Non-exhaust traffic emissions: Sources, characterization, and mitigation measures

Highlights: • NEE are relevant PM sources in urban areas, exceeding EE contribution. • Emission standards and clean air policies must focus on NEE. • Standardized NEE sampling and measurement methods are needed to set regulations. • Among NEE sources, resuspended dust exceeds brake, tyre, and road wear. • EVs have no tailpipe emissions, but still produce a significant amount of NEE. Non-exhaust emissions (NEE) of particulate matter (PM) from brake, tyre, road pavement and railway wear, as well as resuspension of already deposited road dust, account for up to 90% by mass of total traffic-related PM emitted. This review aims at analysing the current knowledge on road traffic NEE regarding sources, particle generation processes, chemical and physical characterization, and mitigation strategies. The literature on this matter often presents highly variable and hardly comparable results due to the heterogeneity of NEE sources and the absence of standardized sampling and measurement protocols. As evidence, emission factors (EFs) were found to range from 1 mg km⁻¹ veh⁻¹ to 18.5 mg km⁻¹ veh⁻¹ for brake wear, and from 0.3 mg km⁻¹ veh⁻¹ to 7.4 mg km⁻¹ veh⁻¹ for tyre wear. Resuspended dust, which varies in even wider ranges (from 5.4 mg km⁻¹ veh⁻¹ to 330 mg km⁻¹ veh⁻¹ for cars), is considered the prevailing NEE source. The lack of standardized monitoring approaches resulted in the impossibility of setting international regulations to limit NEE. Therefore, up until now the abatement of NEE has only been achieved by mitigation and prevention strategies. However, the effectiveness of these measures still needs to be improved and further investigated. As an example, mitigation strategies, such as street washing or sweeping, proved effective in reducing PM levels, but only in the short term. The replacement of internal combustion engines vehicles with electric ones was instead proposed as a prevention strategy, but there are still concerns regarding the increase of NEE deriving from the extra weight of the batteries. The data reported in this review highlighted the need for future studies to broaden their research area, and to focus not only on the standardization of methods and the introduction of regulations, but also on improving already existing technologies and mitigating strategies.