# Advances in High-Precision NO2 Measurement by Quantum Cascade Laser Absorption Spectroscopy
## 2 Materials and Methods
### Description of the Dual-Laser Spectrometer
#### 2.1.2 Performance Assessment

Table 1: Comparison of reported precision for NO\({}_{2}\) measurements by QCLAS and TDLAS. OPL, optical path length; N, number of passes of the laser beam in the MPC; D, distance between the MPC mirrors; P, pressure in the MPC; \(\sigma_{1s}\), Allan deviation at 1 s resolution; \(\sigma_{min}\), minimum Allan deviation; time, integration time corresponding to \(\sigma_{min}\). Figure 4: [PERSON]–[PERSON] deviation plot (1\(\sigma\)) of 12 h measurements of NO\({}_{2}\) equivalent amount fraction obtained by sampling NO\({}_{2}\)-free synthetic air at different sample pressures (30 and 80 mbar) and two distinct operation modes (NO\({}_{x}\) and NO\({}_{2}\) mode). Figure 4 also indicates that, after about 150 s, a drift starts to dominate the instrument's precision. To prevent this drift in the ambient air NO\({}_{2}\) measurements, synthetic air is periodically sampled during 1 min by switching the electronic three-way valve at the instrument's inlet. Amount fractions of NO\({}_{2}\) measured during these periods are linearly interpolated and then subtracted from the measurements, referred to as zero-drift correction. The response time (99%) of the instrument at 0.8 L/min is approximately 45 s, which results in a transition time of 3 min between the measurement periods to correct for zero drift (hereafter zeroing time). The frequency of this \"zeroing\" defines the duty cycle which is obtained by dividing the measurement duration by the total cycle duration. For the measurements presented in Section 3 a duty cycle of 63% was used, corresponding to 5 min of measurement for 3 min of zeroing time. Some experimental conditions require using higher duty cycles (longer measurement periods) to avoid dead-times (e.g., kinetic experiments, transient mixing ratios variations detection, etc.). To quantify the influence of this parameter on the instrument's precision, we corrected the time-series used for obtaining the Allan-Werle plots (Figure 4) with the drift correction procedure describe above. Since these time-series were obtained by continuous sampling of synthetic air without any valve switching, the data are artificially categorized into measurement and zeroing data. Figure 5 presents the drift-corrected time-series measured in NO\({}_{2}\) mode at 80 and 30 mbar with duty cycles of 50% and 90%, corresponding to 3 and 27 min of measurements and 3 min of zeroing time. The associated histograms of the relative frequency count for each dataset is shown on the right. The least spread with a standard deviation (1 \(\sigma\)) of 1.35 pptv is obtained with the duty cycle of 50% at 80 mbar. With the same duty cycle but at 30 mbar, the standard deviation is 2.24 pptv. In each case, however, the precision is good enough for measurements at very low NOx conditions as found in the most remote locations. As mentioned above, the GAW program recommends a LOD (defined as three times the standard deviation) below 5 pptv for measuring in remote conditions. In our case, using a zero drift correction with a duty cycle of 50% yields LODs of 4.1 and 6.7 pptv (equivalent to 1.09 \(\times\) 10\({}^{8}\) and 1.78 \(\times\) 10\({}^{8}\) molecules/cm\({}^{3}\) at standard temperature and pressure conditions, respectively) at 80 and 30 mbar, respectively, at 1 min resolution. Averaging the data over longer time periods helps improving the LOD. For example, working at a resolution of 12 min, which corresponds to two measurement cycles at a duty cycle of 50%, permits LODs of 3.0 and 3.9 pptv at 80 and 30 mbar, respectively.