## 3 Results and Discussion
### Sampling Location and Instrumentation
#### 3.1.3 Vehicle #2 Regeneration  
Figure 6 presents the regeneration of the Euro 6b vehicle at the last part of the WLTC. A non-regenerating WLTC is also plotted for comparison. Figure 6a shows that the speed and the exhaust flow rate were identical for the regenerating and non-regenerating WLTCs. The only difference was in the exhaust gas temperature at approximately 1600 s for the regenerating cycle. Figure 6b plots the particle number emissions. Only the solid particle number emissions > 10 nm at tailpipe (SPN\({}_{10,\text{TP}}\)) and dilution tunnel (SPN\({}_{10,\text{CVS}}\)) are given for the non-regenerating cycle for better clarity of the figure, and they are indistinguishable. The SPN\({}_{10,\text{TP}}\) of the regenerating and non-regenerating cycles were identical up to 1570 s; after that point, the SPN\({}_{10,\text{TP}}\) were higher for the regenerating cycle. The SPN\({}_{23,\text{TP}}\) were at the  
Figure 6: Comparison of regenerating and non-regenerating WLTCs with vehicle #2 (Euro 6b): (**a**) exhaust gas temperature at the tailpipe, speed, and exhaust flow rate. Continuous lines are data from regenerating cycle. Dashed lines from non-regenerating cycle; (**b**) solid particle number (SPN) and total particle number (TPN) emissions of the regenerating cycle. Only the 10 nm SPN at the tailpipe (TP) and dilution tunnel (CVS) is shown for the non-regenerating cycle for better clarity. same level, without any evident existence of a solid core mode as with the Euro 6d-Temp vehicle. The \(>\)10 nm concentration from the dilution tunnel \(\text{SPN}_{\text{10,CVS}}\) were at the same level as the \(\text{SPN}_{\text{10,TP}}\) until approximately 1620 s; after that time, the \(\text{SPN}_{\text{10,CVS}}\) clearly deviated. The total particle number (TPN) concentration started to increase from approximately 1600 s at the dilution tunnel, but the system at the tailpipe measured a significant increase after 1670 s. The \(\text{SPN}_{\text{10,CVS}}\) followed the TPN signals, indicating that they could be volatile artefacts, even though the system had a catalytic stripper. Another fact that supports this assumption is that the \(\text{SPN}_{\text{10,CVS}}\) was increasing when the \(\text{SPN}_{\text{10,TP}}\) was decreasing, a behavior typical of volatile particles. The estimated mass from the EEPS was \(>\)10 mg/m\({}^{3}\), which is at the same level with the mass of Figure 3, where such differences were also noticed. Similar differences between tailpipe and dilution tunnel have been reported a few times: In one case, changing the dilution ratio of the device did not change the concentration of the sub-23 nm particles during the regeneration, indicating that they are non-volatile particles [12]. Another study found similar concentrations of 10 nm particles with evaporation tube and catalytic stripper during a regeneration [50]. A dedicated study with a moped gave some insight for the differences in solid particles \(<\) 10 nm between the tailpipe and the dilution tunnel [51]. It was suggested that the moped emitted nanoclusters (particles in the 1-3 nm size range), which would grow in the transfer tube to sizes detectable by the instruments due to heterogeneous nucleation and the condensation of volatile material either emitted from the moped or desorbed from the transfer tube due to the high exhaust gas temperatures. It was explained that the reason that the particles were not evaporated in the catalytic stripper of the instrument at 350 \({}^{\circ}\)C was due to their low volatility and kinetic restrictions (finite residence time in the catalytic stripper). Thus, for the regeneration tests, it is possible that the differences are due to nanoclusters that reached different sizes at the two locations. These nanoclusters could be due to the injected fuel or even oxidation-driven soot fragmentation at the DPF [52; 53; 54], which may result in the increase in nucleation particles, as indicated in experimental [53] and theoretical [55] studies.