# Advances in High-Precision NO2 Measurement by Quantum Cascade Laser Absorption Spectroscopy
## 2 Materials and Methods  
### Description of the Dual-Laser Spectrometer  
The main target of our instrumental design and development was to reach high precision NO\({}_{2}\) measurements under field conditions with possible extension towards other relevant species, such as NO. The chemistry of these compounds in the atmosphere is strongly interconnected since they share common anthropogenic sources and are involved in rapid cyclic processes. From an air quality perspective, especially for urban or near-road environments, the sum of both is often considered and referred to as NO\({}_{x}\). Considering these aspects, a dual-laser concept has been implemented [32; 37], where two distributed feedback quantum cascade lasers (DFB-QCLs, Alpes Laser SA, St-Blaise, Switzerland) with distinct emission frequencies are combined and coupled into an astigmatic multipass absorption cell (MPC). For completeness, the instrumental setup is described for both lasers. However, since NO\({}_{2}\) is the target substance of the present study, only the results that are relevant for NO\({}_{2}\) are shown. The first laser emits in the spectral range around 1600 cm\({}^{-1}\), corresponding to the \(v_{3}\) ro-vibrational band of NO\({}_{2}\). Similar to previous studies [32; 34], the doublet NO\({}_{2}\) line at 1599.9 cm\({}^{-1}\) is used for concentration retrieval. This doublet-line was chosen for its intensity and because it is well separated from any neighboring water lines (see below). The second laser emits in the 1900 cm\({}^{-1}\) range, and NO concentration are derived using a doublet line at 1900.1 cm\({}^{-1}\). Both lasers are driven in intermittent continuous-wave mode (iCW). This driving approach and its advantages have been described elsewhere [38; 39; 40]. Briefly, the laser driver rapidly turns on and off the current to the lasers, which results in an emission frequency sweep of the DFB-QCL due to the change in the refractive index of the laser device induced by Joule heating. This driving mode has the advantage of lower heat dissipation requirements and thus more compact thermal management solutions compared to the conventional continuous-wave (CW) mode. For NO\({}_{2}\) and NO, the laser drivers apply current to the lasers for 45 and 30 \(\upmu\)s, resulting in emission frequency sweeps over 0.8 and 0.4 cm\({}^{-1}\), respectively. The laser current drivers are triggered by a field-programmable gate array (FPGA) embedded on a commercial system-on-chip (Alpha250, Koheron, Lorient, France). This board features a dual-core ARM processor and a fast analog front-end with dual-channel 14-bit ADCs and 16-bit DACs of 100 MHz bandwidth, and it is coupled to a custom-made extension board. The FPGA firmware (VHDL-Code) and the Linux service routines (C-Code) were custom developed to support the functionalities required for the spectroscopic setup. For higher flexibility in data acquisition, several user-defined time-windows within a spectral scan are supported. The summation of consecutive spectra is implemented in real-time with digital signal processors (DSP) and dual port block random access memory (BRAM). The averaged spectra are then sent to an external computer via a TCP/IP-interface for spectral analysis. Further functionalities to control peripherals, such as laser temperature controllers, sample pressure and temperature sensors, valve switching, etalon-flip, etc. are handled by the custom extension PCB containing additional high-precision ADC, DAC and DIO ports. These functionalities were implemented using the Inter-Integrated Circuit (I2C) communication protocol. The overall instrument is composed of two main units. One module (80 \(\times\) 60 \(\times\) 40 cm) contains the optical setup (Figure 1), while the other compartment (45 \(\times\) 40 \(\times\) 20 cm) houses the electronic hardware (data acquisition (DAQ) unit, power supplies, embedded computer, temperature controllers  and laser drivers).