Tire-Derived Organic Pollutants across Urban Land Uses: Associations with Traffic Exposure

Tire-derived organic pollutants (TDOPs), particularly N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) and its transformation product 6PPD-quinone (6PPD-Q), are increasingly recognized as emerging urban contaminants. However, their distribution and potential drivers in urban soils beyond the immediate roadside environment remain poorly characterized. In this study, 19 TDOPs were quantified in 28 urban soil samples representing four land-use types in Metro Detroit, Michigan, United States. TDOPs were widely detected in all soil samples, and 16 compounds exhibited detection frequencies ≥75%. Very low 6PPD/6PPD-Q ratios and the relative enrichment of N,N′-diphenyl-p-phenylenediamine (DPPD) over 6PPD suggest substantial environmental transformation of 6PPD. No statistically significant differences in the concentrations of TDOPs among land-use types or associations with measured soil properties were detected. In contrast, 6PPD exhibited a strong positive association with traffic-derived exposure metrics. The maximum Spearman rank correlation coefficient was observed for the commercial traffic exposure metric at a distance-decay parameter of 800 m (ρ = 0.76), and the association remained significant after optimization-adjusted permutation testing (pperm = 0.00030) and false discovery rate correction (q = 0.0048). This study demonstrates the widespread occurrence of TDOPs in urban soils beyond the immediate roadside environment and identifies modeled traffic exposure as an important correlate of soil 6PPD concentrations.