Characterization of Road Dust Emissions in Milan: Impact of Vehicle Fleet Speed

Road dust suspension is an increasing concern in terms of being a source of atmospheric particulate matter (PM) in cities, due to its toxicity and the lack of knowledge on emission estimates, impact and mostly with respect to measures to control or mitigate it. Both technological and policy measures have been proposed, but their application is limited by the gap of knowledge on their effectiveness in the real world. This study analyses the real-world emission factors of road dust at ten sites in the city of Milan (Italy), with an emphasis on the impact of different fleet speeds at one particular road. PM10 emission factors were estimated by means of both the EPA method (based on silt loadings) and the vertical profile of dust deposition. Road dust silt loadings varied within 0.006–0.066 g m–2, with the highest loadings found at sites affected by construction works. Typical urban roads were found to have fleet-averaged emission factors within 13–32 mg vehicle–1 km–1, which is in the central range of the literature values in Europe. The emission factors estimated by means of the vertical profile approach were within 19–26 mg VKT–1, which agree quite well with the EPA method if corrected for speed. In fact, a power-law relation was found between fleet speed and emission factor estimates, with an exponent close to 1.5 for speeds within 36–57 km h–1. These results suggest that the limitation of the maximum traffic speed can be effective for mitigating road dust emissions in cities.