**4. Conclusions**

Road dust resuspension is one of the major sources causing PM10 exceedances, with detrimental effects on climate and human health. It has been demonstrated that emission inventories must use locally determined emission factors as these vary not only with weather and road conditions, but also with vehicle fleet. This study offers the first experimental estimates for road dust emission factors in the region of Viana do Castelo. A PM10 emission factor of 49 mg veh−<sup>1</sup> km−<sup>1</sup> was derived for an asphalt-paved road, which is in the range of values documented for the same type of pavement in a few other southern European cities, while much higher emissions (around 300 mg veh−<sup>1</sup> km−1) were found for cobble stone streets. Although sampling took place in only three streets, these were carefully selected to represent different sectors of the city. On the other hand, because composite samples were

obtained from multiple road segments and the fact that the emission factors of the present study are similar to those determined in a previous work for several streets in Oporto, also an Atlantic city with comparable pavements and tra ffic fleet, the representativeness of the values now estimated is broadened.

The chemical composition and the enrichment indices sugges<sup>t</sup> an anthropogenic contribution of tra ffic-related elements, such as Br, Cl, Cr, Cu, P, Pb, S, Sn, W and Zn, especially to the finest road dust fraction (F1, <0.074 mm) from urban streets. In these locations, the fingerprint of marine spray is also identified with higher concentrations of Cl, Na and Mg. Samples from the suburban area presented an extremely severe anthropogenic enrichment in the finest fraction for the element As, possibly linked to agricultural activities and fossil fuel combustion. Quartz was mostly present in the coarser fraction, in which it was the most abundant mineral. Other minerals derived from natural sources were also observed (muscovite, albite, kaolinite, Fe-enstatite and graphite), as well as a significant amount of anthropogenic-related materials (amorphous). SEM and EDS analyses indicated that the main constituents in the amorphous content originated from non-exhaust tra ffic sources (brake, tyre and road abrasion) and from fuel combustion.

The estimation of non-carcinogenic health risks due to exposure to heavy metals indicated that children may experience adverse e ffects due to ingestion of the finest size fraction of road dust. For the suburban location, the risk associated with the ingestion of this finest road dust was above the acceptable target proposed by USEPA. Other samples and size fractions presented a risk by ingestion and dermal contact within a range indicating that managemen<sup>t</sup> measures are required.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1660-4601/17/5/1563/s1, Figure S1. Location of the sampling sites; parameters used in the calculations of Sections 2.3 and 2.4; Figure S2. Cluster analysis of the XRF results of road dust samples (S1 suburban environment influenced by agricultural activities; S2 and S3 urban streets); Figure S2. Enrichment index for elements detected in road dust fractions (F1—below 74 μm, F2—between 74 μm and 1 mm).

**Author Contributions:** Conceptualisation, C.A.; sampling, C.A., E.V. and M.T.; sieving, C.A., C.C. and E.V.; geochemical, mineralogical and morphological characterisation, C.C., P. Á. and F.R.; data treatment, C.A. and C.C.; writing of original draft, C.A. and C.C.; review of original draft, C.A. and C.C., in collaboration with all co-authors. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the project "Chemical and toxicological SOurce PROfiling of particulate matter in urban air (SOPRO)", POCI-01-0145-FEDER-029574, funded by FEDER, through Compete2020 - Programa Operacional Competitividade e Internacionalização (POCI), and by national funds (OE), through FCT/MCTES. An acknowledgement is also given to the Portuguese Foundation of Science and Technology (FCT) and to the POHP/FSE funding programme for the fellowship SFRH/BD/117993/2016. We are also grateful for the financial support to CESAM (UID/AMB/50017/2019), to GeoBioTec (UID/GEO/04035/2019; UIDB/04035/2020), and to FCT/MCTES through national funds, and co-funding by FEDER, within the PT2020 Partnership Agreement and Compete 2020.

**Acknowledgments:** Special thanks are given to the Viana do Castelo City Council, and especially to Ricardo Carvalhido, for all the logistic support.

**Conflicts of Interest:** The authors declare no competing interests.
