## 3 Discussion  
The NO\({}_{2}\) OGS spectrometer was tested in the concentration range from 100 to 1000 \(\upmu\)mol/mol and was found to be in good agreement with the certified values of the reference gases, demonstrating the calibration-free OGS capability of the instrument. The detection limits of the dTDLAS instrument are 0.825 \(\upmu\)mol/mol (at \(\Delta\)t = 86 s) and the relative uncertainty of the results delivered by the instrument is 5.1%, \(k\) = 1. This relative standard uncertainty was derived following GUM [34] principles. These results indicate that the instrument has the potential to measure NO\({}_{2}\) (that can be in the 1 to 1000 \(\upmu\)mol/mol: detection limit of 0.825 \(\upmu\)mol/mol) emitted from the exhaust of cars as further supported by the simulation (HITRAN [45] line data used) in Figure 5 where the interference (due to absorption by other exhaust gas molecules) on NO\({}_{2}\) absorption is insignificant (absorbance  
Figure 4: **(a)** Plot of the NO\({}_{2}\) OGS amount fraction results at different levels as a function of time (at \(\Delta\)t = 86 s, 1\(\sigma\) for 108.4, 305.0 and 983.5 are 6.2, 9.9 and 9.6, respectively); (b) dTDLAS NO\({}_{2}\) amount fraction \(x_{\text{NO2}}\) results as a function-certified values of gas mixtures. in the order of 10\({}^{-5}\) for H\({}_{2}\)O, NO\({}_{2}\), CO\({}_{2}\), and CO: \(L\) = 77.4 cm, \(p\) = 100 hPa, \(T\) = 296 K). As presented, the relative repeatabilities of the NO\({}_{2}\) results at 1000 \(\upmu\)mol/mol and lowest amount fraction of 100 \(\upmu\)mol/mol are 0.1% and 0.5%, respectively. Table 2 holds a summary of the capabilities of the NO\({}_{2}\) OGS instrument presented in this work. The relative uncertainties of the NO\({}_{2}\) results of 5.1% (\(k\) = 1) are dominated (96% of the total uncertainty) by the uncertainty of the line strength value. Therefore, to further reduce the uncertainty of the results, future work will be focused on reducing the uncertainty of the line strength value. Since the dTDLAS results are in good agreement with the certified values (see Figure 4b), it can be concluded that the line strength used for the analysis is validated. The line strength value is taken from the HITRAN database [45]. This first validation of the line strength value was further assessed in a separate analysis to determine the line strength value from the data measured with the 300 \(\upmu\)mol/mol gas mixture, resulting in a line strength value of (5.444 \(\pm\) 0.076)\(\cdot\)10\({}^{-21}\) cm/molecule, \(k\) = 1, at 294.4 K for the NO\({}_{2}\) line 2919.59 cm\({}^{-1}\). It could be noted here that the accuracy of the measured line strength would be dependent on the accuracy of the certified gas mixture used. This line strength value (5.444 \(\pm\) 0.076)\(\cdot\)10\({}^{-21}\) cm/molecule is in agreement with HITRAN's [45] value of (5.425 \(\pm\) 0.271)\(\cdot\)10\({}^{-21}\) cm/molecule, \(k\) = 1, at 296 K, further validating the HITRAN value [45]. In order to test the effect of a new line strength value with a reduced uncertainty, we used the line strength value 5.444\(\cdot\)10\({}^{-21}\) cm/molecule and the standard uncertainty (0.076\(\cdot\)10\({}^{-21}\) cm/molecule: 1.4% relative) to evaluate the uncertainty of the NO\({}_{2}\) results (similar to Table 1), yielding relative uncertainty of 1.7% (\(k\) = 1). The uncertainty of the dTDLAS results currently stands at 5.1%, and a 1.7% relative uncertainty for the NO\({}_{2}\) amount fraction results will fulfill the requirements of portable