The SPIRIT airborne instrument: a three-channel infrared absorption spectrometer with quantum cascade lasers for in situ atmospheric trace-gas measurements

Abstract

trace gases, enabling fast online and precise measurements with compact instrumentation. This technique has been implemented in aircraft [1,2], and is now more and more widespread. Most recently built airborne instruments comprise several variants, i.e., non-resonant cavities using tunable diode lasers (see, e.g., [3][4][5][6] and references therein), quantum cascade lasers (QCL; see, e.g., [7,8] and references therein), or difference frequency generation lasers [9], and resonant cavities for cavity ring-down spectroscopy (CRDS) [10][11][12], off-axis integrated cavity output spectroscopy (OA-ICOS) [13][14][15], or optical-feedback cavity enhanced absorption spectroscopy (OF-CEAS) [16]. Using lasers allows for narrow linewidths, leading to selectivity of trace gases and very low limits of detection. Using QCLs, which emit in the Mid-Infrared (MIR) domain, leads to stronger absorptions than in the near infrared due to the molecular fundamental transitions taking place and, therefore, again to higher sensitivity. Among other advantages, QCLs possess high optical output power, which is useful for long optical path operation. First used in pulsed mode, QCLs begin to be operated in continuous-wave (CW) mode, resulting in negligible linewidths [17] with respect to the molecular absorption line at the operation pressure. As a result, this leads to measurements of intrinsically higher accuracy and sensitivity.For more than 20 years, our group, in collaboration with the French space agency CNES (Centre National d'Etudes Spatiales), has developed a series of balloon-borne instruments using optical spectroscopy techniques with remote (stars, moon) or in situ (embarked lasers) light sources for the detection of stratospheric species [18][19][20]. In the present paper, we present a newly built airborne instrument called SPIRIT (SPectromètre Infra-Rouge In situ Toute altitude). SPIRIT performs ultra-high-resolution (<0.001 cm −1 ) infrared absorption spectroscopy thanks to the use of three MIR CW-QCLs (with FWHM of emission lines lower than Abstract An infrared absorption spectrometer called SPIRIT (SPectromètre Infra-Rouge In situ Toute altitude) has been developed for airborne measurements of trace gases in the troposphere. At least three different trace gases can be measured simultaneously every 1.6 s using the coupling of a single Robert multipass optical cell with three Quantum Cascade Lasers (QCLs), easily interchangeable to select species depending on the scientific objectives. Absorptions of the mid-infrared radiations by the species in the cell at reduced pressure (<40 hPa), with path lengths adjustable up to 167.78 m, are quantified using an HgCdTe photodetector cooled by Stirling cycle. The performances of the instrument are assessed: a linearity with a coefficient of determination R 2 > 0.979 for the instrument response is found for CO, CH 4 , and NO 2 volume mixing ratios under typical tropospheric conditions. In-flight comparisons with calibrated gas mixtures allow to show no instrumental drift correlated with a...

Intercontinental transport of biomass burning pollutants over the Mediterranean Basin during the summer 2014 ChArMEx-GLAM airborne campaign

“…Dry volume mixing ratios (vmr) are deduced using the measured pressure and temperature of the optical cell and using the water vapour mixing ratios measured by a laser absorption spectroscopy hygrometer (WVSS-II from SpectraSensors). Total uncertainties for CO were estimated to be 4.7 ppb using comparisons performed during previous flights with high-altitude instrumented surface stations (Pic du Midi de Bigorre and Puy-de-Dôme, France) belonging to the WMO/GAW network and with a NOAA standard cylinder on board the aircraft (Catoire et al, 2017). They are also checked in the present campaign (see Sect.…”

“…The SPIRIT airborne infrared absorption spectrometer for the measurements of trace gases was mounted on board the Falcon 20. A complete description of the instrument principle can be found in Catoire et al (2017). In brief, it uses continuous-wave distributed-feedback roomtemperature quantum cascade lasers (QCLs), allowing rapid scanning (each 1.6 s) of strong fundamental molecular rotational-vibrational lines lying in the mid-infrared, with ultra high spectral resolution (10 −3 cm −1 ).…”

Evidence of convective transport in tropical West Pacific region during SHIVA experiment

“…The airborne CO and CH 4 measurements were performed with the SPIRIT instrument (Catoire et al, ), and CHBr 3 and CH 3 I with the GHOST instrument (Sala et al, ). Additionally, whole air samples taken in the RV Sonne were analysed for halocarbons, CO and CH 4 .…”

Modification, Characterization and Evaluation of a Quantum/Interband Cascade Laser Spectrometer for simultaneous airborne in situ observation of CH&lt;sub&gt;4&lt;/sub&gt;, C&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;6&lt;/sub&gt;, CO&lt;sub&gt;2&lt;/sub&gt;, CO and N&lt;sub&gt;2&lt;/sub&gt;O

“…and C 2 H 6 providing best-in-class detection sensitivities of 40 ppt and 15 ppt, respectively. The custom-built airborne QCL spectrometer described by Catoire et al (2017) allows for simultaneous observation of CO, CH 4 and nitrogen dioxide (N O 2 ) with in-flight precisions of 0.3 ppb, 5 ppb and 0.3 ppb for a sampling time of 1.6 s. McManus et al (2011) reported on the development of a high-sensitivity trace gas instrument based on quantum cascade lasers and astigmatic Herriott cells with up to 240 m path length. This design is commercially available from AERODYNE RESEARCH INC., Billerica, USA, and allows for simultaneous observation of a multitude of gases, depending on the wavelength of the installed lasers.…”