## 4 Conclusions
The largest differences between SC2 and SC1 were found during the cold period (-3.8 ppb), mainly over urban areas due to titration. The side-effect of cleaner vehicles could lead to higher O3 concentrations and possible exceedances in cities that are currently below the O3 limits. A similar behaviour was also found for PM2.5, where a small increase in PM2.5 concentrations was found over urbanised areas when NOx emissions were reduced. For this reason, it is important to identify the main precursors on which to act and to which extent when air quality plans are designed. Both counterfactual scenarios modelled in this study were used to explore, in a simplified manner, the impact of the Euro 6 standard. SC1, in particular, indicates that pre-RDE Euro 6 vehicles have had a positive contribution to air quality, and SC2 points to the further potential for the latest Euro 6 standard to continue to deliver air quality improvements before vehicles compliant with the new Euro 7 standard (currently under preparation) are gradually introduced into EU vehicle fleets in the second half of the present decade. However, even if this emission reduction potential were fully realised, this would not translate into compliance with current EU air quality standards in many EU urban areas. Further reductions than modelled in this study are in any case possible through a combination of a technology-neutral Euro 7 standards with lower emissions, through the accelerated introduction of zero tailpipe emission light-duty vehicles and also with emission standards and other measures aimed at reducing the emissions from sectors other than road transport (none of which were modelled in our scenarios). The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12094257/s1](https://www.mdpi.com/article/10.3390/app12094257/s1)]([https://www.mdpi.com/article/10.3390/app12094257/s1](https://www.mdpi.com/article/10.3390/app12094257/s1)), ref. [17; 20; 51; 52; 53; 54; 55]. Figure S1: Instrumentation and sampling scheme in VELA laboratory; Figure S2: Test cycles speed profile WLTC; Figure S3, Set up and vehicle in the VELA laboratory; Table S1: Emission Factors for Road Traffic in Edgar; Table S2: Average fuel-specific emission factors of diesel and gasoline Euro 6d-TEMP/Euro 6d vehicles over WLTP; Figure S4: Calculated yearly relative mean differences (%) between Reference Case and Scenario 1 for NO\({}_{2}\). Together with the differences in averaged concentrations between the two simulations for three main periods, i.e., summer period (May till September), transition period (March, April and October) and a winter period (November till February); Figure S5: Same as Figure S4, but for O\({}_{3}\); Figure S6: Same as Figure S4, but for PM\({}_{25}\); Figure S7: Calculated yearly mean relative differences (%) between Scenario 1 and Scenario 2 for NO\({}_{2}\). Together with the differences in averaged concentrations between the two simulations for three main periods, i.e., summer period (May till September), transition period (March, April and October) and a winter period (November till February); Figure S8: Same as Figure S7, but for O\({}_{3}\); Figure S9: Same as Figure S7, but for PM\({}_{25}\). Conceptualisation, A.d.M., P.T. and E.P. ; methodology, A.d.M., P.T. and E.P. ; software, A.d.M., M.C., D.G., O.M. and G.D.O. ; validation. ; A.d.M., P.T., E.P., V.V. and R.S.-B. ; formal analysis, A.d.M., P.T., E.P., V.V. and R.S.-B. ; investigation, A.d.M., P.T., E.P., V.V. and R.S.-B. ; resources, A.d.M., M.C., D.G., O.M. and G.D.O. ; writing--original draft preparation A.d.M., P.T., E.P., V.V. and R.S.-B., writing--review and editing, A.d.M., C.A., P.T., E.P., O.M., G.D.O., V.V., R.S.-B. and V.F. ; project administration, C.A. ; funding acquisition, C.A. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. The authors declare no conflict of interest.