## 4 Conclusions  
In this study, we measured the particle number emissions of two Euro 6 Diesel vehicles. The emissions of both solid and volatile particles were determined using instruments sampling directly from the tailpipe and from the dilution tunnel with different lower cut-points. Three main topics were addressed: (i) sampling location, i.e., differences between tailpipe and dilution tunnel; (ii) differences between solid and total (i.e., solids and volatile) particles; and (iii) emissions during regenerations and contribution to the overall emission levels.  
The first vehicle in order (Euro 6d-Temp) regenerated a second time after 460 km of driving, while the second one (Euro 6b) had not regenerated a second time after 830 km. The solid particle number emissions including the regeneration events for both vehicles were <2 \(\times\) 10\({}^{11}\)\(\#\)/km, with a sub-23 nm particle concentration of 10 to 40%. However, the total (i.e., solids and volatiles) particle number emissions were 15 to 300 \(\times\) 10\({}^{11}\)\(\#\)/km, raising the question of whether these particles can be neglected. The measurements from the dilution tunnel and the tailpipe gave similar results (\(-\)35% to 20%) for the solid particles, but much higher differences were measured for the total particles. The reason was the desorption of particle pre-cursors from the tube that connects the vehicle to the tailpipe.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Vehicle** & \multicolumn{2}{c}{**\#1 (Euro 6d-Temp)**} & \multicolumn{2}{c}{**\#2 (Euro 6b)**} \\ \hline Cycle & WLTC & Overall 1  & WLTC & Overall 2  \\ SPN\({}_{23,\text{TP}}\) & 0.04 & 1.44 & 1.64 & 1.21 \\ SPN\({}_{10,\text{TP}}\) & 0.06 & 1.96 & 2.06 & 1.57 \\ TPN\({}_{10,\text{TP}}\) & 0.16 & 153 & 2.49 & 19.9 \\ TPN\({}_{4,\text{TP}}\) & 0.22 & 304 & 3.37 & 27.3 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Emissions (\(\times\)10\({}^{11}\)\(\#\)/km) of the two vehicles following the type approval cycle and including emissions during regeneration. Only measurements from the tailpipe were considered.  
Even though a good conditioning of the tube was done by driving at high speed, such desorption phenomena were evident even after >1 h of high-speed driving. During the first active regeneration tests, the concentration of volatiles was so high that it interfered with the >10 nm measurement of solid particles from the dilution tunnel, even though a catalytic stripper was implemented in the instrument.  
The key message is that the solid particle emissions of regenerations should be considered in future regulations, even though the weighted emissions did not exceed the current limit in our results. The volatile particles, which might also have health and environmental effects, should also be monitored, as this study and others in the literature showed very high concentrations. We believe that the tailpipe measurement is more representative for such particles. Even for solid particles, the dilution tunnel measurements might be suspect to volatile interference at the instruments at some regeneration events.  
Conceptualization, B.G.; formal analysis, A.M. and B.G.; writing--original draft preparation, B.G.; writing--review and editing, A.M. and T.L. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability Statement: Data are available from the corresponding author.  
This work was partly conducted in the framework of the DownToTen project, which was funded from the European Union's Horizon 2020 research and innovation programme under grant agreement Nr. 724085. Special acknowledgment to [PERSON] and [PERSON] for the experimental support.  
The authors declare no conflict of interest.