## 3 Results and Discussion
### PN Emissions
In contrary, the cumulative PN emissions for driving in the sport mode show a lower increase than in the hybrid mode, because the ICE is operated more continuously, which avoids major particle formation, even though the average fuel consumption is higher. [PERSON] et al. [19] show for a GPF-equipped vehicle that the PN emissions are shaped by accelerations and the engine speed. Similarly, in case of the investigated PHEV of this study, the differentials of the fuel consumption and therefore the engine speed are decisive for PN formation. Furthermore, the observed PN emission increase during motorway driving events is not related to a potential low operating temperature of the PHEV, as in contrast to [7], the test drives are not performed in the electric driving mode. Due to the frequent ICE operation phases in the hybrid and sport driving modes, the operating temperature of the vehicle is sufficiently high during rural and motorway driving. This is proven by the level of the TWC temperatures of +300 \({}^{\circ}\)C during the second half of the tests (e.g., Figure 3). Therefore, the authors conclude that the obtained results are representative in spite of the mentioned relatively small motorway share of the test route. Figure 8 shows a comparison of the average, distance-related PN emission factors of the studied PHEV for driving in the hybrid mode in summer (Figure 8a) and driving the same mode in autumn (Figure 8b). It should be noted that in the test series during summer particle sizes of up to 23 nm are captured, while the test series in autumn include the measurement of UFP with up to 10 nm diameter. Figure 8: Influence of season and sub-23 nm particle measurement on the real-world PN emissions of the studied PHEV considering hybrid driving mode. (**a**) Summer time; (**b**) autumn time, including ultrafine particle (UFP) measurement. Figure 7: Comparison of the cumulative PN emissions and the fuel consumption differentials of a test in the hybrid driving mode (**a**) and a test in the sport driving mode (**b**). Both tests are performed during summer time. The average total PN emission factor of the autumn test series is 9.34 \(\times\) 10\({}^{11}\) #/km, which is approx. 49% higher than the measured average emission factor in the summer tests. While the differences of PN emission factors for rural and motorway vehicle speeds are smaller (6.67 \(\times\) 10\({}^{11}\) #/km and 5.07 \(\times\) 10\({}^{11}\) #/km for rural driving and 1.00 \(\times\) 10\({}^{12}\) #/km and 8.58 \(\times\) 10\({}^{11}\) #/km for motorway driving), the average urban PN emission factor of the autumn tests is approx. 79% higher than in the summer tests (Figure 8). One possible cause for this observation could be the influence of the lower ambient temperature in the autumn test series. According to Table 3, the ambient temperature of the autumn test series is 10-14 \({}^{\circ}\)C and 24-29 \({}^{\circ}\)C during the summer test series. Worldwide harmonized Light vehicles Test Procedure (WLTP) tests of the same vehicle show that for the hybrid driving mode, PN emissions are marginally higher at \(-7\)\({}^{\circ}\)C ambient temperature compared to 23 \({}^{\circ}\)C ambient temperature and for the case of urban PN emissions, the values can be even lower at the lower ambient temperature [20]. Considering these findings and the fact that within the test series of this study the ambient temperature differences are actually smaller, the authors assume that the influence of ambient temperature differences on PN emissions can be neglected in this comparative case. Hence, the significantly higher PN emission factors of the autumn tests are primarily associated with the UFP share, which is particularly dominant at urban vehicle speeds.