# Advances in High-Precision NO2 Measurement by Quantum Cascade Laser Absorption Spectroscopy
## 2 Materials and Methods
### Description of the Dual-Laser Spectrometer
#### 2.1.3 Spectral Interferences Induced by Water Vapor  
Spectral simulations indicate that at sample pressures below 100 mbar, the ro-vibrational absorption lines of NO\({}_{2}\) in the selected spectral region are sufficiently separated from lines of any other atmospheric species. In a previous study, however, [PERSON] et al. (2016) found that an ambient H\({}_{2}\)O amount fractions of 1% resulted in a NO\({}_{2}\) equivalent signal of about \(-\)60 pptv compared to dry air measurements at the same gas pressure of 50 mbar. A similar water-induced bias has also been observed for our instrument. To precisely quantify this bias and mitigate its effect, different experiments at various water concentrations and at different sample pressures were conducted. The effect of humidity on the instrument's NO\({}_{2}\) retrieval was investigated by sampling alternatively dry and humidified synthetic air, both coming from the same source. The step-wise sample humidification was done by using a HovaCAL calibration gas generator (IAS GmbH, Iserlohn, Germany). A full cycle consists in measuring alternatively dry and humid samples, 3 min each. These experiments also showed that measuring dry air before and after several minutes of humid air sampling results in the same NO\({}_{2}\) equivalent signal, which demonstrates the high inertness of the sampling line including the custom-made MPC.  
Figure 6 shows the \(\Delta\)NO\({}_{2}\), corresponding to the NO\({}_{2}\) difference obtained between wet and dry conditions, in function of the H\({}_{2}\)O amount fraction (up to 2% absolute humidity) at different sample pressures ranging between 30 and 80 mbar. The water content was spectroscopically determined using the H\({}_{2}\)O line at 1600.25 cm\({}^{-1}\) and calibrated against a dew-point hygrometer (Thygan VTP 6, Meteolabor AG, Switzerland). The data show that the NO\({}_{2}\) equivalent signal clearly depends on the water amount fraction and the sample pressure. The observed NO\({}_{2}\) bias at an absolute humidity of 1% is in a similar range as that found by [PERSON] et al. [34]. This bias is attributed to a change in the spectral baseline due a very intense H\({}_{2}\)O absorption line located at 1601.2 cm\({}^{-1}\). The tailing of the pressure broadened water absorption induces a steeper curvature of the baseline (compared to the dry situation) covered by the laser scan, causing an offset in the baseline  
Figure 5: Zero-drift corrected and 1 min averaged time-series of 12 h long measurements of synthetic air using two different sample pressures (30 and 80 mbar) and duty cycles of 50% and 90%, respectively. The associated histograms with a Gaussian fit indicate the spread of the data for each cases.  
intensity estimation at the NO\({}_{2}\) line location. An increase in gas pressure causes a further broadening of this water line, thus enhancing this effect. Figure 6 shows that at 30 mbar and 1.5% humidity, the measured NO\({}_{2}\) equivalent signal is approximately \(-10\) pptv, which is close to the LOD of this instrument under typical operating conditions. The optimal operating parameters, however, depend on the measurement conditions: in remote, dry conditions (e.g., free troposphere), where NO\({}_{2}\) and H\({}_{2}\)O amount fractions are both expected to be very low, working at an elevated gas pressure in the MPC ensures the highest possible precision, while near-ground measurements should be done at lower sample pressure to assure high selectivity. For the ambient measurements reported in Section 3.2, we used a simple linear function to determine the correction factor for the humidity effect.