Secondary formation of PM_2.5-bound PFAS in the Pearl River Delta, China: Field evidence and implications for...

Per- and polyfluoroalkyl substances (PFAS) are global pollutants of concern, yet their atmospheric sources and secondary formation mechanisms remain poorly understood. Here, we collected PM_2.5 samples during winter and summer across nine cities in the Pearl River Delta (southern China). Among 44 targeted PFAS in PM_2.5, 15 were detected with a median concentration of 222 pg/m³, with ultra-short-chain PFAS accounting for 42% of the total concentration. Higher Σ₁₅PFAS concentrations occurred in winter (261 pg/m³) than in summer (141 pg/m³), with distinct seasonal compositional patterns. Source apportionment analysis indicated that potential secondary formation contributed an average of 16% to Σ₁₅PFAS. Significant correlations between selected perfluoroalkyl acids (PFAAs) and atmospheric oxidants, aerosol liquid water content, and succinic acid point to aqueous-phase precursor degradation as a potential pathway for PFAS formation. These results suggest that PM_2.5 may act as a sink for PFAS, especially for ultrashort-chain species during winter. Overall, our study provides field-based evidence for potential secondary PFAS formation in ambient aerosols and underscores the importance of controlling precursor emissions to mitigate atmospheric PFAS pollution.