Regulatory regimes seeking to reduce emissions from transport have largely focused on tailpipe emissions—i.e., the criteria pollutants and CO2 that emerge with the exhaust from the tailpipe. However, there is more than 15 years of research showing that the contribution of non-exhaust primary particles to the total traffic generated primary particles is significant in urban areas. Non-exhaust PM factors include tire wear, brake wear, road surface wear and resuspension of road dust. Further, a 2013 review by Denier van der Gon et al., 2013 found that the ratio of non-exhaust to exhaust particles is strongly increasing in the last two decades, due to exhaust emission reductions.
While battery electric vehicles have the obvious advantage of zero tail-pipe emissions, they are not equally advantaged when it comes to non-exhaust emissions. Accordingly, there have been a number of recent studies working to assess the impact of non-exhaust emissions from EVs and suggesting a regulatory or policy response (e.g., earlier post).
Tires generate particles both through the wear of the rubber and through the wear of road surfaces. These processes may depend on tire type, size, and age, vehicle speed and weight, road surface properties, and meteorological conditions (temperature, road wetness, etc.). Tire wear contributes to PM10 even though most of the wear results in larger particles.
Brake wear is due to large frictional heat generation by brake linings. Detailed laboratory tests have shown that 50% of the total wear is emitted as airborne material; the other half directly deposits on the (road) surface and the wheel of the car.
Wear of the road surface varies significantly based on the properties of the asphalt as well as tire type, vehicle type, and speed, as well as road surface conditions. Key properties of the pavement are the
Road wear—pavement-derived PM10—mainly consists of small mineral fragments and therefore is dominated by crustal elements like Si, Ca, K, Fe, and Al. The composition therefore differs depending on the rock material used.
There is a clear lack of data in the field of road transport wear and suspension emissions to conclusively assess its importance for air quality and the impact on human health. It is uncertain how the problem of “non-exhaust” relates to “exhaust” in a relative sense, in terms of both emissions and air quality and in terms of human health. Although generalized mass fractions are available for non-exhaust and exhaust road transport PM emissions, the regional, spatial, and temporal patterns of the former are least known, while the role of suspension versus primary wear emissions is even less characterized.
As the trend toward cleaner technologies with reduced exhaust emissions continues through the use of catalytic converters, diesel particulate filters (DPF), and improved fuels and engines, non-exhaust PM will soon surpass exhaust emissions and may well become dominant by 2020 both in terms of emissions and contributions to air quality. The majority of the experts participating in the workshop (55%) ranked the importance of wear and suspension emission compared to exhaust emissions in the future (2020 and beyond) as dominating in terms of PM mass.
Participants agreed that we need to improve our knowledge about emission, exposure, and health effect of wear particles since this fraction of PM cannot be neglected and its relative and absolute importance is still increasing. It is important to stress to regulators and policymakers that road transport emissions continue to be an issue for health and air quality, despite the encouraging rapid decrease of tailpipe exhaust emissions.—van der Gon et al.
A 2014 study by Weinbruch et al. in Germany quantified the contribution of the three traffic-related components of exhaust, abrasion, and resuspension to curbside and urban background PM10 and PM1 levels based on the analysis of individual particles by scanning electron microscopy.
They collected 160 samples on 38 days between February and September 2009 at a curbside and an urban background station in the urban/industrial Ruhr area. They then classified 111,003 particles studied in detail based on size, morphology, chemical composition and stability 14 particle classes: traffic/exhaust; traffic/abrasion; traffic/resuspension; carbonaceous/organic; industry/metallurgy; industry/power plants; secondary particles; (aged) sea salt; silicates; Ca sulfates; carbonates; Fe oxides/hydroxides; biological particles; and other particles.
The traffic/exhaust component consisted predominantly of externally mixed soot particles and soot internally mixed with secondary particles. The traffic/abrasion component contained all particles with characteristic tracer elements (Fe, Cu, Ba, Sb, Zn) for brake and tire abrasion. The traffic/resuspension component was defined by the mixing state and comprises all internally mixed particles with a high proportion of silicates or Fe oxides/hydroxides which contain soot or abrasion particles as minor constituent.
In addition, silicates and Fe oxides/hydroxides internally mixed with chlorine and sulfur-containing particles were also assigned to the traffic/resuspension component.
The total contribution of traffic to PM10 was found to be 27% at the urban background station and 48% at the curbside station;the corresponding values for PM1 were 15% and 39%.
The relative share of the different traffic components for PM10 at the curbside station was 27% exhaust, 15% abrasion, and 58% resuspension (38%, 8%, 54% for PM1).
For the urban background, the following relative shares were obtained forPM10: 22% exhaust, 22% abrasion and 56% resuspension (40%, 27%, 33% for PM1).
Compared to earlier studies, Weinbruch et al.observed a significantly lower portion of exhaust particles and a significantly higher portion of resuspension particles. The high abundance of resuspension particles underlines their significance for the observed adverse health effects of traffic emissions and for mitigation measures.
