1 Department of Environmental Science and Engineering, University of Science and Technology of China, Hefei 230026, China; State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, China.
2 Department of Chemistry and Biochemistry, Concordia University, 7141 Sherbrooke Street West, Montreal, QC H4B 1R6, Canada.
3 Department of Environmental Science and Engineering, University of Science and Technology of China, Hefei 230026, China; Department of Anesthesiology, The First Affliated Hospital of USTC, Division of Life Sciences and Medicine, University of Science and Technology of China, Hefei, Anhui 230001, China.
4 Anhui Provincial Engineering Laboratory of Water and Soil Pollution Control and Remediation, School of Ecology and Environment, Anhui Normal University, Wuhu, Anhui 241002, China.
5 Division of Ambient Air Monitoring, China National Environmental Monitoring Centre, Beijing 100012, China.
6 Department of Civil and Environmental Engineering, Northeastern University, Boston, MA, USA.
7 Department of Environmental Science and Engineering, University of Science and Technology of China, Hefei 230026, China.
8 School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China.
9 Department of Environmental Science and Engineering, University of Science and Technology of China, Hefei 230026, China; State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, China; Beijing National Laboratory for Molecular Sciences (BNLMS), Beijing 100190, China. Electronic address: liuqifan@ustc.edu.cn.

Transformed from p-phenylenediamines (PPDs) antioxidant, PPD-derived quinones (PPD-Qs) have recently been recognized as emerging contaminants due to their potential negative impacts on the environment and human health. While there have been measurements of airborne PPD-Qs, the size distribution of PPD-Qs and the impact of particle size on PPD transformation chemistry remain largely unknown. Here, through the measurements of atmospheric particles in three megacities in China (Beijing, Xi'an, and Hefei), we find that PPD-Qs are widely distributed in these cities. Further analysis of the size-fractioned particles in Hefei indicates that 48 % of PPD-Qs reside in coarse particles. Given that previous studies mainly focus on the measurement of PPD-Qs in fine particles, the previously reported PPD-Q concentrations and the corresponding human exposure dosages are likely to be significantly underestimated. Furthermore, the ratio of PPD-Q to PPD concentration (PPD-Q/PPD) for particles with size range of 0.056 - 0.1 µm is up to 3 times higher than that with size range of 10 - 18 µm, highlighting the key role of particle size in determining the atmospheric oxidation reactivity of PPDs. Model simulations reveal a size-dependent pattern for the estimated concentration of particulate PPD-Qs in human body. In addition, we also demonstrate that PPD-Qs can induce the formation of cellular reactive oxygen species, suggesting that they may pose risks to human health. Overall, our results emphasize the importance of considering the particle size effect when evaluating the reaction potential and exposure risk of airborne PPD-Qs.