Tracks of travel: unveiling tire particle concentrations in Swiss cantonal road soils

Summary

Tire wear particles (TWP) originating from tire abrasion on roads are a major source of microplastics to the environment. Together with associated pollutants like polycyclic aromatic hydrocarbons and trace metals, TWP are emitted to roadside soils in the immediate vicinity of road networks. Our study aimed at quantifying TWP number and mass concentrations and investigating particle features in low-traffic roadside soils using a novel particle-based analytical approach. On the example of fifteen Swiss cantonal roadside soils, with average daily traffic volumes of 2,290 vehicles per day− 1, we sampled composite samples from distances of 1, 2, 5 and 10 m to the roadside. TWP were extracted via density separation and wet-chemical sample purification. TWP analysis was performed using microscope images and trainable Weka segmentation image analysis. Furthermore, associated road pollutants like polycyclic aromatic hydrocarbons, benzop[a]pyrene and trace metals were analysed using TQ GC-MS/MS and ICP-MS. We found average concentrations of 111,000 TWP per kg soil dry weight (TWP kg− 1) highest values reaching 615,000 TWP kg− 1 and mean TWP masses of 52.7 ± 83.2 mg TWP kg− 1. TWP had a minimal Feret diameter of 62.8 ± 45.6 μm on average and showed mean circularity values of 0.7 ± 0.2, resulting in elliptic particle morphology. TWP concentrations and sizes decreased with increasing distance from the road. Positive relationships were found between TWP numbers and polycyclic aromatic hydrocarbons, benzop[a]pyrene and zinc concentrations in roadside soils. However, a moderate relationship to speed limits was identified. We were able to demonstrate that even in low-traffic areas, roadside soils act as an environmental sink for high concentrations of TWPs and associated pollutants and that spatial distribution and the spread of TWP to soils strongly dependent on the distance to the road.