Beyond Substituted p -Phenylenediamine Antioxidants: Prevalence of Their Quinone Derivatives in PM 2.5 .

Substituted para -phenylenediamine (PPD) antioxidants have been extensively used to retard oxidative degradation of tire rubber and were found to pervade multiple environmental compartments. However, there is a paucity of research on the environmental occurrences of their transformation products. In this study, we revealed the co-occurrence of six PPD-derived quinones (PPD-Qs) along with eight PPDs in fine particulate matter (PM 2.5 ) from two Chinese megacities, in which N , N '-bis(1,4-dimethylpentyl)- p -phenylenediamine quinone (77PD-Q) was identified and quantified for the first time. Prevalent occurrences of these emerging PPD-Qs were found in Taiyuan (5.59-8480 pg/m 3 ) and Guangzhou (3.61-4490 pg/m 3 ). Significantly higher levels of PPDs/PPD-Qs were observed at a roadside site, implying the possible contribution of vehicle emissions. Correlation analysis implied potential consistencies in the fate of these PPD-Qs and suggested that most of them were originated from the transformation of their parent PPDs. For different subpopulation groups under different exposure scenarios, the estimated daily intakes of PPD-Qs (0.16-1.25 ng kg bw -1 day -1 ) were comparable to those of their parent PPDs (0.19-1.41 ng kg bw -1 day -1 ), suggesting an important but overlooked exposure caused by novel PPD-Qs. Given the prolonged exposure of these antioxidants and their quinone derivatives to traffic-relevant occupations, further investigations on their toxicological and epidemiological effects are necessary.