## 3 Results and Discussion
### On-Road Measurement of NH\({}_{3}\) and N\({}_{2}\)O Emissions  
Figures 6 and 7 illustrate the emissions of N\({}_{2}\)O and NH\({}_{3}\) measured on the road using the two on-board systems, the OBS-ONE-XL and the PEMS-LAB. As for the laboratory experiments, the emission profiles recorded by the two instruments were highly comparable, with the N\({}_{2}\)O concentrations measured presenting better agreement than the concentrations of NH\({}_{3}\) (see Figure 6). The correlation of the N\({}_{2}\)O concentrations measured by the instruments was again very good, with an R\({}^{2}\) > 0.9. However, the correlation of the NH\({}_{3}\) concentrations was poor. Although both instruments reported the emission events at the same time (see Figure 7), the OBS-ONE-XL provided better peak resolution for the NH\({}_{3}\) than the PEMS-LAB. This shows that the PEMS-LAB system, using an 8 cm\({}^{-1}\) spectral resolution and without an optimization for the NH\({}_{3}\) measurement, struggled to provide  
Figure 5: **(Top panel)** NH\({}_{3}\) emission profiles measured using the SESAM (blue), the OBS-ONE-XL (grey) and the MEXA-ONE-QL-XX (yellow) over the WHVC at 35 \({}^{\circ}\)C. Correlation of the NH\({}_{3}\) concentrations measured by the OBS-ONE-XL plotted against that measured by the SESAM (**central panel**) and against the MEXA-ONE-QL-XX (**bottom panel**). reliable readings of NH\({}_{3}\) under the challenging conditions represented by the HD-CNG's exhaust, which has high water content. The N\({}_{2}\)O and NH\({}_{3}\) emissions measured from the HD-CNG on-road and in the laboratory were substantially different from those reported from a Euro VI HD Diesel vehicle equipped with a Diesel Oxidation Catalyst (DOC), a Selective Catalytic Reduction system (SCR) and an Ammonia Slip Catalyst (ASC) [19]. This is because the emissions of N\({}_{2}\)O and NH\({}_{3}\) from the two different powertrains are related to different chemical and physical processes. In TWC-equipped vehicles, such as the HD-CNG tested in this study, their emissions are linked to a series of catalytic reactions that take place on the TWC, involving CO, NO, H\({}_{2}\) and H\({}_{2}\)O [37; 38]. On the other hand, Diesel HDVs' emissions of N\({}_{2}\)O and NH\({}_{3}\) are linked to the use of DOC, SCR and/or ASC after-treatment systems and the use of urea aqueous solution to reduce NOx emissions (for more detailed information, see [PERSON] et al., 2021 [3] and the reference therein). The N\({}_{2}\)O emissions from the HD-CNG were lower than those reported for the HD Diesel vehicle. However, NH\({}_{3}\) emissions were more frequent and had higher concentrations than those reported for the Euro VI HD Diesel. One of the three on-road tests presented a particular NH\({}_{3}\) emission profile (see Figure 7). During this test, the vehicle was kept on idle for about 600 s to resume the OBD connection of the PEMS system. The NH\({}_{3}\) concentrations measured by the instruments increased, reaching values close to 500 ppm. As Figure 7 illustrates, the emissions measured by the OBS-ONE-XL showed a sort of harmonic oscillation. Although the PEMS-LAB reported higher concentrations, it did not show the quickly oscillating behavior. In order to clarify whether this behavior might be related to an interference or to a malfunctioning of the specific instrument, a dedicated idle test was performed in the  
Figure 6: (**Left panel**) N\({}_{2}\)O emission profiles measured on-road using the OBS-ONE-XL (grey) and the PEMS-LAB (orange) over the real-driving conditions. The left panel also shows a close-up on the first 400 s. (**Right panel**) Correlation of the N\({}_{2}\)O concentrations measured by the OBS-ONE-XL plotted against that measured by the PEMS-LAB. Figure 7: NH\({}_{3}\) emission profiles measured during an on-road test using the OBS-ONE-XL (grey) and the PEMS-LAB (orange) over the real driving conditions. laboratory using both a QCL-IR (MEXA-ONE-QL-NX) measuring NH\({}_{3}\) at 10 Hz frequency and the FTIR SESAM measuring at 1 Hz.