## 5 Discussion  
The main observation of this study is that during cold start of spark ignition engines, the CO\({}_{2}\) concentrations measured on a wet basis or estimated from a dry basis may have large differences. This is the first study to quantify this effect. The differences of the first seconds, which originate from the dry-wet correction, were up to 10% (Figure 3, Table 4). However, for the rest of the test cycle the differences were within experimental uncertainties (\(\pm\)2%). Such comparisons are rare in the literature, even though the engine cold start topic is widely investigated (see Introduction). One study that compared a heated NDIR with the corrected dry laboratory measurement found 7% for a gasoline vehicle, but it was attributed to the heated NDIR water corrections [37]. Another study that compared the FTIR with the laboratory analyzers showed that the differences of the gasoline vehicles were higher than of the diesel vehicles for CO\({}_{2}\)[26]. Figure 5: Vehicle Gasoline Direct Injection (G-DI). Left panels: First 600 s of WLTC. Right panels: NEDC (New European Driving Cycle). Upper: H\({}_{2}\)O concentrations. Middle: Exhaust gas temperatures. Low: Speed profiles. Dashed boxes indicate the time period where the H\({}_{2}\)O concentrations reached similar levels. The dashed line is the dew point of gasoline exhaust gas (53 \({}^{\circ}\)C). The differences were not company, laboratory, or instrument dependent (Figure 4), but they were principle of measurement dependent, i.e., they were seen only between wet- and dry-based measurements. The various equations for the dry-wet correction factor (e.g., RDE or ISO) had a small difference (Figure 2) that could not explain the cold start differences in concentrations. Only using measured H\({}_{2}\)O the agreement between wet and corrected measurements was good. The conversion factors based on H\({}_{2}\)O or CO\({}_{2}\) and CO had differences 4%-12% for the start of a test cycle, but \(<\)3% for the rest of the cycle (Table 3). We believe that the differences during the first seconds of a test are due to condensation of water on the cold surfaces of the vehicle exhaust pipes (assuming that the instruments sampling lines are heated). When the surfaces reach the dew point temperature of the exhaust gas, the differences become negligible (as speculated in Figure 5). The water condensation at the tailpipe has been observed by some researchers [17; 20]. The low H\({}_{2}\)O concentration at the beginning of a cold start test is something that has been seen [19], but no special attention has been given as until recently measurements from the tailpipe were not required by the legislation. In order to further investigate this hypothesis, steady cycles at constant speed points were conducted with G-PFI #2 vehicle starting with cold engine at 23 \({}^{\circ}\)C (Figure 6). During the first 150 s the FTIR CO\({}_{2}\) concentration was higher than the laboratory corrected CO\({}_{2}\), indicating water vapor condensation. When the exhaust gas temperature reached 53 \({}^{\circ}\)C the two concentrations were similar. For the time period of 150-210 s the exhaust gas temperature remained relatively constant indicating that both condensation and evaporation were taking place. The two CO\({}_{2}\) concentrations continued to be at the same level until the end of the 50 km/h point (exhaust gas temperature \(<\)66 \({}^{\circ}\)C). When the vehicle accelerated to 100 km/h, the exhaust gas temperature exceeded 100 \({}^{\circ}\)C and the FTIR CO\({}_{2}\) concentration was lower than the laboratory corrected CO\({}_{2}\), indicating evaporation of condensed water and that the \(K_{d\omega,RDE}\) correction cannot capture it. At the end of the 100 km/h point the two concentrations were at the same level. When the vehicle accelerated to 130 km/h the exhaust gas temperature further increased but the two CO\({}_{2}\) concentrations remained at the same level.