## 3 Results
### Scenario 1 versus Scenario 2
#### 3.2.2 Ozone (O\({}_{3}\))  
Reference Case versus Scenario 1  
In this section, we analysed O\({}_{3}\) concentrations in terms of the daily 8 h maximum indicator to exclude the low night-time O\({}_{3}\) values in urban areas. The production of O\({}_{3}\) depends on the availability of NO\({}_{x}\) and VOCs, both emitted by road transport, among other sectors. Whether reductions in NO\({}_{x}\) or VOC emissions will lead to lower O\({}_{3}\) concentrations depends on the location and type of chemical regime, i.e., whether it is a NO\({}_{x}\)-limited or a VOC-limited regime. For NO\({}_{x}\)-limited regimes (e.g., locations downwind of urban environments), lowering NO\({}_{x}\) emissions results in lower O\({}_{3}\) peak concentrations, whereas in VOC-limited areas (e.g., highly polluted urban areas), the opposite occurs, leading to higher O\({}_{3}\) concentrations. On the other hand, lowering VOCs and keeping NO\({}_{x}\) constant always leads to reduced O\({}_{3}\) values. Figure 4a shows the difference in annual mean O\({}_{3}\) concentrations between the Reference Case and SC1. What it interesting to see is the general increase in O\({}_{3}\) concentration (up to 2.5 ppb, \(\sim\)8.6%, Figure S5 ES), particularly in densely populated areas (e.g., the Benelux, London, Liverpool, Manchester, Madrid, Barcelona, Paris, Milan, Rome, Naples, Lisbon, Porto, the Ruhr area, Berlin, and Dublin), which was associated with lower NO\({}_{2}\) and NMVOCs emissions. This side-effect of cleaner vehicles could lead to higher O\({}_{3}\) concentrations and possible exceedances in cities that are currently below the O\({}_{3}\) limits. Figure 3: (a) NO\({}_{2}\) concentrations (\(\upmu\)g/m\({}^{3}\)) at city level for the Reference Case, and the two counterfactual emission reduction scenarios, together with the number (#) of exceedance days (Exc) of the WHO recommended concentration value for the city center of Milan (b) and Paris (c). The underlying reason for this increase is that less O\({}_{3}\) is removed by NO (NO\({}_{\rm x}\) titration), therefore augmenting O\({}_{3}\) values in VOC-limited zones, as mentioned earlier. NO\({}_{\rm x}\) titration is an important removal mechanism of O\({}_{3}\) during night and wintertime [37]. Therefore, we see in general an increase in O\({}_{3}\) values by SC1, especially during the cold period in Figure 4b over Eastern Europe and the Po Valley (up to 19.2%). During the warm period (May to September), there is a slight reduction of O\({}_{3}\) concentration over non-urban areas for the countries in the Mediterranean basin (Spain, France, Italy, up to \(\sim\)2 ppb) where the concentrations are usually the highest. In principle, these reductions in O\({}_{3}\) could lead some regions to fulfil the current legal limit of 120 \(\rm{\upmu g/m^{3}}\) (8 h maximum). Yet, higher O\({}_{3}\) values were found, due to the reduction of NO\({}_{\rm x}\) over the densely populated areas, as described above. What is interesting to see is that the seasonal variations of O\({}_{3}\) concentrations are more distinct than for NO\({}_{2}\), as described earlier. An explanation for this is that the emissions of NO\({}_{\rm x}\) by traffic are rather constant throughout the year, whilst O\({}_{3}\) is chemically formed by the oxidation process of volatile organic compounds (VOCs) in the presence of NO\({}_{\rm x}\) (NO + NO\({}_{2}\)) and its formation is driven by the sunlight's intensity. The latter has a strong seasonal profile. Figure 4: Calculated yearly mean differences in O\({}_{3}\) concentrations (ppb) between Reference Case and Scenario 1 (**a**), together with the differences for three main periods, i.e., (**b**) cold/winter period (November to February), (**c**) warm/summer period (May to September) and (**d**) mild/transition period (March, April, and October). Scenario 1 versus Scenario 2  
Yearly mean O\({}_{3}\) concentrations by SC2 are somewhat lower than SC1, with a minimum of 0.6 ppb.