## 3 Results and Discussion
### Sampling Location and Instrumentation
#### 3.1.2 Vehicle #1 Regenerations
In summary, during the active regeneration of the Euro 6d-Temp vehicle, the following particle classes were measured: (i) volatile particles that appeared when the tailpipe temperature was around 200 \({}^{\circ}\)C with a mean size between below 10 nm; (ii) core particles due to the incomplete combustion of the injected fuel with a mean size around 10 nm; and (iii) soot particles with a mean size > 23 nm. While the volatile and soot particle emissions are commonly reported in the literature, the existence of a core mode during active regenerations has not been identified before. The term \"core\" is used in lack of another term to characterize these particles. It is highly likely that these particles are heavy hydrocarbons from incomplete combustion during the late fuel injection to regenerate the filter. High concentrations of polycyclic aromatic hydrocarbons (PAHs) have also been detected during active regenerations [49]. Regeneration studies have identified fuel-originated hydrocarbons [45], but low-volatility oil hydrocarbons have been also measured in significant amounts [10]. Figure 5 presents the first regeneration test of the same vehicle #1. The car was at 130 km/h, and after approximately 650 s, the regeneration initiated, as shown by the increase in the exhaust flow rate and the exhaust temperature at the tailpipe. The solid  
Figure 4: Emissions during the last regeneration event of vehicle #1 (Euro 6d-Temp) at 120 km/h. particles \(>\) 23 nm at the tailpipe (SPN\({}_{\text{23,TP}}\)) and the dilution tunnel (SPN\({}_{\text{23,CVS}}\)) started increasing gradually. A similar increase was noted for the solid 10 nm particles at the tailpipe (SPN\({}_{\text{10,TP}}\)), which were approximately 50% higher than the 23 nm concentrations. What was rather unexpected was the high increase in the solid 10 nm particles at the dilution tunnel (SPN\({}_{\text{10,CVS}}\)), which started almost from the beginning of the regeneration (750 s) when the exhaust gas temperature at the tailpipe was 250 \({}^{\circ}\)C. During the regeneration, the speed of the car changed from 130 km/h to 120 km/h (1250 s). The exhaust gas temperature dropped slightly, but remained elevated at around 350 \({}^{\circ}\)C. The difference between 10 nm and 23 nm particle number concentrations at the tailpipe remained at similar levels as before (50%), while the 10 nm concentration at the CVS came closer to the rest of the instrument's concentrations. At the end of the regeneration (1350 s), all SPN concentrations reached the same level in both sampling locations. With the exception of the SPN\({}_{\text{10,CVS}}\) concentration, the two regenerations had similar behavior. In both cases, the duration was around 700 s, the exhaust gas temperature reached 350 \({}^{\circ}\)C, and there was a considerable number of particles below 23 nm. It is not clear why the SPN\({}_{\text{10,CVS}}\) was different at the two regenerations. Volatile artefacts (i.e., volatiles that penetrate through the catalytic stripper) is one possible explanation. The total concentration of volatiles at the dilution tunnel (TPN\({}_{\text{6,CVS}}\)) is not known because the instrument (EEPS) was saturated in most part of the regeneration. The fitting of the non-saturated parts of the size distribution during times that the total concentration was saturated gave a geometric mean diameter (GMD) of 37 nm with geometric standard deviation of 1.49 and concentration of 2 \(\times\) 10\({}^{8}\) #/cm\({}^{3}\). Assuming unit density, this gave a mass concentration of more than 11 mg/m\({}^{3}\). The mass of the volatiles at the last regeneration of Figure 4 (without such strange behavior) was 1.5 mg/m\({}^{3}\), (GMD = 29 nm, GSD = 1.45, concentration 6 \(\times\) 10\({}^{7}\) #/cm\({}^{3}\)). In this regeneration, there were solid cores, so any breakthrough of volatiles could have grown the core particles to the measurement range of the 10 nm counter. This will be further elaborated later. When the regeneration was finished, only soot particles \(>\) 23 nm were detected.