## 2 Experimental Section
Each QCL modulates the wavelength over a range of about 0.5 cm\({}^{-1}\), and acquires an absorption signal with a spectral resolution of about 0.001 cm\({}^{-1}\), which corresponds to the laser linewidth. The absorption signal is detected with a non-cooled InAsSb photovoltaic detector. The gas is sampled with a flow rate of approximately 3.3 L min\({}^{-1}\). The PC serves as the user interface for the operation of the analyzer, the calculation, and to display the measured concentrations. In this configuration, the device's measurement ranges are 0-1000 ppm for N\({}_{2}\)O and 0-1500 ppm for NH\({}_{3}\). The estimated LoD, 3\(\times\) standard deviation, of the instrument used in this test is 0.06 ppm and 0.09 ppm, respectively. The acquisition frequency is 10 Hz [25]. The PEMS-LAB is an on-board metrology experimental platform conceived to measure gases and particle number from vehicle tailpipe emissions in real driving conditions on road. It includes: a portable FTIR analyzer (p-FTIR); a PEGASOR Particle Sensor PPS-M for solid particulate number (PN) measurement; a dedicated weather station for ambient temperature, pressure and relative humidity measurement; an EFM module. For the purpose of this campaign, the PEMS-LAB's EFM module was not used. A PTFE tube is used to sample raw exhaust directly from the vehicle tailpipe, and it is heated at 220 \({}^{\circ}\)C to avoid condensation and/or adsorption of hydrophilic compounds (e.g., NH\({}_{3}\)). The sampling rate is 8.5 L min\({}^{-1}\). The frequency of acquisition is 1 Hz. The p-FTIR consists of a cell with an internal volume of 200 cm\({}^{3}\), a fixed optical path of 2 m, and windows of BaF\({}_{2}\), and uses a mercury cadmium telluride (MCT) thermoelectrically (Peltier) cooled detector. The cell works at 180 \({}^{\circ}\)C and atmospheric pressure. A shock resistant interferometer with a spectral range of 900-4200 cm\({}^{-1}\) and a resolution of 8 cm\({}^{-1}\) completes the instrument. At VELA7, the vehicle was tested under the World-Harmonized heavy-duty Vehicle Cycle (WHVC) using four different ambient temperatures: 35 \({}^{\circ}\)C, 23 \({}^{\circ}\)C, 0 \({}^{\circ}\)C and \(-7 {}^{\circ}\)C. This cycle is a transposition on the chassis dyro of the homologation Type Approval cycle (WHTC--World Harmonized Transient Cycle) that is performed only on the engine at the dyro test bench. The speed profile of the test cycle is reported in Figure 2 above. All the tests were performed with the vehicle carrying 50-60% of its maximum allowed payload at test start. Vehicle coolant temperature matched the ambient temperature (\(\pm\)3 \({}^{\circ}\)C) when the test started. It should be noted that due to a technical problem, the PEMS-LAB was not used during the laboratory test performed at 23 \({}^{\circ}\)C.  
VELA7 is a climatic test cell that comprises a 2-axis roller dynamometer (72\({}^{\prime\prime}\) inches) dedicated to heavy-duty vehicle testing. The exhaust gas was connected to the full dilution tunnel, with a 9 m transferline feeding a full dilution tunnel with constant volume sampler (CVS). Criteria pollutants NOx, CO, HC and CH\({}_{4}\) were measured using AVL AMA i60 analyzers (both the raw exhaust and the dilution tunnel). VELA7 is also equipped with a laboratory grade FTIR spectrometer (Nicolet Antaris IGS Analyzer--Thermo Electron Scientific Instruments LLC, Madison, WI, USA)--SESAM--that allows for the measurement of NH\({}_{3}\) and N\({}_{2}\)O. The SESAM was connected along the OBS-ONE-XL and the PEMS-LAB to the vehicle's tailpipe. It uses a heated PTFE sampling line (191 \({}^{\circ}\)C). This FTIR was equipped with a multipath gas cell with 2 m of optical path and a downstream sampling pump (6.5 L min\({}^{-1}\) flowrate), and had the acquisition frequency of 1 Hz with a working pressure of 860 hPa. The FTIR was made up with a Michelson interferometer (spectral resolution: 0.5 cm\({}^{-1}\), spectral range: 600-3500 cm\({}^{-1}\)) and a liquid-nitrogen-cooled mercury cadmium telluride detector.