Disentangling Traffic Emissions in Road Airborne Particles across China: 6PPD-Q as a Molecular Marker for Tire Wear...

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.