# Advances in High-Precision NO2 Measurement by Quantum Cascade Laser Absorption Spectroscopy
## 2 Materials and Methods
### Description of the Dual-Laser Spectrometer
A thermo-chiller (T-Three, Solid State Cooling System, Wappingers Falls, NY, USA) is used to stabilize the temperature of the water-cooled elements mounted in series (laser housings, optical board, lasers current drivers and thermoelectric (TEC) controllers). The DFB-QCLs are mounted in a custom-made compact laser housing (CLH) with embedded TEC element. A few millimeters after the CLH, an AR coated aspheric lens (Black Diamond-2, Thorlabs Inc., Newton, NJ, USA) is placed to collimate the output laser beam. This external collimation solution was chosen to allow more flexibility in the beam shaping. Afterwards, the two laser beams are combined by a custom-made IR/IR beam combiner (LohnStar Optics Inc., Escondido, CA, USA) anddirected onto a series of plane and spherical, gold-coated mirrors used to further steer and shape the combined IR beams to appropriately fulfill the coupling and beam matching conditions of the MPC (see, e.g., [41]). After multiple reflections, the IR beam exits the MPC and is aimed at a MIR detector (PVI-4 TE-6, VIGO System, Ozarow Mazowiecki, Poland) with a high-bandwidth pre-amplifier (PIP-DC-200M). The concentrations of NO and NO\({}_{2}\) are retrieved by fitting a Voigt function to the measured spectra using the Levenberg-Marquardt least-squares algorithm implemented in LabView (National Instruments Inc., Austin, TX, USA). The transmission spectrum background (baseline) is fitted using a polynomial function. The spectral line intensity and the pressure broadening coefficients are taken from the HITRAN2016 database [42], whereas the sample pressure and temperature are measured and taken into account in the fitting procedure. An example of a transmission spectrum for NO\({}_{2}\) with the associated fitted curves using Voigt profiles are shown in Figure 2. To convert the time scale into frequency scale, a two inch long solid germanium etalon with a free spectral range (FSR) of 0.0243 cm\({}^{-1}\) can be placed in the beam path. For the highest stability, but especially when measuring at very low NO\({}_{2}\) concentration levels (\(<\)0.1 ppb), where the detector noise and optical fringes dominate the NO\({}_{2}\) absorption signal, a frequency locking mechanism is implemented using a neighboring water absorption line at 1600.25 cm\({}^{-1}\) (Figure 2). For this, the retrieved central position of this water absorption line is used as a reference for determining the position of the NO\({}_{2}\) line, assuming a constant frequency separation between both as given in the HITRAN database. Besides this passive frequency lock, we also implemented an active locking, where the water absorption line position is used as feedback signal to the laser TEC to adjust the heat-sink temperature such that this position remains constant (\(\pm 1\times 10^{-4}\) cm\({}^{-1}\)). For ambient air measurements, the water concentration is high enough to assure a good signal-to-noise ratio (SNR) so that its absorption line can always be used in both frequency locking mechanisms. Figure 1: Top view of a 3D representation of the optical setup. Abbreviations: CLH, compact laser housing; BC, IR/IR beam combiner; TL, trace laser; FFM, flat flip-mirror; GE, germanium etalon; BSM, beam shaping in-coupling mirrors (f indicates the focal length of spherical mirrors); MPC, multipass cell (without closing lid); D, mid-infrared detector. The laser beam path from the CLHs to the first spot on the MPC’s back mirror and the path from the last spot to the detector are also indicated (colored line).