## 3 Results
### Nitrogen Monoxide (NO) and Nitrogen Dioxide (NO\({}_{2}\))  
NO and NO\({}_{\rm x}\) (sum of NO and NO\({}_{2}\)) are typically measured with chemiluminescence analyzers (CLA) developed in early 1970s [120]. NO reacts with O\({}_{3}\) and the excited NO\({}_{2}\) molecules spontaneously return to normal state emitting light (chemiluminescence) [136]. The sensitivity for NO is influenced by the existence of CO\({}_{2}\) or H\({}_{2}\)O in the sample gas, because the excited NO\({}_{2}\) can react with CO\({}_{2}\) and H\({}_{2}\)O (quenching effect) [128]. The effect is \(<\)1%, in particular for systems with H\({}_{2}\)O sensors that can compensate for H\({}_{2}\)O concentrations up to 14% [137]. The light can also be filtered before being measured by the photomultiplier to minimize interference [130]. The CLA can be used to measure NO\({}_{\rm x}\). In this case, a carbon or active metal-based furnace converts NO\({}_{2}\) into NO (NO\({}_{\rm x}\) converter) [120; 136]. Regulations require a conversion efficiency of \(>\)95%. However, NH\({}_{3}\) can also be converted to NO in the converter. Theconversion efficiency depends on the converter material, the temperature and the concentrations of NO and NH\({}_{3}\). With a 5:1 ratio of NH\({}_{3}\) to NO, an error of 30% was estimated for NO\({}_{\rm x}\)[120]. Nevertheless, laboratory CLA today has a cross-sensitivity of <1 ppm for 1000 ppm NH\({}_{3}\). NH\({}_{3}\) could also cause a loss of conversion efficiency of the NO\({}_{\rm x}\) converter over time. On the other hand, NO\({}_{\rm x}\) can be converted to N\({}_{2}\) or NH\({}_{4}\)NO\({}_{3}\) (ammonium nitrate), resulting in a lower concentration [120]. Ammonium nitrate formation is very rapid even at room temperature if NH\({}_{3}\) and HNO\({}_{3}\) are present. HNO\({}_{3}\), in turn, is readily formed when NO\({}_{2}\) dissolves in liquid water. Ammonia scrubbers can be used to avoid this problem with CLA [138]. The disadvantages over FTIR are the extra expenses required for an ozonizer, NO\({}_{2}\) to NO converter with converter efficiencies >95% (which need to be checked every month according to EU Regulation), possible NO\({}_{2}\) losses in the chiller, influence due to quenching from water and CO\({}_{2}\)[136; 139]. FTIR was also found to provide accurate measurements of NO\({}_{\rm x}\) in the presence of NH\({}_{3}\)[120]. Table 14 summarizes the studies that the FTIR NO\({}_{\rm x}\) reading was compared to calibration cylinders or CLA. The comparisons with calibration gases were within \(\pm\)5%, while with reference CLA within \(\pm\)10%, with a few exceptions. For example, a study had 30% difference due to different exhaust flows used for the FTIR and the PEMS [110]. Higher differences were reported when the response of the FTIR was slow [34]. Comparison of tailpipe FTIR with dilution tunnel bags were also within \(\pm\)10%, with only a few exceptions. In some cases, the laboratory analyzers were suspected [98]. The comparisons of FTIR and CLA for NO\({}_{2}\) did not always give acceptable differences, with differences exceeding 10% in particular at different sampling locations [140]. One explanation might be due to the fact that NO\({}_{2}\) is not directly measured but calculated from the difference between NO\({}_{\rm x}\) and NO. Wrong CLA NO\({}_{2}\) measurements can also originate from different time delays in the analyzer for NO and NO\({}_{\rm x}\)[141], and as mentioned before, converter-related side reactions. Partly lower NO\({}_{2}\) concentrations peaks due to reaction of NO\({}_{2}\) with soot contamination have also been reported [141]. Finally, despite the advantages of FTIR spectroscopy, the fundamental interference of NO\({}_{2}\) measurement by water vapor remains. It should be added that NO\({}_{2}\) (and NH\({}_{3}\)) are considered \"sticky\" substances and the sampling setup is very important.