## 5 Conclusions and Recommendations
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* Recent research findings show that different after-treatment technologies have an important effect on the level and the chemical composition of the emitted particles, and highlight the importance of particle filter device conditions and regular checking to maintain best performance. **Maintenance of after-treatment systems is key to maintaining best performance. **
* The role of real driving emissions is clearly identified, with quite different emissions factors largely depending on engine operating mode. **Particular attention should be paid to pursue the use of real-world driving cycles to determine EF. **
* For non-equipped diesel vehicles, the use of biofuels can reduce BC emissions by 30% and could be an option to achieve the legal air quality target or limit values sooner. * Even if brake, tyre and road wear emit mainly coarse particles, a non-negligible fine fraction of PM is emitted. The TSP emissions per km are larger than current Euro VI emissions and a similar fraction of BC is observed in exhaust and no-exhaust PM emissions. Brakes also produce ultrafine particles, metals and PAHs, and temperature greatly affects dust PM emissions. BC emissions from brakes are correlated to PM\({}_{1}\) emissions. * Under particular driving conditions, UFP below 60 nm can be emitted during braking phases due to tyre emissions; but compared to DPF-equipped cars, emissions remain of minor importance. * There is no widely used after-treatment system to control brake, tyre and road wear emissions. The type of materials and the behaviour of the driver are often cited as key to reducing emissions. Some companies have developed brake particle collection systems that would reduce brake mass and number emissions by 80% to 90%, respectively. * **For particle number measurements, it is important to define the methodology details (sampling and dilution, measurement instrumentation, relevant sizes, etc.) and conduct inter-laboratory exercises to determine the reproducibility of a proposed method. **
* **PM resuspension from the road should be addressed and analysed in terms of carbonaceous composition avoiding double counting in emission inventories**. In principle, these types of emissions are not yet regulated, although they are responsible for a large fraction of total road traffic emissions. Such emissions depend on meteorology (wind, temperature, humidity, precipitation) and site climatology/location (land use in the vicinity). Ultrafine particles containing carbon are known to have adverse effects on health. Even if the solid fraction (BC/EC) of a particle has been reduced by after-treatment systems, the processes involving secondary species formation are still poorly known and this organic matter dominates the carbonaceous particulate emission of vehicles. A topic still poorly addressed so far in this research field is the role of physicochemical interactions between organic and inorganic species during the dilution process of vehicular emissions. From a research perspective, we must emphasise the need to better understand the processes at work during the dilution phase of the emissions with both adequate measurement systems and modelling tools. The evolution of the mixing state of emitted particles is also an important issue in terms of their impact on health as well as on radiative impacts. Conceptualisation, B.B. ; methodology, B.B. ; investigation, B.B. ; writing--original draft preparation, B.B. ; data analysis, B.B. and J.-P.P. ; writing--review and editing, N.A., J.-P.P., F.C., J.-M.A., P.T., E.P., D.S. and B.N.M. ; visualisation, B.B. ; supervision, B.B. ; project administration, B.B. ; All authors have read and agreed to the published version of the manuscript. This research received no external funding. **Institutional Review Board Statement**: Disclaimer--The U.S. Environmental Protection Agency (US EPA) through its Office of Research and Development collaborated in the research described here.