Modification, Characterization and Evaluation of a Quantum/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

Abstract

Abstract. Achieving an improved understanding of the anthropogenic influence on climate due to man made greenhouse gas emissions is of major interest for the global civilization. Sources, sinks and transport of climatologically-relevant gases in the Earth's atmosphere are still insufficiently understood, implying a fundamental need for accurate, spatially and temporally dense observations. Tunable diode laser absorption spectroscopy is a widely used technique for in situ sensing of atmospheric composition. Mid-infrared spectrometers have become commercially available, since continuous wave quantum cascade (QCL) and interband cascade lasers (ICL) today achieve excellent performance, stability and high output power at typical ambient conditions. Aircraft deployment poses a challenging environment for these newly-developed instruments. Here, we demonstrate the successful adaption of a commercially available QCL/ICL based spectrometer for airborne in-situ trace gas measurements. The instrument measures methane, ethane, carbon dioxide, carbon monoxide, nitrous oxide and water vapor simultaneously, with high 1σ-precision (740 ppt, 205 ppt, 460 ppb, 2.2 ppb, 137 ppt, 16 ppm, respectively) and high frequency (2 Hz). We estimate a total measurement uncertainty of 2.3 ppb, 1.6 ppb, 1.0 ppm, 7.4 ppb and 0.8 ppb in CH4, C2H6, CO2, CO and N2O, respectively. The instrument enables truly simultaneous and continuous (zero dead-time) observations for all targeted species. Frequent calibration allows for a measurement duty cycle ≥ 90 % while retaining high accuracy. A 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 U.S.. 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 excellent agreement of the QCL/ICL based instrument to these concurrent observations within the combined measurement uncertainty.