## 3 Results and Discussion
### NH\({}_{3}\) and N\({}_{2}\)O Emissions Measurements at Different Ambient Temperatures
Moreover, the sampling lines need to be kept above 100 \({}^{\circ}\)C to avoid water condensation, which leads to the loss of hygroscopic compounds such as NH\({}_{3}\), resulting in wrong estimations of the emissions. Most FTIR systems, such as those used in the present study, operate at 191 \({}^{\circ}\)C. On the other hand, in the presence of HNCO, a high sample line temperature can decompose the molecule producing NH\({}_{3}\)[31]. Thus, some instruments, such as the OBS-ONE-XL and the MEXA-ONE-QL-NX used in the present study, operate at 113 \({}^{\circ}\)C.  
Figure 4 shows that the NH\({}_{3}\) emissions of the HD-CNG began at the catalyst light-off. The emissions then continued during high acceleration events. As in the case of N\({}_{2}\)O, this is in line with what has been previously reported for light-duty positive ignition vehicles equipped with TWC, including CNG vehicles [32]. The NH\({}_{3}\) emissions increased as the temperature decreased. The duration of the first emission peak during the catalyst light-off was also longer. A similar behavior has been shown for positive ignition light-duty engines equipped with TWC tested at sub-zero temperatures [33; 34]. The three instruments presented highly comparable NH\({}_{3}\) emission profiles under all the studied conditions, with the exception of the PEMS-LAB at \(-7\)\({}^{\circ}\)C, where the PEMS-LAB may have suffered from the high water content in the exhaust. The SESAM and the PEMS-LAB, both measuring using the same principle, FTIR spectroscopy, and at the same measuring rate, 1 Hz, presented closer emission profiles for the tests at 35 \({}^{\circ}\)C and 0 \({}^{\circ}\)C than the OBS-ONE-XL. The correlation of the NH\({}_{3}\) concentrations measured by these two instruments was good, resulting in R\({}^{2}\) of 0.87 and 0.96 for the tests at 35 \({}^{\circ}\)C and 0 \({}^{\circ}\)C, respectively. The R\({}^{2}\) and \(-7\)\({}^{\circ}\)C was 0.45, probably due to the water interference. The OBS-ONE-XL, which measures and reports the concentrations at a 10-Hz frequency, showed higher maximum NH\({}_{3}\) concentrations, with the NH\({}_{3}\) peaks being sharper and better defined. The same pattern was previously reported during an intercomparison study where NH\({}_{3}\) emissions were measured from a TWC-equipped light-duty vehicle with a QCL-IR measuring at a 10-Hz frequency and a FTIR measuring at 1 Hz [35]. It should also be noted that while the SESAM and the PEMS-LAB are used to measure a wide variety of compounds, the OBS-ONE-XL has been developed and optimized for the measurement of N\({}_{2}\)O and NH\({}_{3}\). This optimization includes among other important elements a dedicated quartz filter that reduces adsorption of NH\({}_{3}\). The effects of this optimization can be seen in the faster response time of the OBS-ONE-XL compared to the other two systems, as well as in the absence of the tailing on the measured peaks after the vehicle accelerations, which can be attributed to NH\({}_{3}\) adsorption. All these elements lead to some differences in the concentrations measured with the two different techniques (IRLAM and FTIR). However, very good correlations were obtained at 0 \({}^{\circ}\)C and \(-7\)\({}^{\circ}\)C, and a good correlation at 23 \({}^{\circ}\)C, where the R\({}^{2}\) values were >0.9 and 0.7, respectively. The largest differences between the OBS-ONE-XL and the FTIR instruments were found at 35 \({}^{\circ}\)C, with R\({}^{2}\) = 0.43. However, a second test performed at the same temperature showed a substantially better correlation, R\({}^{2}\) = 0.72, between the OBS-ONE-XL and the SESAM (see Figure 5) and a very good correlation (R\({}^{2}\) = 0.90) with a laboratory grade QCL-IR (MEXA- ONE-QL-NX) used in the same experiment. The results are in line with, or better than, those obtained when comparing the NH\({}_{3}\) concentrations measured by two laboratory-grade FTIRs [34; 36].