OECD: Non-exhaust particles to become the leading source of PM_2.5 pollution from road traffic

14 December 2020

Non-exhaust emissions will soon overtake vehicle exhaust as the leading source of fine particles released into the air by road traffic, according to a report on non-exhaust particle emissions released by the Paris-based Organization for Economic Co-operation and Development (OECD) [4987]. Heavy electric vehicles with long-distance batteries could compound the problem, despite the fact they produce no exhaust emissions.

Non-exhaust particle emissions from road traffic consist of airborne PM generated by the wearing down of brakes, clutches, tires and road surfaces, as well as by the suspension of road dust. As particulate matter emitted from exhaust sources decreases with the uptake of electric vehicles, the majority of PM released into the air by road traffic could come from non-exhaust sources as early as 2035, according to the OECD report.

The report finds that the total amount of non-exhaust particulate matter (PM_2.5 and PM₁₀) emitted by passenger vehicles worldwide is likely to rise by 53.5% by 2030. These estimates are based on the assumption that the urban vehicle fleets will continue to grow—the OECD projects the demand for urban passenger travel will more than double by 2050.

While electric vehicles eliminate exhaust emissions, the report shows that they are not likely to provide substantial benefits in terms of non-exhaust emissions reductions. Regenerative braking systems can reduce brake wear, but tire wear, road wear, and road dust resuspension remain significant sources of non-exhaust emissions from electric vehicles. Non-exhaust emissions from these sources can in fact be higher for electric vehicles than for their conventional counterparts, as the heavy batteries in electric vehicles imply that they typically weigh more than similar conventional vehicles. This is particularly the case for electric vehicles with greater autonomy (driving range) that require larger battery packs.

The amount of non-exhaust PM a vehicle emits is determined by many factors, including vehicle weight, driving styles, the material composition of brakes, tires and roads, and the amount of dust on road surfaces. Lightweight electric vehicles with a driving range of about 100 miles (161 km) emit an estimated 11-13% less PM_2.5 than conventional vehicles in the same segment. However, heavier electric vehicles with battery packs enabling a range of 300 miles (483 km) emit an estimated 3-8% more PM_2.5 than equivalent conventional vehicles.

These findings invite a reappraisal of the net social benefits that can be expected from the use of electric vehicles, and suggest that electric vehicles should not be exempted from tolls and congestion charges aimed at reducing road traffic emissions, said the OECD. Instead, road traffic regulations should consider both exhaust and “non-exhaust” emissions from all vehicles and should take into account factors like vehicle weight and tire composition. Policy makers should also favor measures that reduce driving distances, limit urban vehicle access and encourage public transport, walking and cycling.

Non-exhaust PM emissions have been largely ignored by environmental policies. The evidence presented in the report is intended to bring attention to this overlooked environmental policy issue and provide preliminary insights regarding how to address them. The findings also underline the need to establish standardized approaches to measuring non-exhaust particulate matter and to develop a better understanding of how factors like vehicle characteristics influence the amount of PM generated.

Source: OECD