## 3 Results and Discussion
### Emission Levels  
Figure 8 plots the excess sub-23 nm particles not counted with the current SPN procedure in the regulation, which considers only particles larger than 23 nm. Lowering the cut-off size to 10 nm would increase 10-40% the emissions for both vehicles in the concentration range of interest (>10\({}^{10}\) #/km). At lower number concentrations in a few cases, higher percentages can be seen (the 80% values are from two WLTCs). Appendix A gives examples of where the sub-23 nm particles come from (mainly due to cold start). One noticeable exception, not shown in the figure, is vehicle #2's excess sub-23 nm concentration of cold idle (16 times higher than the >23 nm concentrations) and of the hot idle (2.2 times the >23 nm concentrations). The emission levels of the two vehicles are presented in Figure 9a,b, respectively. The emissions are given separately for the WLTC, 120 km/h, the same tests with active regeneration, and the overall emissions. For vehicle #1, the overall emissions are the emissions after the first regeneration until the second regeneration (including the second regeneration event) (460 km) (see also Figure 2). For vehicle #2, the overall emissions include all emissions over the 830 km driven. Each bar gives the solid particle number (SPN) emissions > 23 nm, and >10 nm, as well as the total particle number (TPN) emissions > 10 nm and >4 nm. Only the results from the tailpipe were considered in the figures for the reasons discussed in the previous section. Figure 7: Three tests at 120 km/h at 330 km (1), 390 km (2), and 610 km (3): (**a**) Total particle number (TPN) emissions at the tailpipe (TP) or the dilution tunnel (CVS); (**b**) Speed and exhaust gas temperature at the tailpipe. For vehicle #1 (Euro 6d-Temp), the emissions (solid and total particles) were low for tests without regeneration (<10\({}^{11}\)\(\#\)/km). The emissions during regeneration, though, were very high (>10\({}^{14}\)\(\#\)/km). The overall TPN emissions were dominated by the emissions measured over the regeneration event. The TPN-weighted emissions were still more than two orders of magnitude higher than the overall SPN emissions, which were still below the Euro 6 limit. For vehicle #2, the SPN emissions with and without regeneration were not very different and remained below the Euro 6 limit. The TPN increased by more than two orders of magnitude during the regeneration. The overall TPN emissions were one order of magnitude above the Euro 6 limit. High emissions of volatiles are commonly reported in the literature. Most studies have measured a nucleation mode with mean size < 20 nm [10, 39, 41, 44, 46, 53, 59] and concentration around 1-2.5 \(\times\) 10\({}^{13}\)\(\#\)/km [13, 18, 39, 41] but also higher 2-8 \(\times\) 10\({}^{14}\)\(\#\)/km [9, 10, 46, 53, 60]. Table 2 summarizes the emissions of the two vehicles for the WLTC and the overall emissions. Focusing on the >23 nm SPN, the WLTC emissions were very low for the Euro 6d-Temp car, and one fourth of the limit (6 \(\times\) 10\({}^{11}\)\(\#\)/km) for the Euro 6b. The overall (i.e.,  
Figure 8: Excess of sub-23 nm particle concentrations calculated as (SPN\({}_{10,\text{TP}}-\text{SPN}_{23,\text{TP}}\))/SPN\({}_{23,\text{TP}}\). Figure 9: Solid particle number (SPN) and total particle number (TPN) emissions measured from the tailpipe with counters of different lower cut-points. The results are plotted separately for the worldwide harmonized light vehicles test cycle (WLTC) and 120 km/h, without and with active regeneration, and the overall (i.e., weighted with one regeneration event) emissions: (**a**) Vehicle #1 (Euro 6d-Temp); (**b**) Vehicle #2 (Euro 6b). weighted emissions including one regeneration event) >23 nm SPN emissions for both vehicles were one-fourth of the limit. The increase in the emissions considering the sub-23 nm particles was 25-30% for the Euro 6b vehicles and 36% for the Euro 6d-Temp (48% for the WLTC).