# Advances in High-Precision NO2 Measurement by Quantum Cascade Laser Absorption Spectroscopy
## 4 Conclusions  
We designed a high-precision quantum cascade laser based mid-infrared spectrometer (QCLAS) for NO\({}_{2}\) detection. The instrument reaches a precision (1\(\sigma\)) of 0.8 pptv for NO\({}_{2}\) after 150 s integration time. For this, we developed a fully custom-made astigmatic multipass absorption cell (MPC) with 110 m optical path length and employed a low-noise laser driver. The mirror design of the MPC involved optimization based on the investigation of simulated beam re-circulation patterns. The instrument's response time (99%) is 45 s, which allows not only resolving rapid concentration changes, but also applying regular drift corrections by switching between sample- and zero-air measurements. This approach was demonstrated to maintain a limit of detection (LOD) for NO\({}_{2}\) below 5 pptv, which complies with the highly challenging GAW data quality objectives. Furthermore, water-induced spectral interferences on NO\({}_{2}\) detection were investigated under various humidity levels and pressure conditions. We show that operating the instrument at low sample pressure (30 mbar) allows minimizing the water induced bias near the LOD even for an absolute humidity of up to 2%. Furthermore, we show that the instrument can be operated in dual mode to simultaneously measure both NO\({}_{2}\) and NO. The NO\({}_{x}\) precision of 20 pptv after 150 s integration time can be be useful for ambient air quality monitoring and studies of the nitrogen cycle. In addition, we also investigated different calibration methods (NO titration based, high-concentration cylinder dilution and permeation) during an inter-comparison campaign conducted at a suburban air quality monitoring station. We found significant differences between the calibration methods, which underlines the difficulty of handling NO\({}_{2}\) and the importance of improved calibration methods. These discrepancies represent the main source of uncertainties of the measurements. The inter-comparison campaign involved the newly developed QCLAS as well as three other NO\({}_{2}\) instruments: a chemiluminescence detector (CLD), a cavity attenuated phase shift spectrometer (CAPS) and a second QCLAS. While the two QCLAS instruments were in excellent agreement, the CLD showed a clear overestimation of the measured NO\({}_{2}\) compared to the QCLAS. Significant and unexpected differences between the QCLAS and CAPS NO\({}_{2}\) measurements were also observed. These were associated with photo-chemistry dominated conditions with a clear day/night cycle. Further measurements under various environmental conditions could help to validate the measurement quality of our instrument and to investigate biases on NO\({}_{2}\) detection in the visible range. Figure 9: Intercepts and slopes of the linear regression of ambient air measurement data for the CLD, CAPS and QCL-MIRO vs. the QCL-Empa using three different calibration methods. The error bars represent the fit uncertainties (1 \(\sigma\)). Overall, our results demonstrate that QCLAS is a highly attractive alternative, in terms of both precision and selectivity, to existing instruments based on more commonly used techniques for NO\({}_{2}\) detection. [PERSON] designed and developed the instrument under the guidance of [PERSON] and with input by [PERSON]; [PERSON] designed and developed the electronics hardware; [PERSON] developed and implemented FPGA and DAQ functionalities; [PERSON] developed the spectral analysis and hardware control software; [PERSON] performed the experiments and evaluated the data; [PERSON] and [PERSON] participated in the ReGaS instrument deployment and data analysis; and [PERSON], [PERSON] and [PERSON] designed the research, managed and supervised the project and discussed the results. All authors have read and agreed to the published version of the manuscript. This research was funded by the Empa and by the EMPIR Project MetNO2. The EMPIR initiative is co-funded by the European Union's Horizon 2020 Research and Innovation Programme and the EMPIR Participating States. Not applicable.