Secondary formation of PM2.5-bound PFAS in the Pearl River Delta, China: Field evidence and implications for precursor control

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/m 3 , with ultra-short-chain PFAS accounting for 42% of the total concentration. Higher Σ 15 PFAS 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 Σ 15 PFAS. 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.