National-scale emissions, multimedia fate, and future projections of tire-derived 6PPD in China

The tire antioxidant N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) has emerged as a contaminant of increasing environmental concern because of its widespread use in vehicle tires and the ecological risks associated with its transformation products. However, national-scale assessments linking 6PPD emissions, environmental distribution, and multimedia fate remain largely unavailable for China despite its extensive road network and rapidly growing vehicle fleet. Here, we developed an emission-factor-based framework to quantify 6PPD emissions from vehicle tire wear across 333 prefecture-level cities in China from 2000 to 2020 and coupled these emissions with a multimedia fate model covering 58 secondary river basins. National emissions increased from 0.0064 Mt in 2000 to 0.045 Mt in 2020, with rural roads contributing the largest share of 56.5%. Soil was identified as the dominant receiving compartment, accounting for 57.0% of total environmental loading. Modeled river concentrations agreed well with field observations in major river systems. Multimedia fate analysis revealed that atmospheric degradation and water-air exchange dominated environmental transport, whereas direct atmospheric deposition contributed negligibly. Future projections indicate that annual 6PPD emissions may reach 0.15 Mt by 2060 under the Shared Socioeconomic Pathway 2 scenario. This study provides the first national source-to-receptor assessment of 6PPD in China, offering a quantitative basis for understanding the environmental fate of tire-derived contaminants and supporting future management of 6PPD.