Blog | Non- Exhaust Traffic Emissions: An Increasing Concern in Air Quality Studies

04 Mar Blog | Non- Exhaust Traffic Emissions: An Increasing Concern in Air Quality Studies

Among the air pollutants, atmospheric aerosol – also known as particulate matter (PM) – emerges as one of the major challenges to air quality. PM is acknowledged as one of the highest risk factors for human health, causing cardiovascular and respiratory diseases and leading to premature deaths. PM particles are emitted by a huge variety of sources, both of natural and anthropogenic origin. In urban areas, where most of the European population lives, traffic is one of the main sources of PM. Traffic emissions can be divided into:

• Exhaust emissions, from internal combustion engine pipes.
• Non-exhaust emissions (NEE), from the wear and tear of vehicle components such as tyres, brakes, and mechanical parts, as well as from road dust resuspension.

Over the last decades, exhaust emissions have been drastically reduced thanks to stringent emission regulations, such as the use of low sulphur fuels, diesel particulate filters, and catalytic converters. Additionally, the progressive electrification of the EU vehicle fleet is expected to lead a further lowering of exhaust emissions.

On the other hand, the contribution of non-exhaust emissions has been drastically underestimated or ignored. Recent studies have underlined the relevance of this component, which is up to 50% of total traffic-related PM emissions. With the transition to hybrid or electric vehicles, the impact of NEE will be more and more important, becoming substantially the only one to face. Indeed, these vehicles have a higher weight, leading to a more effective road abrasion and tyre wear due to enhanced friction, as well as an increased brake wear. Particles emitted by brake and tyre wear contain a lot of transition metals, which have a high potential of inducing inflammation responses in biological tissues and/or other toxic effects. Metals can be incorporated in the road dust and are easily transferred to air, soil, and water, impacting both humans and the ecosystem. NEE are also a potential source of micro- and nano-plastics, which are highly detrimental for the environment. Therefore, mitigating the environmental and health-related impacts of NEE is an open issue that must be faced towards the switch to a more sustainable mobility.

At the state of the art, one of the biggest challenges is the lack of comprehensive data on the individual components of non-exhaust emissions, such as tyres, brakes, and road dust resuspension. Among them, the resuspended road dust (RD) is by far the least known component. This knowledge gap is mainly due to the lack of dedicated instrumentation and standardised procedures to characterize it. RD are collected using several approaches, going from manual sweeping to on-site resuspension chambers or fully equipped mobile vans. Moreover, the chemical composition of RD is highly variable and depends on the site, meteorology, traffic conditions, fleet, road surface, and many other factors. Consequently, emission inventories for RD are often inaccurate, limiting the effectiveness of air quality modelling and policymaking.

To implement effective emission mitigation strategies, a detailed understanding of PM sources is essential. For this purpose, the investigation starts with high-time resolution measurements of PM in real-world conditions (e.g. on the street). These samples are then analysed to retrieve the chemical composition (e.g. elements, ions, carbonaceous components). However, these studies are particularly complex and challenging because multiple sources can contribute to the particles collected in real-world samples. Suitable models (like e.g., receptor models) are then needed to perform source apportionment studies, which allow to disentangle the different sources and to assess their contribution to each sample. In this way, is then possible to identify the contribution of RD to our samples.

The Environmental Physics research group of the University of Milan (Italy) is specifically targeting the issue related to RD estimates. From the experimental point of view, we developed MORDOR (Measurement Of Resuspended Dust On Road), which is a resuspension chamber designed and realised at the University of Milan with the aim of reaching a robust assessment of the road dust contribution to non-exhaust traffic emissions. To this end, the most suitable setup for the device was identified and a sampling procedure for real-world measurements was developed. In the chamber, road dust from the ground is resuspended in the air, through the use of fans. The resuspended PM is then collected on filters, which can be subsequently analysed for the chemical composition. A series of tests were performed in the laboratory to characterise as much as possible the device in terms of e.g., wall losses, sampling efficiency, and suppression of ambient air particle contributions. MORDOR has been successfully employed on the field in several measurement campaigns, which provided an estimate of the emission factor of resuspended road dust and the chemical fingerprint of the source.  

In addition, the group is also implementing advanced receptor modelling approaches for source apportionment studies. Indeed, traditional receptor models are not able to exploit all the available experimental information on the chemical and physical properties of PM. The group developed cutting-edge modelling approaches able to analyse data collected with different time resolution and in multiple size classes, as well as aerosol optical parameters. These models allow a better identification and quantification of the PM sources, which also include non-exhaust emissions.