nand 0.8\u2009 ppb in CH4 , C2H6 , CO2 , CO and N2O , respectively.\nThe instrument enables simultaneous and continuous observations for all\ntargeted species. Frequent calibration allows for a measurement duty cycle\n ≥90 \u2009%. Custom retrieval software has been implemented and instrument\nperformance is reported for a first field deployment during NASA s\nAtmospheric Carbon and Transport – America (ACT-America) campaign in fall\xa02017\nover the eastern and central USA. This includes an inter-instrumental\ncomparison with a calibrated cavity ring-down greenhouse gas analyzer\n(operated by NASA Langley Research Center, Hampton, USA) and periodic flask\nsamples analyzed at the National Oceanic and Atmospheric Administration\n(NOAA). We demonstrate good agreement of the QCL- and ICL-based instrument to\nthese concurrent observations within the combined measurement uncertainty\nafter correcting for a constant bias. We find that precise knowledge of the\n δ13C of the working standards and the sampled air is\nneeded to enhance CO2 compatibility when operating on the 2227.604\u2009 cm−1 13C16O2 absorption line.