**4. Conclusions**

This paper analyzed the early-stage effects on air quality of the new traffic policy in Milan, the so-called Area B. The concentrations of nitrogen oxides (NOx) and nitrogen dioxide (NO2), which are mainly primary pollutants, have been considered as proxies of pollution emissions.

The first hypothesis in the introduction inquires about the presence of a significant effect on the air quality of the city. As a first point, the preliminary results show that concentrations during spring and summer 2019 are lower than during the same seasons in the previous five years, hinting for a reduction effect due to the policy. On the other side, a similar reducing trend has been observed in various neighbouring cities around Milan, which belong to a homogeneous meteorological, social, and economical cluster within the Po valley. Their similar behavior is used here as an areal common trend capturing both weather and anthropogenic components. Our approach, which adjusts for local weather conditions and the areal common trend, does not provide a further reduction effect for any station comparing to this trend. Instead, in Senato station, which is inside the historical city centre and was already covered by Area C, the estimates provide a strong, but moderately statistically significant, increase for both considered pollutants. This is coherent with the fact that the restriction introduced is very limited as it concerns just some classes of old vehicles, which are a small percentage of the entire vehicle pool, both in terms of number of cars and emissions.

Since the first research hypothesis is confirmed just to a minor extent and with an opposite sign with respect to what was expected, the second research hypothesis, concerning the homogeneity of the possible effects, assumes now only a technical scope. It is confirmed just for what concerns the positive direction of the changes, but not for their significance. In fact, among all the estimated permanent effects, only Senato station is significant at 5%. Moreover, the estimated transitory effects are always not significant at any confidence level.

The above facts hint that, compared to the common trend of the considered area, Milan air quality is improving slowly, and, in this sense, the first phase of Area B seems to have a negative effect on air quality. Due to the limited scope of this first phase and its progressiveness, it is not unexpected to find a limited or a zero effect. Nonetheless, the negative effect needs some more explanations.

Although finding the ultimate motivation for this is not the aim of this paper, a discussion follows. Firstly, the statistically significant increase found is limited in space and is located inside the previously introduced restricted Area C. It may be possible that this further restriction increased congestion of public transport buses, which are often very old vehicles, or to the aforementioned adaptation shocks. This could explain only a part of the results. In fact, this first point is also related to the other sources of nitrogen oxides. According to INEMAR [25], road traffic is about 68% of the total emissions. Hence, a transition to house heating green techniques slower in Milan comparing the other considered cities could have an influence on this result. Moreover, also the other stations experienced a comparative deterioration of air quality and the second-worst station is Città Studi, which is an urban background station, hence with limited relation to local traffic congestion. Second, the increase due to road traffic may have temporal dynamics. Since the traffic restriction is limited to business hours, there may be an increase in congestion early in the morning and in the late evening, affecting the daily average.

In conclusion, although environmental protection policies are in general a fundamental step for sustainability improvement, in some cases, they may not be sufficient or their implementation may be misleading. In our case, we considered only the early-stage of a policy, which is progressive in time. Hence, the results of this paper may be regarded as physiological, provided that they characterize only the initial part of the policy implementation and are improved soon. It follows a recommendation to the municipal governmen<sup>t</sup> to develop the policy more strongly.

Additional research could be developed in the future. In particular, the effect on traffic congestion inside Area C could be investigated further using historical data related to the vehicle movements crossing the access points. Moreover, the use of a multivariate approach, which includes other pollutants such as PM10 and PM2.5, and spatio-temporal modeling could highlight hidden effects, which are not visible considering the single stations. Eventually, the extension to hourly data could consider both the presence of intra-daily effects and explaining the spatial dynamics related to traffic.

**Author Contributions:** Conceptualization, P.M., A.F., M.P. and M.M.; methodology and formal analysis, P.M., A.F. and M.P.; software, P.M. and M.P.; data curation and original draft preparation, P.M.; literature review, P.M. and A.F.; review and editing and validation and supervision P.M., A.F., M.M. and M.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors are grateful to the editors and three anonymous referees for their comments and suggestions.

**Conflicts of Interest:** The authors declare no conflict of interest.
