This paper presents a set of targeted tyre emissions studies carried out within the UK Department for Transport’s (DfT) Brake and Tyre Emissions programme. The work was aimed at improving the measurement of airborne particles generated by tyres, and at examining the factors that influence particle number and particle mass emission. It also explored physical tread wear.
To achieve this, a revised sampling duct system was developed with high extraction flow and partial wrap-around of the tyre, and a coarse hard-wearing surface was applied to the chassis dyno roller. The sampling system supplied Total PN4, PN10 (volatile and non-volatile), PM2.5, and particle size instrumentation.
Several tyre types were selected to represent a broad range of sizes, constructions, manufacturers, compounds, and mileages. Tests were performed on a dedicated chassis dynamometer testing facility using PG42, WLTC, and RDE based cycles, together with additional cycles designed to investigate the influence of temperature, speed, and braking. Tread depth and tyre mass were recorded before and after the test programme to determine wear rates, and macro particle sampling was undertaken to assess particle size distribution beyond the airborne PM2.5 range.
Airborne particle measurements showed that tyre PN is dominated by volatile ultrafine particles below 10 nm, with the non-volatile PN10 fraction representing only a small proportion of the total. PM2.5 mass from tyres was generally low and often near the detection limit, with most of the physical wear mass present as large particles (>50 μm) rather than as respirable material. Wear rates varied across tyres but showed no consistent relationship with airborne PM2.5 or PN10. Tyre temperature had the clearest influence on airborne PN emissions: elevated temperatures and high speed/braking conditions produced higher volatile and non-volatile PN.
Overall, the study provides improved understanding of tyre-related airborne particle formation, the limitations of PM2.5 and PN10 as regulatory indicators, and the role of tyre temperature and operating conditions in determining emissions. The findings support the development of future tyre wear measurement methods and associated regulatory frameworks.