## 4 Conclusions  
A Euro 6 PHEV is investigated on PN and NO\({}_{\rm x}\) emissions in multiple test drives with variations in ambient conditions and driving mode on a defined real-world driving test route in the city of Stuttgart. Within each test series, the average fuel consumption and emission factors per driving situation show a good level of reproducibility. This observation is of high relevance since the user's charge and driving behavior may impact the operation frequency of the combustion engine and, thus, the emission from the vehicle. Therefore, an analysis based on these quantities is reasonable in order to identify the main causes of emission formation for the different PHEV test conditions.  
Across all performed tests the real-world NO\({}_{\rm x}\) emissions of the PHEV are mainly shaped by the cold start emission peak from the first activation of the ICE. The trip-average NO\({}_{\rm x}\) emission factors are on a negligible level though.  
With regard to the particle emissions during a specific test run, the phases of non-constant ICE operation and higher electric driving shares might cause a higher release of UFP due to the reoccurring start phases of the ICE. This was not observed in the present case. Neither the initial peak particle number concentration at the different start phases of the combustion engine nor the particle size distributions show significant deviations to the ICE-dominant operation phases. Therefore, the intermediate shut-down of the combustion engine during the urban part of the trip is a gain with regard to the release of particles into the nearby environment. Since the combustion engine operation phases mainly occur during motorway drives (high power required), the use of PHEVs can reduce the impact of road transport on the local air quality in cities.  
Figure 14: CFs for NO\({}_{\rm x}\) emissions of the performed real-world driving tests with the studied PHEV.  
However, hybrid mode driving in motorway situations seems to be critical, as the average PN emissions can be higher than driving more ICE-dominant, exceeding the current Euro 6 CFs in some cases. This highlights the complexity of an ecological assessment of PHEVs as higher electric driving shares and lower fuel consumptions do not necessarily lead to decreasing all pollutant emissions. Furthermore, the UFP share can contribute to a 50% increase in cold start PN emissions, also not meeting the current Euro 6 not-to-exceed limits for the urban test parts. This finding needs to be confirmed by other studies, as there are remaining uncertainties regarding the PEMS CPC measurement precision. Further studies with real-world driving tests of modern GPF equipped PHEVs are required to prove if the particle filter efficiently reduces the particle release in the highlighted (motorway and cold start) driving situations. Moreover, future studies should further focus on measuring PN emissions of PHEVs especially including UFP measurements in a wider range of ambient temperatures and real-world driving situations outside of the current RDE legislation.  
Conceptualization, M.F., S.E. and T.S.; formal analysis, M.F. and T.S.; investigation, F.E. and T.G.; writing--original draft preparation, M.F., S.E. and T.S. All authors have read and agreed to the published version of the manuscript.  
This research received no third-party funding.  
Not applicable.  
All data are summarized in the tables.  
The project \"UrMo Digital\" was financed via the DLR transport research program. The APC was funded by the DLR publication fund. Furthermore, the authors acknowledge [PERSON] and [PERSON] for giving their valuable comments regarding the draft manuscript.  
The authors declare no conflict of interest.