## 2 Materials and Methods
### FTIR Description
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Compound** & **Measurement Principle** & **GTR15 (LD)** & **EU LD** & **EU HD** & **PEMS** \\ \hline PN & VPR + PNC & CVS & CVS & +PFDS & TP \\ PM & Gravimetric (filter) & CVS & CVS & +PFDS & \\ CO\({}_{2}\) & NDIR & bags & bags & +CVS, +TP & TP \\ CO & NDIR & bags & bags & +CVS, +TP & TP 3 \\ THC & FID or HFID (for diesel) & bags & bags & +CVS, +TP & TP 3 \\ CH\({}_{4}\) & NMC or GC combined with FID & bags & bags & +CVS, +TP & TP 3 \\ NMHC & Calculated (THC\(-\)CH\({}_{4}\)) & (bags) & (bags) & calculated & TP 3 \\ NO\({}_{\rm x}\) & CLA or NDUV & bags & bags & +CVS, +TP & TP \\ NO & CLA or NDUV (only bags) & CVS or bags & & & \\ NO\({}_{2}\) & NDUV, QCL (or CLA) or (NO\({}_{\rm x,bags}\)-NO\({}_{\rm CVS}\)) & CVS & & & \\ N\({}_{2}\)O & GC with ECD, QCL-IR, NDIR, FTIR 1 & CVS or bags & & & \\ NH\({}_{3}\) & LDS or QCL or FTIR 2 & TP & TP 4 & & \\ C\({}_{2}\)HgOH & Impinger + GC, FTIR, PAS, PTR-MS, dir. GC & CVS & & & \\ CH\({}_{2}\)O & DNPH + HPLC, FTIR, PTR + MS & CVS & & & \\ CH\({}_{3}\)CHO & DNPH + HPLC, FTIR, PTR + MS & CVS & & & \\ \hline \hline \end{tabular}  
* interferences \(<\) 0.1 ppm; 2 with interference \(<\) 2 ppm at max CO\({}_{2}\) and H\({}_{2}\)O; 3 only for heavy-duty vehicles; 4 LDS or FTIR for HD engines.C\({}_{2}\)H\({}_{5}\)OH = ethanol; CH\({}_{3}\)CHO = acetaldehyde; CH\({}_{4}\) = methane; CLA = chemiluminescence analyzer; CO = carbon monoxide; CO\({}_{2}\) = carbon dioxide; DNPH = 2.4-Dintrophenylhydrazine; ECD = electron-capture detection; FID = flame ionization detection; FTIR = Fourier transform infrared spectrometry; GC = gas chromatography; CH\({}_{2}\)O = formaldehyde; HFID = heated flame ionization detection; HPLC = high-performance liquid chromatography; IR = infrared; LDS = laser diode spectrometer; N\({}_{2}\)O = nitrous oxide; NDIR = non-dispersive infrared spectrometry; NDUV = non-dispersive ultra-violet spectrometry; NH\({}_{3}\) = ammonia; NMC = non-methane cutter; NMHC = non methane hydrocarbons; NO = nitrogen oxide; NO\({}_{2}\) = nitrogen dioxide; NO\({}_{\rm x}\) = nitrogen oxides; PAS = photoacoustic spectrometry; PM = particulate matter; PN = particle number; PNC = particle number counter; PTR-MS = proton transfer reaction—mass spectrometry; QCL = quantum cascade laser; THC = total hydrocarbons; VPR = volatile particle remover.  
\end{table} 
Table 1: Pollutants, principle of measurement, and the sampling option for light-duty (LD) vehicles and heavy-duty (HD) engines in the European Union (EU) regulation and the global technical regulation (GTR 15) for LD vehicles. PEMS (portable emissions measurement systems) refer to the EU regulation for both LD and HD vehicles. Measurements can be conducted directly from the dilution tunnel with constant volume sampling (CVS), the bags, the proportional partial flow dilution system (PFDS), or the tailpipe (TP), depending on the regulation. Because the path that one beam travels is a fixed length and the other is constantly changing as its mirror moves, the signal which exits the interferometer is the result of these two beams \"interfering\" with each other. The resulting signal is called an interferogram (i.e., a plot of light intensity versus optical path difference). The interferograms measured are then Fourier transformed to yield a spectrum (i.e., a plot intensity versus frequency/wavenumber). There is also a laser (not shown in the figure) whose light follows the infrared beam. This laser light is used to measure the optical path difference of the interferometer. The spectral resolution (in cm\({}^{-1}\)) depends on the inverse of the optical path difference. This gives the wavelength accuracy (or Connes' advantage) compared to dispersive instruments where the scale depends on the mechanical movement of diffraction gratings. Thus, the FTIR can give very accurate frequencies in the spectrum--this enables processing techniques such as spectral subtraction.