## 2 Materials and Methods
### FTIR Description
It was shown that for automotive applications, a resolution of 0.5 cm\({}^{-1}\) is the best compromise to obtain suitable fineness of the spectra (required to build robust calibration method) without compromising signal to noise ratio [8]. At lower resolutions (e.g., 1 cm\({}^{-1}\)), water's absorbance bands may create interference problems that affect the detection limits of many compounds [112]. The accuracy and detection limits of the individual compounds measured are dependent on the intensity of the respective absorbance bands and their interference, but also on the detector [113, 114]. For most compounds, the detection limit is well below 1 ppm for 1 s resolution [55, 115]. The resolution of 0.5 cm\({}^{-1}\) is achievable only with a photonic detector (e.g., Mercury Cadmium Telluride--MCT) that needs to be cooled down with liquid N\({}_{2}\). Peltier cooled MCT or heating detector (e.g., Deuterated TriGlycide Sulfate--DTG)) are less sensitive and reactive compared to photonic detectors. However, they might be more suitable for portable systems. Another advantage of the FTIR is the multiplex advantage (or Fellgett advantage) because all wavelengths are collected simultaneously and thus a spectrum can be obtained very quickly. In contrast, with \"dispersive spectroscopy\" a monochromatic light beam shines at a sample, the absorbed light is measured, and this is repeated for each different wavelength. The throughput advantage (or the Jacquinot advantage) arises because, unlike dispersive spectrometers, FTIR spectrometers have no slits, which attenuate the infrared light, resulting in a higher signal-to-noise ratio. Other advantages of the FTIR systems are: measurement of many pollutants simultaneously, real-time operation, possibility to measure undiluted exhaust gas, the fast response time (at least as fast as the conventional analyzers) for transient engine operation, low minimum detection limits, large dynamic range for dilute and direct sampling, high accuracy with negligible cross-sensitivity [31]. On the other hand, the interference primarily from H\({}_{2}\)O and CO\({}_{2}\) can be high, and the signal-to-noise ratio can be affected by external vibra  
Figure 1: Principle of operation of FTIR (Fourier transform infrared) spectroscopy. IR = infrared. tions. Figure 2a plots absorbance of CO and different nitrogen compounds (lower part), and CO\({}_{2}\) and H\({}_{2}\)O (upper part) in arbitrary scales for better visualization. The example is based on the reference library acquired with a HR-FTIR MKS Multigas analyzer 2030 (Andover, MA, USA) at 191 \({}^{\circ}\)C, 0.95 atm and with a 5.11 m optical path measurement cell. Such reference spectra can be found from instrument manufacturers or from the Hitran (Cambridge, MA, USA) database [116; 117]. It is evident that there are areas with overlap (e.g., NO\({}_{2}\), NH\({}_{3}\) and H\({}_{2}\)O). CO\({}_{2}\) measurement is relatively straightforward, but the selection of the bands needs to be conducted considering H\({}_{2}\)O interference, and between CO absorbance peaks. CO requires the careful selection of infrared absorbance bands to stay within the desired absorbance range and in an area between H\({}_{2}\)O and potential N\({}_{2}\)O absorption region. NO and NO\({}_{2}\) absorb only in regions with very high levels of water interference, as well as CO and CO\({}_{2}\) interferences. Thus, for these compounds, the selection of the absorption bands needs to be conducted between, e.g., H\({}_{2}\)O absorption peaks, in the area between ca. 1850 and 1940 cm\({}^{-1}\) for NO, and between 1570 and 1650 cm\({}^{-1}\) for NO\({}_{2}\). Figure 2b provides the example of NH\({}_{3}\) calibration method, for which a rather straightforward selection of the absorption bands is possible within an area where no interference is expected (between 900 and 970 cm\({}^{-1}\)).