## 4 Conclusions  
In this work we studied the contribution of NMVOCs, PM, NO\({}_{\times}\), and NH\({}_{3}\) emitted by vehicles to the formation of Secondary Aerosol and urban air pollution in Europe. We performed model simulations with the Edgar emission inventory (V5) and changed the EU 2015 (a combination of Euro 1 to Euro 6) emission characteristics of light-duty vehicles with two different counterfactual emission reduction scenarios, one where all vehicles are  
Figure 8: The number (#) of days for which the WHO PM\({}_{2.5}\) limit values of 15 \(\upmu\)g/m\({}^{3}\) (**a**,**b**) and PM\({}_{10}\) European limit value of 50 \(\upmu\)g/m\({}^{3}\) are exceeded for Warsaw and Milan (**c**,**d**). modelled as Euro 6b standard (SC1) and one where all vehicles meet the most stringent Euro 6d-TEMP/6d standard (SC2). The results for both counterfactual scenarios show that total NOx, NMVOC, and NH3 emissions by diesel and gasoline vehicles are reduced when compared to the reference scenario (with a mix of Euro 6 and pre-Euro 6 vehicles in the national vehicle fleets). For NOx, we found a reduction of almost 50% in total emissions for Italy, Spain, Germany, and France. Since diesel vehicles are responsible for the largest contribution of NOx emissions as compared to cars using other fuel types, replacing all diesel cars to Euro 6b or Euro 6d-TEMP results reduces the NOx emissions by a factor of two for countries such as Germany, Spain, France, and Italy. This is very relevant, as these countries (together with Poland and UK) are responsible for the largest road transport emissions. A strong decrease was also found in total emissions per country and for NMVOCs and NH3 for both counterfactual scenarios. Road transport emissions of PM2.5, NMVOC, NOx, and NH3 in the Reference Case are higher than in either SC1 or SC2. NOx and NMVOC emissions by SC2 are lower than SC1 over urban areas, whilst for some areas NH3 mass emissions are higher by SC2, depending on the diesel share of the fleet. The impact of these emission changes on concentration highlights that for yearly mean NO2 concentrations, the Reference Case shows higher NO2 values than SC1 over urban areas, roads, and highways, being up to 5.7 \(\mu\)g/m3. During the cold, warm and transition periods, the differences between SC1 and the Reference Case are similar to the yearly differences. Also, NOx concentrations by SC2 are lower than SC1, up to 41% (4.6 \(\mu\)g/m3), mainly over urban areas. At the city level, we showed that both SC1 and SC2 have a significant impact on reducing NOx exposure levels. Yearly mean PM2.5 concentrations are higher in the Reference Case (up to 2.2 \(\mu\)g/m3). The largest differences were found over the Benelux and the Po Valley. During the cold, warm and transition periods, the differences between SC1 and the Reference Case are similar to the yearly differences. Similar, yearly PM2.5 concentrations by SC2 are lower than SC1, up to -7% (-1 \(\mu\)g/m3). Note that when we analysed the PM2.5 concentrations for individual days, we found a reduction of up to 8 \(\mu\)g/m3 for the city of Milan on 26 October (daily average PM2.5 is 56.2 \(\mu\)g/m3 by REF). This highlights the importance of analysing the daily limit values as well. For cities for which PM2.5 concentrations are problematic, the number of days for which the daily limit values are exceeded dropped drastically when Euro 6d and Euro 6d-TEMP emission factors are implemented. Throughout the year, SC1 shows higher O3 concentrations as compared to the Reference Case (up to -2.5 ppb) over urban areas, with the largest differences found during the cold period. The reason for this is that when NOx emissions are reduced in VOC-limited areas, higher O3 concentrations are found, because less O3 is removed by NOx titration. During the warm period, higher O3 monthly mean values were found by the Reference Case over southern Europe, with the exception of the Benelux and urban areas.