## 3 Results  
Figure 2 compares the two PMP systems at the tailpipe (TP) and the dilution tunnel (CVS) at the first and last tests of the measurement campaign. The last test was approximately after 8 h of laboratory testing (driving) in one week. No other vehicles were tested in the facility during this period. Focusing on the first test (left side), the tailpipe and the CVS SPN >23 nm concentrations were low but at the same level. The SPN > 4 nm concentrations at the TP and CVS were at the same level during the engine start (not shown here, approximately 100 s) and for a few minutes (200 s). At approximately 300 s, when the exhaust gas temperature exceeded 250 \({}^{\circ}\)C, they started to deviate. The difference exceeded one order of magnitude (reaching \(3.3\times 10^{10}\) p/s or \(2.6\times 10^{12}\) p/km). The 10 nm CVS results were between the 23 nm and 4 nm results, while the 10 nm TP results were closer to the 4 nm TP results. This first test confirmed the appearance of nonvolatile particles at the CVS, not seen at the tailpipe, mentioned in the previous study [11]. The previous study confirmed that the particles measured at the CVS are nonvolatile because higher or lower dilution, or a catalytic stripper, did not change their concentration [11]. For the same reason it was also assumed that they are not formed inside the PMP system. It was not clear though when (where) and how they were formed. Figure 2, right side, shows that after 8 h of testing the concentration at the CVS was identical to the one at the TP (\(2.5\times 10^{8}\) p/s or \(2.5\times 10^{10}\) p/km). The tailpipe levels were 3 times lower than at the last test (also the 23 nm concentration, which is not shown) compared to the first test (\(8\times 10^{8}\) p/s or \(6.4\times 10^{10}\) p/km). Figure 3 plots the results with cold dilution (i.e., heaters off) at PMP #2 measuring from the dilution tunnel (CVS cold dilution, orange lines, test 4). The results of two tests (1 and 5) with the heaters on for PMP #2 are also plotted for comparison (red lines). The emission rates from the tailpipe (heaters on, blue lines, test 4) are also shown to put the results into context. With the heaters off, the CVS results of the 23 nm and 4 nm CPCs were very close after 600 s, indicating that the particles had a mean size larger than 23 nm. The difference of the 4 nm CPCs with the heaters on and off (compare  
Figure 2: Solid particle number (SPN) emission rates at the tailpipe (TP) and the dilution tunnel (CVS) at the first (Test 1) and last test (Test 7) of the measurement campaign (default settings). The temperature at position 1 (TP) is also plotted. SPN 4 and TPN 4) is relatively small (less than a factor of 2) which can be attributed to experimental uncertainties and the particle losses in the PMP #2 system when the heaters were on and off [10]. This finding indicates that particles pre-existed and the volatile compounds heterogeneously nucleated and condensed on existing particles, rather than homogeneously nucleated. Figure 4 plots measurements with cold dilution at tailpipe. When a cold diluter was added upstream of the PMP #1 system at the tailpipe (cold mixing dilution 6:1 at TP, test 3), the concentration remained at low levels without indications of nonvolatile particles appearance. When the transfer tube was open at the side of the tailpipe (open transfer tube) the concentrations of particles >4 nm at the tailpipe and the dilution tunnel were similar (Figure 4, right side, test 2). The exhaust gas temperature close to the tailpipe immediately after this dilution (of approximately 2:1) was around 120-140 \({}^{\circ}\)C. The sudden drop of the temperature from 140 \({}^{\circ}\)C to 120 \({}^{\circ}\)C was due to a movement of the transfer tube that changed the dilution. The main conclusion of these tests is that with sudden cold dilution at the tailpipe the particles are not formed or do not grow to the measurement range of the systems.