Non-exhaust particulate matter from road traffic

Background
Our group has been at the forefront of work internationally to characterise and quantify non-exhaust emissions from road traffic1. This has involved three separate and essentially complementary approaches to identification of particles in the atmosphere, and has been underpinned by laboratory studies of tyre wear2 and brake wear3, and by the chemical characterisation of road surface dusts. We propose to address this topic, and also that of fleet electrification.

In summary, the three approaches comprise the following:

(a) The use of chemical tracers in conjunction with size-fractionated sampling of airborne particulate matter collected simultaneously at roadside and nearby urban background (so as to determine the traffic increment by difference) and the application of appropriate factors to estimate total particle mass associated with brake wear, tyre wear and resuspended particles4. Road surface wear, which is often considered as a separate source of particles is not identified separately as road wear particles are typically embedded within tyre wear particles and not present as an external mixture. This method has been deployed successfully on a busy London roadside.

(b) Measurement of particle size distributions with fast-response across a wide range from 15 nm to 10 µm diameter followed by application of Positive Matrix Factorisation (PMF). Deployment of this technique at Marylebone Road, London, allowed the identification of four traffic-related particle classes and five particle classes present in the background London atmosphere5. The size distributions and associations with other pollutants allowed identification of the particle sources including specific non-exhaust particles. PMF can also be applied to multi-element particle chemical composition data, often revealing one or more non-exhaust source factors6.

(c) The application of Aerosol Time-of-Flight Mass Spectrometry. Brake wear and tyre wear particles have been generated in laboratory experiments and used to “calibrate” an Aerosol Time-of-Flight Mass Spectrometer (ATOFMS) for their measurement2,3. Particles of an appropriate size and chemical profile have been measured in the atmosphere but quantification of this technique requires optimisation.

These techniques are most readily applicable to fleet-integrated emissions of particulate matter. This is because individual brake pads and tyres can differ very substantially between manufacturers and performance specifications, and hence application of a chemical composition or size distribution based on a limited set of measurements can be highly misleading. However, sampling from a full vehicle fleet on a busy highway averages across compositional and size differences to yield composite data7.

Objectives of Research
(1) To use field measurements at a range of locations to quantify emissions of brake wear, tyre wear and resuspension particles from road traffic.

(2) To estimate average emission factors for light duty and heavy duty vehicles.

(3) To compare emission factor estimates with those used in the NAEI.

We have established collaborations with leaders in this field in the United States (Jamie Schauer, Costas Sioutas, Phil Hopke) and Europe (Xavier Querol, Fulvio Amato) and joint work will be undertaken with one or more of these groups with a view to comparing techniques.