## 3 Results and Discussion
### Sampling Location and Instrumentation
#### 3.1.2 Vehicle #1 Regenerations  
Figure 4 plots the emissions during the last regeneration event with the Euro 6d-Temp vehicle that took place during the 120 km/h constant speed driving. After more than 20 min at 120 km/h (at 1300 s), the regeneration initiated: The exhaust flow increased, and the tailpipe exhaust temperature started increasing slowly. The total particles started increasing at approximately 1400 s, somewhat earlier at the dilution tunnel (TPN\({}_{6,\text{CVS}}\)). The exhaust gas temperature at the tailpipe was around 200 \({}^{\circ}\)C when the total particles at the dilution tunnel started increasing sharply. The TPN\({}_{6,\text{CVS}}\) emissions at the dilution tunnel exceeded the levels of the 4 nm counter at the tailpipe (TPN\({}_{4,\text{TP}}\)). The emissions of the 4 nm counter were double than of the 10 nm counter (TPN\({}_{10,\text{TP}}\)) when TPN\({}_{10,\text{TP}}\) saturated, which suggests that the peak (mean size) of the nucleation mode was below 10 nm and closer to 4 nm. The solid particles started increasing at approximately 1500 s (exhaust gas temperature 300\({}^{\circ}\)C). The agreement between dilution tunnel and tailpipe was very good for both 23 nm and 10 nm counters (lines almost indistinguishable). The 10 nm emissions were 50% higher than the 23 nm emissions during the regeneration event. After the regeneration (2000 s and later), the 23 nm and 10 nm counters were at the same levels, indicating only a soot (accumulation) mode with a mean size > 23 nm. It is worth mentioning that at the end of regeneration (2000 s), the solid emissions increased, while the total emissions decreased. Nevertheless, the total particle emissions remained elevated, even after the regeneration, thus suggesting desorption of nucleating material after the regeneration. Figure 3: Differences between the SPN instruments at the dilution tunnel (CVS) and the tailpipe (TP). Results from vehicle #1 (Euro 6d-Temp) are plotted with circles and from vehicle #2 with squares (Euro 6b). Solid symbols are the results from the 23 nm counters, while open symbols from the 10 nm counters. An active regeneration is usually divided into the following phases [39]: (i) warm-up until the DPF has reached the appropriate temperature for soot oxidation; at this phase, the DPF filtration efficiency is high, and consequently, the SPN emissions are low (1300-1500 s in Figure 4); and (ii) the soot oxidation phase, where the trapped soot is oxidized (1500-2000 s). This phase is subdivided in the \"increasing SPN emissions\" (1500-1600 s) phase where the DPF filtration efficiency starts to drop, and the \"stabilization phase\" (1600-2000 s), where the DPF is empty, and the filtration efficiency is low [13,40]. The formation of volatile particles depends on the exhaust gas temperature. The general understanding is that they appear when the temperature is appropriate to convert enough SO\({}_{2}\) from SO\({}_{3}\) at the DOC [39,41,42,43] and/or to desorb enough sulfates from the aftertreatment devices [44,45,46,47]. As the exhaust gas is cooled down in the diluter, these sulfates form nucleation mode particles, which grow to the measurement range of the instruments, especially in the presence of hydrocarbons. The appropriate temperature is typically reached before the soot oxidation phase, and the volatile particles increase earlier than the solid particles [39,41], but not always [13]. Their concentration depends on the available pre-cursors, the sampling and dilution conditions, and the possible soot sink for the particles and vapors [48]. At the end of the regeneration in Figure 4, the exhaust gas temperature decreased, the soot particles increased, and the volatile particle concentration immediately dropped, as expected. However, their concentration was still much higher (1-2 orders of magnitude) than the concentration of solid particles because the exhaust gas temperature remained high enough.