## 3 Results
### Emissions Scenarios  
In this section, we analysed the differences in terms of road transport emissions between the Reference Case and the two scenarios, i.e., SC minus REF. The impact of road transport emission reductions is analysed for the 27 EU Member States (EU27) plus Norway, Switzerland, and the United Kingdom. We consider EU27 + countries because these countries follow a common trend in the implementation of transport technologies. As described earlier, emission changes apply only to gasoline and diesel passenger cars and light-commercial vehicles. In general, PM\({}_{2.5}\), NMVOC, NOx, and NH\({}_{3}\) road transport emissions in the Reference Case are higher than in SC1, and those in SC1 are higher than in SC2 (see Figure 1). The unit of the emissions in the EMEP model are given in mg/m\({}^{2}\). The reductions in the emissions are clearly visible over highways and dense urban areas where road transport emissions mostly occur. For some countries (e.g., Germany, the Netherlands, Switzerland, Poland), higher NH\({}_{3}\) emissions reductions are found as compared to other countries (Spain, France). This can be explained by the difference in the share of diesel vehicles in each country's fleet and the fact that gasoline vehicles are responsible for most of the NH\({}_{3}\) emissions from light-duty vehicles. For example, Germany, the Netherlands, Poland, and the UK are the countries showing largest NH\({}_{3}\) reductions because these countries have a relatively low share of diesel vehicles (33.2, 16.8, 30.5, and 38.3% for the year 2017, respectively [31] and the pre-Euro 6 EDGAR/COPERT emission factor of NH\({}_{3}\) for gasoline vehicles is higher than the one attributed to Euro 6 gasolines, so that, when moving all pre-Euro 6 gasoline vehicles to Euro 6 gasoline vehicles, the mass emitted in Scenario 1 is lower than in the Reference case. For countries with a higher diesel share of the fleet (>56%, such as in the Baltic states, France, Luxembourg, Austria, or Spain), the change of scenario barely affects the total NH\({}_{3}\) mass emissions as the reduction on the gasoline contribution is lower than the increase of the diesel contribution. The increase of the NH\({}_{3}\) emission factor for diesel vehicles results from the more extensive use of the selective catalytic reduction (SCR) catalysts needed for large de- NOx conversion. Figure 1: _Cont._Figure 1: Yearly differences in road transport emission totals (mg/m\({}^{2}\)) between the Reference Case and Scenario 1 for PM\({}_{2.5}\) (**a**), NMVOCs (**b**), NO\({}_{\rm x}\) (**c**), and NH\({}_{3}\) (**d**), together with the differences between Scenario 1 and Scenario 2 for NMVOCs (**e**), NO\({}_{\rm x}\) (**f**), and NH\({}_{3}\) (**g**). It is interesting to note that general NH\({}_{3}\) emissions in SC1 are lower than SC2. In SC2, vehicles need to comply with stringent NO\({}_{\rm x}\) limits on the road. For that aim, diesel vehicles make a more intensive use of the after-treatment systems that need to operate throughout the whole driving duration, namely SCR catalysts. As a consequence of a wider operation of the SCR, NH\({}_{3}\) can be emitted, even more so if the vehicles do not mount ammonia slip catalysts. In Euro 6 pre-RDE vehicles (as in SC1), diesel vehicles resulted in very low NH\({}_{3}\) emissions, if any [20; 32]. Between REF and SC1, the largest differences in NMVOCs are found over urban areas of the UK, the Netherlands, Poland, and Czechia, where gasoline vehicles have a relative high share of the total passenger car fleet, as diesel cars emit lower NMVOC emissions than gasoline engines [33] and Euro 6 gasoline vehicles emit less than pre-Euro 6 ones. However, Spain has a high diesel share (56.9%), and the relative difference in NMVOC emissions between the Reference Case and SC1 is up to \(\sim\)30% lower by SC1 for Spain and up to \(\sim\)25% lower for Czechia, considering the mass emissions on the whole country (not shown).