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The detected electrical signals are amplified, first by pre-amplifiers with a fixed gain and second by an automatically adjusted gain in the main electronic module, and digitized by a 16-bit analog-to-digital converter.

As a multichannel tunable laser spectrometer, SPIRIT consists of laser sources, an optical cell, where the trace gas is continuously sampled, photodetectors, and an electronic system controlling laser functioning and data acquisition. The instrumental principle for ground measurements with one single channel and the process to retrieve molecular concentrations of the trace gases have been described previously [ ].

The present paper only deals with the details related to the three-channel airborne version. Figure shows 3D drawings of the SPIRIT instrument integrated in the aircraft rack, using a computer-aided design (CAD).

The instrument weighs 102 kg, its dimensions are 95.8 × 55.9 × 65.4 cm 3, and its power consumption is less than 1.2 kW. A schematic representation of the operation principle is given in Fig. SPIRIT operates with three Continuous-Wave Distributed-Feedback Room-Temperature QCLs (CW-DFB-RT-QCLs from Alpes Lasers SA; see characteristics given hereafter, as shown in Table ), corresponding to the three measurement channels, coupled to one optical cell and two detectors only—one for the measurement channels and the other one for the reference channels. The home-made main electronic module receives commands from a laptop computer via an RS-232 link and drives the current and temperature controllers for each of the three QCLs. Each temperature controller consists of a Peltier thermoelectric module that maintains individually each laser chip at a constant temperature, with a typical precision of 1 mK, allowing for laser single-mode emission at a precise wavenumber.

A water–glycol circuit regulated at a constant temperature (13 °C) allows for removing excess heat from each Peltier cooler and the Stirling detector cooler, using a liquid pump (Ecocirc Laing) and a liquid–air thermal exchanger (LA-115 Laird Technologies). The three QCLs are sequentially switched on/off and synchronized by the data acquisition system.

For each QCL, the current controller applies a saw-tooth ramp of several mA during 4.1 ms to sweep a spectral micro-window of interest for the species to quantify. This ramp is repeated 110 times for measurement spectra followed by eight times for reference spectra. The whole sequence containing measurements for the three channels are thus recorded within 1.6 s, including averaging and transfer of these data to the laptop computer.

All along this paper, volume mixing ratios (vmr) are used as relevant quantities, defined as mole fractions relative to the air (usually expressed in part per billion in volume—ppbv), which is a common practice in the atmospheric community. Roughly calculated preliminary vmr (only based on absorption depths of the line centers corresponding to known vmr measured in the laboratory) are inferred and displayed online to be able to modify live the flight trajectory. In addition, the data are stored on the computer for post-processing, leading to retrievals of the trace-gas vmr, as detailed in Sect.  Security monitor pro serial doc. ( ).

Fig. 4 a Example of experimental ( black line) and simulated ( red line) transmission spectra for CO at 31.7 °C and 32.0 hPa, with pathlength of 134.22 m, leading to CO vmr of 89.1 ± 0.9 ppbv. B Experimental ( black line) and simulated ( red line) second derivatives of this transmission signal.

C Minimum residuals of the direct transmission ( pink line) and of the second derivative ( blue line) In principle, the absorption measurements may be considered absolute and do not require a calibration if accuracy is expected to be about that of the spectroscopic database. The uncertainties on the spectroscopic parameters [ ], essentially the lines strengths and air broadening factors (5% maximum each), affect the accuracy of the trace-gas concentrations by 5 and 2.5%, respectively, as derived from a sensitivity analysis. Vmr are derived knowing the total pressure p and the temperature T of the optical cell. These are measured with accuracies of 0.2 hPa and 1 K, leading to 0.3 and 0.4% accuracies in the inferred vmr, respectively, according to a sensitivity analysis. The dry vmr of the trace gases are then deduced using the water vapor mixing ratios measured by SPIRIT at 1277.270 cm −1 (on the CH 4–N 2O channel), and the H 2O vmr agree with measurements reported by the core instrumentation installed on the aircraft within 10%. Depending on the plane and on the altitude level, this instrumentation is a dew or a frost point temperature hygrometer (CR2 from Buck Research), a lyman-alpha fluorescent spectrometer, or a tunable diode laser absorption spectrometer (WVSS-II from SpectraSensors).