Non-exhaust particles such as ground up tyres, road and brake dusts are likely more numerous and more toxic than diesel emissions

https://uk-air.defra.gov.uk/assets/documents/reports/cat09/1907101151_20190709_Non_Exhaust_Emissions_typeset_Final.pdf

“Consequently, on the basis of current emissions inventory
estimates and toxicity evidence, non-exhaust particles from the UK road traffic fleet should be
considered as potentially having a greater public health impact than the exhaust particles.”

“Regenerative braking does not rely on frictional wear of brake materials so vehicles using
regenerative braking totally or partially, for example electric vehicles, should have lower brake
wear emissions. However, tyre and road wear emissions increase with vehicle mass, which
has implications for any vehicle with a powertrain that is heavier (for example due to additional
battery and hardware mass) than the equivalent internal-combustion-engine vehicle it
replaces.”

 

Executive Summary
Non-exhaust emissions (NEE) from road traffic refers to particles released into the air from
brake wear, tyre wear, road surface wear and resuspension of road dust during on-road
vehicle usage. These emissions arise regardless of the type of vehicle and its mode of power,
and contribute to the total ambient particulate matter burden associated with human ill-heath
and premature mortality. No legislation is currently in place specifically to limit or reduce NEE
particles, so whilst legislation has been effective at driving down emissions of particles from
the exhausts of internal-combustion-engine vehicles, the NEE proportion of road traffic
emissions has increased. Data from the UK National Atmospheric Emissions Inventory
indicate that particles from brake wear, tyre wear and road surface wear currently constitute
60% and 73% (by mass), respectively, of primary PM2.5 and PM10 emissions from road
transport, and will become more dominant in the future. Currently they contribute 7.4% and
8.5% of all UK primary PM2.5 and PM10 emissions. Therefore to achieve further gains in PM2.5
and PM10 air quality in relation to road transport sources requires attention to reducing nonexhaust emissions, not solely a focus on lowering exhaust emissions.
The magnitudes of non-exhaust emissions are, however, highly uncertain, particularly when
compared to data for exhaust emissions. Emissions vary widely according to brake, tyre and
road-surface material, and with driving style. The NEE emission factors used in inventories
have a wide span of uncertainty – greater than a factor of two is typical – including uncertainty
in splits between PM10 and PM2.5 size fractions. The emission factors are also largely based
on data from the 1990s and have not changed as vehicle designs and fleet composition have
changed, in contrast to the regularly updated factors used for exhaust emissions.
The available data indicate that brake, tyre and road-surface wear contribute approximately
equally to UK sources of NEE, and are predominantly derived from cars because of the much
greater vehicle-km travelled for this class of vehicle. NEE particles are also an important
source of metals to the atmosphere; the national inventory estimates NEE contributions of
47% and 21% for Cu and Zn, primarily associated with brake and tyre wear, respectively. The
national inventory does not include estimates of road dust resuspension.
NEE are especially important in urban environments. The national inventory indicates that half
of NEE occurs on urban roads, owing to the greater braking per km than on non-urban roads.
Emissions may also be high in areas such as trunk-road exits. Tyre-wear emissions are
estimated to be greatest on high-traffic trunk roads and motorways (both urban and rural).
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Considerable measurement evidence shows NEE increase concentrations of PM10 and PM2.5
and some metals at roadside although precise quantification of the NEE contribution is difficult.
Data from London Marylebone Road indicate an NEE contribution (including resuspension) of
4-5 g m-3
to the roadside increment in PM, mostly in the coarse particle fraction (PM10-2.5).
Other studies, including dispersion modelling, also indicate total NEE contributions, including
resuspension, of up to several g m-3 of PM10 at busy roadsides, and in the region 1-2 g m-3
for urban background in central London.
The most effective mitigation strategies for NEE are to reduce the overall volume of traffic,
lower the speed where traffic is free-flowing (e.g. trunk roads and motorways), and promote
driving behaviour that reduces braking and higher-speed cornering. Resuspension of particles
from the road surface can be lowered by reducing the material that is tracked onto public road
surfaces by vehicle movements in and out of construction, waste-management and similar
sites; and potentially by road sweeping, street washing and application of dust suppressants
to street surfaces, although the impacts on airborne PM from trials of these latter approaches
have so far proven inconsistent and any benefits have been short-lived.
Regenerative braking does not rely on frictional wear of brake materials so vehicles using
regenerative braking totally or partially, for example electric vehicles, should have lower brake
wear emissions. However, tyre and road wear emissions increase with vehicle mass, which
has implications for any vehicle with a powertrain that is heavier (for example due to additional
battery and hardware mass) than the equivalent internal-combustion-engine vehicle it
replaces. The net balance between reductions in brake wear emissions and potential
increases in tyre and road wear emissions and resuspension for vehicles with regenerative
braking remains unquantified, and will depend upon road type and driving mode, as both
influence the balance between the different sources of emissions. In locations where brake
wear makes a major contribution to overall NEE, it seems likely that there will be a net benefit,
but this has yet to be demonstrated. Other as yet unproven technological mitigation methods
include trapping brake wear particles prior to emission, and mandating formulation of lowwear/low-emission tyres, brake pads and road surfaces.
AQEG recommends as an immediate priority that NEE are recognised as a source of ambient
concentrations of airborne PM, even for vehicles with zero exhaust emissions of particles.
A further priority is to work towards a consistent approach internationally for measurement of
NEE and to update and narrow the uncertainties in their emission factors. Such a programme
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of work could form the basis for subsequently including criteria on brake and tyre wear
emissions in future type approvals and regulations governing formulation.
AQEG also recommends that further studies be conducted to quantify the efficacy of technical
solutions on NEE reductions; in particular, to understand gains from use of regenerative
braking versus potential increased tyre and road wear due to additional mass of vehicles
incorporating such braking.