Particle-based quantification of tire and road wear particles in roadside agricultural soils

The widespread emissions of tire and road wear particles (TRWP) as particulate pollutants, originated from tire abrasion on roads, represent a potential threat to the soil ecosystems. TRWP can accumulate in roadside soils over time and are suspected to have hazardous effects through released toxic additives and metals, demonstrating the urgent need for implementable analytical methods allowing the quantification of TRWP in soils. Up to now, there is a lack of reliable “real-world” TRWP data in roadside agricultural soils. Furthermore, previous research on TRWP in soils reported mass-based concentration, which does not include necessary information on TRWP shape and size distribution. We have determined the TRWP concentrations in 75 topsoil samples from agricultural fields adjacent to roads (1 and 5 m distance) in the Rhine-Main metropolitan area (Hesse, Germany). The sampling locations are close to federal highways, state roads and country roads covering a wide range of daily traffic volumes. TRWP were extracted from soil matrix via combined density separation method that uses high- (NaI, ρ = 1.8 g cm^-3) and low (NaCl, ρ = 1.1 g cm^-3) density solutions as well as subsequent sample purification via Urea/Thiourea treatment and Fenton reaction. The particle-based identification of TRWP was based on their characteristic black color, accessed by optical microscopy and subsequent image analysis using a machine learning approach. This method allows full TRWP quantification and single particle characterization including systematic information on particle size and morphology. The particle-based data can be further used to perform TRWP mass-estimations using 3D particle data derived from microscopy z-stacking and assumed particle densities. Our method shows a mean recovery of 85% with a detection limit of 30 µm and no blank contamination. So far, our preliminary results show higher TRWP concentrations in locations closer to the road and decreasing concentrations with increasing distances from the road. We detected TRWP concentrations (particles per kg) exhibiting mean values of 90,000 p kg^-1 for 1 m distance and 1,000 p kg^-1 for 5 m distance. Furthermore, estimated TRWP masses for both distances show mean values of 100 mg kg^-1 & 0.2 mg kg^-1, respectively. At this stage, we can conclude that there is no dilution of TRWP quantities by agricultural tillage practices and TRWP concentrations are within comparable range to roadside soils.