Fossil-dominated secondary organic aerosol (SOA) formation in coastal China: size-divergent pathways of aqueous Fenton reactions versus gas-phase volatile organic compound (VOC) autoxidation

Elucidating size-dependent formation mechanisms of secondary organic aerosols (SOAs) remains a critical research gap in atmospheric chemistry. Here, we analyzed water-soluble compounds in size-segregated aerosol samples (0.056–18 µm) collected at a coastal site in southern China. Radiocarbon (14C) isotope analysis reveals that fossil sources dominate SOAs in both fine (95.8 %) and coarse (80.4 %) modes, while the small quantity of biogenic SOAs mostly exist in the coarse mode (74.1 %). Fine-mode oxygenated organic carbon (OOC) correlates strongly with polar carbonyl compounds (e.g., glyoxal, methylglyoxal, acetone, and MVK + MACR), while coarse-mode OOC exhibits better correlations with nonpolar aromatic hydrocarbons (e.g., toluene, C8 aromatics, C9 aromatics, styrene) and biogenic volatile organic compounds (VOCs) (e.g., monoterpenes, isoprene), indicating that the sources of fine- and coarse-mode OOC are different. Multivariate analyses incorporating inorganic ions, pH, water-soluble iron ions, aerosol liquid water content, and O3 revealed divergent size-dependent mechanisms, emphasizing the significant role of aqueous-phase reactions in fine-mode OOC formation, particularly the key contribution of water-soluble Fe ions (r2 = 0.74), while coarse-mode OOC exhibited a notable correlation with O3 (r2 = 0.63). Combining the information on VOC precursors and key components, our study elucidates the fact that aqueous-phase reactions play a key role in fine-mode OOC, especially the Fenton reaction, while gas-phase VOC autoxidation plays an important role in coarse-mode OOC generation. By examining OOC formation across a wide range of particle sizes, our study highlights the critical need for mode-specific treatment of SOA generation in atmospheric chemical transport modeling.