## 3 Results
### Hydrocarbons (HCs) and Methane (CH\({}_{4}\))
For FTIR, in general, the analysis of infrared spectra works best for components with strong and sharp absorbance bands [113]. It is progressively more difficult for FTIR to speciate hydrocarbons as the number of carbons increases for two reasons [162]: (i) the individual molecular rotation-vibration lines coalesce into broader bandshapes. (ii) they progressively overlap with each other. Table 15 summarizes the comparisons of FTIR with FID systems for THC. There is a high scatter of the results. The differences are typically within 15%, but underestimation by a factor of 2 has also been reported [110; 163]. Another study found FTIR measuring 34% higher than the FID from the dilution tunnel [164]. The explanation was the reduced sensitivity of FID to methanol and formaldehyde. It should be emphasized that this is a comparison of two technics, which are both estimating THC. None of them are specifically quantifying every single HC. FID is non-specific, and FTIR is non-exhaustive. Table 16 summarizes the FTIR studies with CH\({}_{4}\). The differences were around 5% when measuring calibration gases and 10% compared to FIDs with NMC when measuring CH\({}_{4}\) from the tailpipe. The difference to other methods (GS) was within 5%. A few older studies found higher differences (up to 18%) at levels of <10 ppm CH\({}_{4}\). To put results into perspective of those older studies, when the FTIR was used at the tailpipe and then to measure the CH\({}_{4}\) from the bags, the differences remained the same (slope 0.88 and \(R^{2}\) 0.96) [165], indicating uncertainties with the FTIR. Similarly, a study that measured CH\({}_{4}\) from cylinders found a 3.5 ppm offset due to interference from other gases [166]. The same study found more than double emissions compared to chemical ionization mass spectrometry (CI-MS). Table 14 summarizes the FTIR studies with NMHC. The differences were around 5%, with some cases having >30% differences. For these cases, an uncertainty analysis revealed that the most likely reason for the differences was the reference method. The reference method uncertainty was around 80-100 ppm (almost 50-100%), while for FTIR was around 25 ppm (10-15%) [162]. Another study, even assuming ethane and ethylene to have an error of 10% and the other components to have an error of 50%, found that the FTIR had a lower absolute error than FID for NMHC [167]. A promising proposal to measure real-time NMHC and NMOG with FTIR is to use a group of HCs, as a surrogate to NHMC and then combined it with ethanol, acetaldehyde and formaldehyde to calculate NMOG [162]. That study also showed that typically the top five organic compounds account for about 60% of the HC emissions with CH\({}_{4}\) the most abundant, and the top 10 compounds account for 80% for fuels ranging from gasoline to 85% ethanol/gasoline blends. The same study concluded that FTIR can measure NMHC and NMOG within 5% to the regulatory method [162]. Only a few studies compared ethanol emissions from FTIR and other methods. In one study, the differences were <10% for emission levels ranging from 100-1000 mg/km [160]. Another study with three FTIR found on average \(-\)7% (0% to \(-\)12%) compared to PTR-Qi-ToF-MS during the cold start phase (emissions 150 mg/km) [168]. Regarding methanol many studies in the 1990s gave good results: slopes 0.72-1.10 (FTIR vs. impinger + CS) [169, 170, 16, 171, 26]. One study compared alcohols and found an agreement of 22% (FTIR vs. impinger + CS) for emission levels up to 250 mg/km and fuels up to E100 [172].