# Advances in High-Precision NO2 Measurement by Quantum Cascade Laser Absorption Spectroscopy
## 3 Results and Discussion
### Ambient Air Measurements  
Figure 8a shows the ambient NO\({}_{2}\) amount fractions measurements for the 10 days of inter-comparison obtained by the QCL-Empa instrument, with the CDC calibration factors. We chose to present the data with these factors as they result in the highest ambient NO\({}_{2}\) concentrations, which thus give the upper limit for the absolute differences between the instruments (Figure 8b). The amount fractions of NO (measured by CLD) and O\({}_{3}\) are also shown. During the measurement period, the NO\({}_{2}\) mixing ratios ranged from 0.5 to 50 ppb, a high variability as expected from a site close to emission sources. In terms of O\({}_{3}\) and NO\({}_{2}\) concentration levels, the measurement period can be divided into two five-day sub-periods: the first period (24 January-27 January) is characterized by higher and more stable O\({}_{3}\) and NO\({}_{2}\) mixing ratios and much lower NO mixing ratios, while the second period (27 January-2 February) shows on average lower NO\({}_{2}\), but with maximum values similar to the first period. The differences between the commercial NO\({}_{2}\) instruments and the QCL-Empa measurements are shown in Figure 8b. The same differences normalized by NO\({}_{2}\) (QCL-Empa) are presented in Figure 8c. Figure 7: Calibration results: (**a**) Set versus measured NO\({}_{2}\) amount fractions for the four instruments and the three different calibration methods deployed. The colored lines show the least squares linear regression for the QCL-Empa instrument. (**b**) Summary of the intercepts and slopes of the linear fit of the data in (**a**). The error bars represent the 2 \(\sigma\) uncertainties of the fit. The width of the colored area represent the combined uncertainty of the fit and of the set span NO\({}_{2}\) concentration for the QCL-Empa calibration (k = 2). Besides the calibration factor, ambient air data were further corrected for formation of NO\({}_{2}\) in the instruments individual inlet by the reaction between NO and O\({}_{3}\). The amount of NO\({}_{2}\) formed this way is estimated from the inlet residence time, the reaction rate constant between the reactants and their concentrations. The results of this calculation (in terms of NO\({}_{2}\) amount fractions) were subtracted from the data of each instruments. The magnitude of this correction ranges from near 0 (mostly during night-time) up to 1 ppbv (for 6.5 s residence time in the CLD inlet line at 100 ppbv O\({}_{3}\) and 5 ppbv NO). Finally, the QCL-Empa data were corrected for the H\({}_{2}\)O interference (see Section 2.1.3) using the correction factor determined at 60 mbar, the pressure used during the field measurements. This pressure was chosen as a comprise among precision, long-term stability and selectivity. The agreement between the two QCL instruments is remarkable with an average normalized difference (Figure 8) of \(-0.2\pm 1.4\%\). This is not unexpected, since both instrument share largely the same technology. For the first period (having more stable and higher  
Figure 8: Time-series measured during the field campaign: (**a**) ambient NO\({}_{2}\) (measured by the QCL-Empa with the CDC calibration factor), NO (measured by the CLD) and O\({}_{3}\); (**b**) absolute differences between NO\({}_{2}\) measured by the CLD, CAPS and QCL-MIRO and the QCL-Empa; and (**c**) absolute differences normalized by the NO\({}_{2}\) (QCL-Empa) values (the CLD data being further divided by two, for better visibility). NO\({}_{2}\) mixing ratios), the normalized difference is \(0.1\pm 0.3\%\). The CAPS is in reasonable agreement with the QCL-Empa with an average normalized difference of \(3.4\pm 3.9\%\). The main contribution to this difference lies in the offset of the CAPS measurements (compared to the QCL-Empa), as shown below.