Adaptation and performance assessment of a quantum and interband cascade laser spectrometer for simultaneous airborne in situ observation of CH<sub>4</sub>, C<sub>2</sub>H<sub>6</sub>, CO<sub>2</sub>, CO and N<sub>2</sub>O.
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- "Tunable laser direct absorption spectroscopy is a widely used technique for the in situ sensing of atmospheric composition. Aircraft deployment poses a challenging operating environment for instruments measuring climatologically relevant gases in the Earth's atmosphere. Here, we demonstrate the successful adaption of a commercially available continuous wave quantum cascade laser (QCL) and interband cascade laser (ICL) based spectrometer for airborne in situ trace gas measurements with a local to regional focus. The instrument measures methane, ethane, carbon dioxide, carbon monoxide, nitrous oxide and water vapor simultaneously, with high 1s-1σ precision (740ppt, 205ppt, 460ppb, 2.2ppb, 137ppt and 16ppm, respectively) and high frequency (2Hz). We estimate a total 1s-1σ uncertainty of 1.85ppb, 1.6ppb, 1.0ppm, 7.0ppb and 0.8ppb in CH4, C2H6, CO2, CO and N2O, respectively. The instrument enables simultaneous and continuous observations for all targeted species. Frequent calibration allows for a measurement duty cycle ≥90%. Custom retrieval software has been implemented and instrument performance is reported for a first field deployment during NASA's Atmospheric Carbon and Transport-America (ACT-America) campaign in fall 2017 over the eastern and central USA. This includes an inter-instrumental comparison with a calibrated cavity ring-down greenhouse gas analyzer (operated by NASA Langley Research Center, Hampton, USA) and periodic flask samples analyzed at the National Oceanic and Atmospheric Administration (NOAA). We demonstrate good agreement of the QCL-and ICL-based instrument to these concurrent observations within the combined measurement uncertainty after correcting for a constant bias. We find that precise knowledge of the δ<sup>13</sup>C of the working standards and the sampled air is needed to enhance CO<sub>2</sub> compatibility when operating on the 2227.604<sup>ĝ'1</sup> <sup>13</sup>C<sup>16</sup>O<sub>2</sub> absorption line."
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