## 3 Results and Discussion
### NH\({}_{3}\) and N\({}_{2}\)O Emissions Measurements at Different Ambient Temperatures  
The tests performed in the VELA7 already show that the HD-CNG presented N\({}_{2}\)O emissions during a short fraction of time during the catalyst light-off. For that reason, Figure 3 shows the first 300 s of the test, in which all N\({}_{2}\)O emissions took place. This is in line with what has been previously reported for light-duty positive ignition vehicles equipped with TWC [22]. The concentrations and emission profiles changed slightly between the warm temperatures (35 \({}^{\circ}\)C and 23 \({}^{\circ}\)C) and the cold temperatures (0 \({}^{\circ}\)C and \(-7\) \({}^{\circ}\)C), and higher N\({}_{2}\)O emissions were presented at warmer temperatures compared to the cold ones. Nonetheless, the overall emission pattern, with N\({}_{2}\)O emissions taking place only during the catalyst light-off, was not affected by the ambient temperature. All three instruments presented highly comparable N\({}_{2}\)O emission profiles under all the studied conditions, with the exception of the PEMS-LAB at \(-7\)\({}^{\circ}\)C. The higher noise present for the PEMS-LAB at this very low temperature could be explained by the high concentration of water that can be present in the exhaust of a CNG engine at this cold temperature due to a decrease in the air to fuel ratio or due to water being condensed in the very cold exhaust lines. At high concentrations, the water can be a source of cross-interference due to the lower spectral resolution of the instrument (8 cm\({}^{-1}\)) compared to the SESAM (0.5 cm\({}^{-1}\)). In fact, it has been shown that for resolutions lower than 0.5 cm\({}^{-1}\), the absorbance bands of water may create interferences, affecting the detection limits of many  
Figure 3: **(Left panels)** N\({}_{2}\)O emission profiles measured using the SESAM (blue), PEMS-LAB (orange) and OBS-ONE-XL (grey) over the first 300 s of WHVC at 35 \({}^{\circ}\)C, 23 \({}^{\circ}\)C, 0 \({}^{\circ}\)C and \(-7\) \({}^{\circ}\)C. **(Right panels)** Correlation of the N\({}_{2}\)O concentrations measured by the SESAM plotted against the N\({}_{2}\)O concentrations measured by the PEMS-LAB (orange) and against the OBS-ONE-XL (grey). The PEMS-LAB’s trend line is represented by a solid black line and the OBS-ONE-XL’s trend line is represented by a dashed black line. compounds [28]. Although to a much lower extent, the interference on the PEMS-LAB measurement can already be noticed during the test performed at 0 \({}^{\circ}\)C (see Figure 3). The R\({}^{2}\) of the N\({}_{2}\)O concentrations measured by the PEMS-LAB compared to those measured by the SESAM, was >0.9 for the tests at 35 \({}^{\circ}\)C and 0 \({}^{\circ}\)C, resulting in a good correlation for the two instruments. A similar result was also obtained for the N\({}_{2}\)O concentrations measured by the OBS-ONE-XL, with R\({}^{2}\) ranging from 0.85 to 0.95 for the four temperatures tested. These figures are comparable to the correlation obtained for the N\({}_{2}\)O concentrations measured by two laboratory-grade FTIRs [22]. Even though the correlation was good for the range of concentrations measured, it seemed that the correlation of the instruments was better at concentrations up to 50 ppm N\({}_{2}\)O. This suggests that some factors, such as response time and time resolution, may have played a role in the obtained results. The measurement of NH\({}_{3}\) emissions from vehicle exhaust has always been considered challenging as compared to other regulated gases [29; 30]. The main reason is that NH\({}_{3}\) can readily adsorb on the setup's surfaces if the sample is not properly handled. For that reason, EU 582/2011 and GTR-15 prescribed the measurement of NH\({}_{3}\) emissions from heavy- and light-duty vehicles to be performed at the tailpipe. This prevents NH\({}_{3}\) from being adsorbed on the CVS walls or other possible cold points of the transferline.