Fossil-Dominated SOA Formation in Coastal China: Size-Divergent Pathways of Aqueous Fenton Reactions versus Gas-phase VOC Autoxidation

Abstract. Elucidating size-dependent formation mechanisms of secondary organic aerosols (SOA) 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. