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Disentangling Traffic Emissions in Road Airborne Particles across China: 6PPD-Q as a Molecular Marker for Tire Wear Particles
Original title: DisentanglingTrafficEmissions in Road Airborne Particlesacross China: 6PPD‑Q as a Molecular Marker for Tire Wear Particles
Summary
Scientists studying air near roads in 16 Chinese cities found a chemical marker (called 6PPD-Q) that can reliably track tire wear particles — tiny bits of rubber and chemicals that flake off tires as you drive. The surprising finding: exhaust from tailpipes only accounts for about a quarter of traffic-related air pollution, while tire wear, brake dust, and road dust make up the rest — meaning that even as we shift to electric cars, this "nonexhaust" pollution (which can include microplastics) will still need attention to protect air quality and health.
Tire wear particles (TWPs) are a major source of nonexhaust emissions, yet their distinction from other traffic-related particles remains challenging. This study applied a molecular marker-based approach to identify and quantify TWPs in road airborne particles. We analyzed p-phenylenediamines (PPDs) in car air filters from 16 Chinese megacities, cross-validated them with rubber-derived TWPs, and quantified TWPs’ contributions to traffic-related sources. Seventeen of the twenty-three PPDs (Σ17PPDs) were detected (58.9–5000 ng/g), with N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine quinone (6PPD-Q) and 4-nitrodiphenylamine contributing 36 ± 11% and 20 ± 7%, respectively, indicating the predominance of oxidation products. Rubber-derived TWPs were detected in all samples (1.52–36.4 mg/g), showing strong correlations with Σ17PPDs (r = 0.595, p < 0.001) and similar spatial patterns, supporting their coemission from tire wear. 6PPD-Q was identified as a molecular marker for airborne TWPs based on its high detection frequency, statistically significant correlation with rubber-derived TWPs, and relatively high environmental stability. Source apportionment indicated that resuspended road dust, brake wear, TWPs, and vehicular exhaust made comparable contributions of roughly one-quarter each, with nonexhaust sources collectively exceeding 75% of traffic-related emissions. These findings highlight the dominant role of nonexhaust sources in near-road urban environments and the need for their inclusion in future air quality management strategies.