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.