Composition and evolution of tire antioxidants and their quinone derivatives in summer PM2.5 of a megacity under climate warming

Substituted p-phenylenediamines (PPDs) are widely employed as antioxidant additives in rubber products, particularly in tire manufacturing, to delay oxidative degradation. However, increasing attention has been drawn to their transformation products—quinone derivatives of PPDs (PPD-Qs)—which have been detected in diverse environmental matrices, including urban runoff, road dust, snow, and atmospheric particles. PPD-Qs, especially 6PPD-quinone (6PPD-Q), exhibit significant toxicity for aquatic organisms and are now recognized as emerging contaminants of concern. Despite this growing awareness, research on the occurrence, evolution, and atmospheric transformation of PPDs and PPD-Qs remains limited, particularly under real-world meteorological conditions such as elevated temperature and oxidant levels. Substituted p-phenylenediamines (PPDs) are widely employed as antioxidant additives in rubber products, particularly in tire manufacturing, to delay oxidative degradation. However, increasing attention has been drawn to their transformation products—quinone derivatives of PPDs (PPD-Qs)—which have been detected in diverse environmental matrices, including urban runoff, road dust, snow, and atmospheric particles. PPD-Qs, especially 6PPD-quinone (6PPD-Q), exhibit significant toxicity for aquatic organisms and are now recognized as emerging contaminants of concern. Despite this growing awareness, research on the occurrence, evolution, and atmospheric transformation of PPDs and PPD-Qs remains limited, particularly under real-world meteorological conditions such as elevated temperature and oxidant levels. In this study, we systematically investigated the occurrence, chemical composition, transformation behaviour, and influencing factors of PPDs and PPD-Qs in fine particulate matter (PM2.5) during the summer season (June–August) from 2018 to 2024 in urban Shanghai, which is a representative megacity in eastern China. Via the use of ultrahigh-performance liquid chromatography coupled with high-resolution mass spectrometry (UHPLC-HRMS), we identified 25 PPD-related compounds in total. Among these, 7 parent PPDs and 6 corresponding PPD-Qs were selected for quantitative analysis. The concentrations of individual PPDs and PPD-Qs ranged from 45.7 to 1580 pg/m3 and 83.9 to 1130 pg/m3, respectively, indicating their widespread occurrence in urban atmospheric particles. Both the total concentration of PPDs (ΣPPDs) and that of PPD-Qs (ΣPPD-Qs) exhibited significant interannual increases over the study period. This trend was highly associated with the increasing frequency and intensity of summer heatwave events observed in recent years. Notably, the concentration ratio of 6PPD-Q to its parent compound 6PPD—used as an indicator of transformation degree—increased notably in years with more intense heatwaves, highlighting the temperature-sensitivity of oxidative pathways. Further analysis revealed that the ambient temperature and ozone (O3) concentration play crucial roles in shaping the atmospheric behaviour of PPDs. Temperature acts through two pathways: (1) a direct nonphotochemical mechanism that promotes the release of PPDs from surfaces such as road dust or tires, and (2) an indirect mechanism that enhances O3 formation, in turn accelerating the oxidation of PPDs into PPD-Qs. Moreover, O3 contributes to transformation via classical photochemical oxidation reactions, particularly under intense solar radiation. This dual mechanism underscores the complex interplay between meteorological conditions and atmospheric chemical processes affecting PPD fate. To further evaluate potential human exposure, we estimated the daily intake (DI) of PPDs and PPD-Qs via inhalation for both adults and children. From 2018 to 2024, the total DI of PPDs and PPD-Qs for both adults and children showed a clear year-by-year increasing trend. Under the median exposure scenario, the annual DI values from 2019 to 2024 were approximately 1.14, 1.50, 1.85, 2.37, and 2.98 times higher than that in 2018, respectively. These findings indicate that climate-related intensification of oxidative conditions is not only altering the chemical landscape of PM2.5 but is also driving an upward trend in human exposure to previously overlooked toxicants. Our findings provide critical insights into the environmental behaviour of tire-derived PPDs and their transformation products under real-world atmospheric conditions. The persistent and increasing levels of PPD-Qs in PM2.5, particularly during heatwaves, raise concerns regarding their potential risks to human health and ecosystems through inhalation and deposition pathways. This study also offers a systematic basis for environmental risk assessment and underscores the need for targeted regulatory strategies to monitor and mitigate emissions of tire-associated chemicals, especially within the contexts of climate change and urban air pollution control.