A new article has been published in Energies (MDPI) summarizing the findings of a literature review commissioned by ISPRA – Ministry of the Environment, focusing on non-exhaust particulate matter from passenger cars. This refers to particulate emissions not from the tailpipe, but from brake and tire wear and the resuspension of particles caused by vehicle movement on road surfaces.
Non-Exhaust Particulate Emissions from Road Transport Vehicles
The article, written by researchers from CNR-STEMS in collaboration with the Sustainable Mobility Team at Innovhub SSI, lays the groundwork for ongoing testing activities within the framework of a service agreement with ISPRA. These tests aim to determine emission factors for this type of particulate matter, which is still scarcely documented in the scientific literature. The Sustainable Mobility Team coordinates the joint efforts of CNR-STEMS and Innovhub SSI, with the goal of providing ISPRA – Ministry of the Environment with updated data on emission factors for pollutants and GHGs from vehicles. These data are essential for the periodic update of Italy's National Emission Inventory and for verifying the country’s compliance with international commitments under the UN Framework Convention on Climate Change (UNFCCC) and the UNECE Convention on Long-Range Transboundary Air Pollution (UNECE-CLRTAP).
As part of the Zero Pollution Action Plan of the Green Deal, the European Commission has set the goal of reducing premature deaths caused by particulate matter (PM2.5) emissions by at least 55% by 2030, compared to 2005 levels. To achieve this goal, the Commission plans to introduce stricter limits on emissions and ambient concentrations. In urban areas, road vehicles are a significant source of particulate emissions. These originate from engine exhaust, brake and tire wear, road surface degradation, and resuspension of road dust.
Particulate formation due to tire wearOver recent decades, the enforcement of stricter tailpipe emission limits has driven the adoption of technologies that significantly reduce exhaust particulate emissions. In the future, the rise of electric mobility is expected to bring these emissions close to zero. However, non-exhaust emissions have increased in recent years due to the proliferation of Sport Utility Vehicles (SUVs)—which have grown about tenfold globally—and electric vehicles, which tend to be heavier than their internal combustion counterparts. This trend has resulted in a more modest reduction in PM10 and PM2.5 emissions from the transport sector (limited to −49% and −55%, respectively, between 1990 and 2020) compared to other pollutants emitted solely from exhaust. The aim of this article is to provide an updated overview of non-exhaust particulate characterization, incorporating insights from recent scientific literature to address this critical environmental and public health challenge.
Non-exhaust particulate matter accounts for 66% of the coarse fraction (particles between 2.5 and 10 μm) and approximately 29% of the fine fraction (particles between 0.2 and 2.5 μm) of particulate concentrations in urban areas. These emissions are not linked to specific vehicle technologies or fuel types and significantly contribute to airborne particulate pollution. Currently, emission standards do not regulate non-exhaust particulate emissions from vehicles already in use. Although the forthcoming Euro 7 standards will introduce limits for brake and tire wear emissions, the reference methodology for tire wear is still being defined, and the methodology for brake emissions is currently limited to laboratory testing. In the coming years, new methods more representative of real-world conditions are expected to be developed, taking into account variables such as brake and tire types and road surface composition. These parameters, combined with the variability in test environments (laboratory vs. road) and in measurement and sampling systems, contribute to uncertainty in the results.
Example of resuspended particulate sampling methodologyThe article provides an overview of non-exhaust particulate characterization for each emission source—brakes, tires, and resuspension—describing current sampling methodologies and measured emission factors:
Emissions from brake wear are strongly influenced by brake pad type and braking conditions, including test type, vehicle weight, and sampling method;
Emissions from tire wear are often related to tire mass loss. However, this mass loss does not directly correspond to atmospheric emissions. Resuspended particles affect on-road tire emission measurements, while laboratory tests are limited by their inability to fully replicate real-world tire wear conditions;
Emission factors for resuspended road dust are less dependent on measurement methods but are significantly influenced by weather conditions, traffic levels, and road maintenance and cleaning practices.
Measuring non-exhaust particulate remains a current scientific challenge due to the difficulties in sampling within dynamic, contamination-prone environments. Both regulatory bodies and manufacturers require specific testing to identify particulate emissions from tires and brakes. Therefore, in the near future, standardized methods in controlled environments will be adopted to provide unambiguous measurements. However, due to the need to perform tests directly on vehicles to obtain emission values that reflect real-world conditions, more complex measurement systems will also be considered—isolating brakes and tires without altering test integrity.
The article is available in open access in Energies 2024, 17(16), 4079.
Authors
Maria Antonietta Costagliola, Luca Marchitto, Rocco Giuzio – Institute of Science and Technologies for Sustainable Energy and Mobility (STEMS-CNR)
Simone Casadei, Tommaso Rossi, Simone Lixi, Davide Faedo – Innovhub—Stazioni Sperimentali per l’Industria S.r.l, Sustainable Mobility Team