Deposition rates and air concentrations of tire and road wear particles near a motorway in Germany

Tire and road wear particles (TRWP) are an important component of non-exhaust emissions from road traffic. They are internal mixtures of tire material, road wear, brake wear, and soil minerals. In contrast to tire material analysed by bulk analytical techniques, there is little literature on the air concentrations and deposition rates of TRWP. The main purpose of our study was to determine deposition rates and air concentrations of TRWP at a motorway in Germany. TRWP were collected with passive samplers and analysed by scanning electron microscopy. Number deposition rates varied between approximately 100 particles cm⁻² day⁻¹ at 3.5 m distance and 30 particles cm⁻² day⁻¹ at 30.5 m distance from the road, and mass deposition rates between 7.8 mg m⁻² day⁻¹ and 2.8 mg m⁻² day⁻¹, respectively. Air concentrations of TRWP calculated from the mass deposition rates varied between approximately 3.8 μg/m³ (3.5 m distance) and 0.95 μg/m³ (30.5 m distance). The influence of potential particle misclassification, density of TRWP, and volume approximation on the deposition rates and air concentrations is small. Air concentrations of TRWP observed in this study are reasonably consistent with the results of previous electron microscopy studies, but are significantly higher than the values used in risk assessment. This discrepancy clearly shows that further exposure measurements are required. The input of organic additives such as benzothiazole and N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) as well as carbon, sulfur and zinc through deposition in road soils should be taken into account when assessing the environmental impact of TRWP.