## 3 Results and Discussion
### Sampling Location and Instrumentation
#### 3.1.3 Vehicle #2 Regeneration
Figure 7 presents tests with the Euro 6b vehicle at 120 km/h at different days. The particle number emissions are plotted in Figure 7a, while the speed and exhaust gas temperature at Figure 7b. Based on the signals of speed and exhaust temperature, the tests were very repeatable. However, this was not the case for the particle emissions, which were dropping over time (distance driven). Even though the instrument at the tailpipe had lower cut-point than at the dilution tunnel (4 nm vs. 6 nm), the TPN emissions at the dilution tunnel were much higher than at tailpipe for the same test. For example, at the first test, the emissions at the dilution tunnel reached 7 \(\times\) 10\({}^{12}\) #/s, while at the tailpipe, they were 5 \(\times\) 10\({}^{10}\) #/s (two orders of magnitude lower) for the same test. Over time, the concentrations were decreasing. At the third test (after 280 km from the first test), the instrument at the tailpipe did not increase, while the instrument at the dilution tunnel had a small increase reaching 3 \(\times\) 10\({}^{10}\) #/s. This series of tests demonstrates the high variability of the volatile particles. There can be a huge difference in the measured TPN between sampling locations, different tests, as well as between different days. The variability in the emissions between different days comes from the desorption of the deposited material at the aftertreatment and tailpipe of the vehicle and, in case of CVS, the tube that connects the vehicle with the dilution tunnel. For the same day, the difference between the tailpipe and the dilution tunnel is probably due to the deposited material at the tube between the vehicle and the dilution tunnel. The data in Figure 7 clearly show that the transfer line can be a huge source of nucleation mode particles. What is important to mention at this point is that all these tests with vehicle #2 were conducted after the tests with vehicle #1, which ended with a regeneration event (plus one WLTC) with the exhaust gas temperature at the tailpipe reaching 350 \({}^{\circ}\)C during the regeneration (see protocol in Figure 2). Furthermore, the first test with vehicle #2 had a regeneration at the last part of the WLTC, and the temperature reached 400 \({}^{\circ}\)C. One would expect that the desorption of any deposited material at the subsequent tests would be minimal, as the exhaust gas temperature at the 120 km/h tests only reached up to 275 \({}^{\circ}\)C. The results indicate that this may not be the case. There was still enough material to be desorbed at temperatures around 250 \({}^{\circ}\)C that took more than 1.5 h of driving at 120 km/h to reach low levels. Some of this material was probably deposited during the testing of the 330 km with this car (typical WLTCs). The literature has also highlighted the need of stabilization time to the variability of the volatile particles measurements [56]: Many studies have found that it can take more than 1 h to stabilize the concentration of volatile particles [42; 57], similar to what was demonstrated in our study. Other studies have also found higher concentration of volatile particles at the dilution tunnel compared to the tailpipe due to the desorption from the tube between the vehicle and the tailpipe [9; 58].