CO measured by SPIRIT during airborne campaigns

An infrared absorption spectrometer called SPIRIT (SPectromètre InfraRouge 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). Absorptions of the mid-infrared radiations by the species in the cell at reduced pressure (30-40 hPa), with path lengths adjustable from 117.45 to 167.78 m are quantified using a HgCdTe photodetector cooled by Stirling cycle. Measurements of CO, NO2, CH4 and CO2 volume mixing ratios were performed during the period 2011-2021.
Two rovibrational lines of CO were successively used, namely 2179.772 cm-1 in 2011-2016, and 2183.224 cm-1 in 2019-2021. The performances of SPIRIT concerning CO are the following: a linearity response between 0.6 and 600 ppbv with a coefficient of determination R2 > 0.992; an in-flight 1sigma precision below 2% at 1.6 s sampling. In-flight comparisons with calibrated gas mixtures allow to show no instrumental drift correlated to atmospheric pressure and temperature changes (when vertical profiling) and to estimate the overall uncertainties in the measurements of CO to be 0.9 ppbv, or about 4 ppbv when uncalibrated (1.6 s sampling).
SPIRIT has been operated on Falcon-20 aircraft from DLR (Germany) and on Falcon-20 and ATR-42 from SAFIRE (CNRS-CNES-Météo France). With support from various projects (SHIVA and DACCIWA from the FP7 European Commission; TC2, ChemCalInt, GLAM-CHARMEX-MISTRALS and MAGIC from INSU-CNES-Météo France), 74 flights with 209 hours of measurements were conducted over three continents (Europe, Asia, and Africa), including two inter-continental measurements (Europe-Asia and Europe-Africa). 3D trajectories mapped by CO levels was plotted for each flight and is reported in paper submitted to Earth System Science Data.