The role of EVs. As reported earlier here (earlier post) a recent literature review by a team from the University of Edinburgh (Timmers et al.) concluded that, when factoring in the additional weight and non-exhaust PM factors, total PM10 emissions from electric vehicles (EVs) are equal to those of modern internal combustion engine vehicles (ICEVs); for PM2.5 emissions, EVs deliver only a negligible reduction in emissions.
A new Rotterdam-specific study by a pan-European team (Tobollik et al.) used Health Impact Assessment (HIA) to quantify co-benefits of GHG mitigation policies in Rotterdam. The effects of two separate interventions (10% reduction of private vehicle kilometers and a share of 50% electric-powered private vehicle kilometers) on particulate matter (PM2.5), elemental carbon (EC) and noise (engine noise and tire noise) were assessed using Years of Life Lost (YLL) and Years Lived with Disability (YLD).
The baseline was 2010 and the end of the assessment 2020. The team found that both interventions were associated with a decreased exposure to noise, with the effects of 50% electric-powered car being slightly higher. However, they found that the two interventions had marginal effects on air pollution, because already implemented traffic policies will reduce PM2.5 and EC by around 40% and 60% respectively, from 2010 to 2020.
Another new approach to the problem comes from the team of Hooftman et al. in Belgium. They reported in an open-access paper in the journal energies on their analysis of non-exhaust emissions of passenger vehicles, both conventional (diesel and gasoline) or electric, on air quality levels in an urban environment in Belgium.
Hooftman et al. took a broader view—they proposed a method to compare the air quality impact of different vehicle technologies on a larger scale than solely based on regulated emission, addressing the most important un-regulated pollutants as well as non-exhaust emissions.
Specifically, they proposed a method to assess the contribution of EVs to urban air quality in Belgium, compared to conventional vehicles of the same weight class; they modeled the effect on human toxicity (HT), photochemical ozone formation (POF) and particulate matter formation (PMF). In addition, they simulated disability adjusted life years (DALY) to assess the healthy years lost due to poor urban air quality. The scope of the study was thus not limited to the use-phase of a vehicle.Among their main conclusions were:
Unregulated non-exhaust emissions have significant and dominant impacts throughout the analyzed categories.
EVs were the best alternative to diesel and gasoline vehicles across all categories. EVs have no exhaust emissions and reduced non-exhaust pollutants as well as low electricity generation associated emissions. Hooftman et al.concluded that EVs tend to emit up to eight times less non-exhaust PM than diesel vehicles and at least two times less than gasoline powertrains—findings at opposition with some of the earlier studies on the topic.
Non-exhaust emissions require active regulation. Either this is achieved by using alternative materials during production of both tires, brakes and pavements, or by introducing alternative technologies such as regenerative braking in ICEs to reduce braking wear. Tires should be subject to technological pushes in order to mitigate wear and tire composition.
Policy makers should enforce further stringent regulations in the transportation sector regarding emissions as well as promote the usage of alternative means of passenger transport. Such a change would highlight the benefits, both environmental, economic and social of these alternative means (such as human powered and electric two-wheelers).
Denier van der Gon, H.A.C., Gerlofs-Nijland, M.E., Gehring, R., Gustafsson, M., Janssen, N., Harrison, R.M., Hulskotte, J., Johansson, C., Jozwicka, M., Keuken, M., Krijgsheld, K., Ntziachristos, L., Riediker, M., Cassee, F.R. (2013) “The policy relevance of wear emissions from road transport, now and in the future e an international workshop report and consensus statement.” J. Air Waste Manag. doi: 10.1080/10962247.2012.741055
Stephan Weinbruch, Annette Worringen, Martin Ebert, Dirk Scheuvens, Konrad Kandler, Ulrich Pfeffer, Peter Bruckmann (2014) “A quantitative estimation of the exhaust, abrasion and resuspension components of particulate traffic emissions using electron microscopy,” Atmospheric Environment, Volume 99, Pages 175-182 doi: 10.1016/j.atmosenv.2014.09.075
Victor R.J.H. Timmers, Peter A.J. Achten (2016) “Non-exhaust PM emissions from electric vehicles,” Atmospheric Environment, Volume 134, Pages 10-17, doi: 10.1016/j.atmosenv.2016.03.017
Myriam Tobollik, Menno Keuken, Clive Sabel, Hilary Cowie, Jouni Tuomisto, Denis Sarigiannis, Nino Künzli, Laura Perez, Pierpaolo Mudu (2016) “Health impact assessment of transport policies in Rotterdam: Decrease of total traffic and increase of electric car use,” Environmental Research, Volume 146, doi: 10.1016/j.envres.2016.01.014
Nils Hooftman, Luis Oliveira, Maarten Messagie, Thierry Coosemans and Joeri Van Mierlo (2016) “Environmental Analysis of Petrol, Diesel and Electric Passenger Cars in a Belgian Urban Setting,” energies, Vol. 9, No. 2 doi: 10.3390/en9020084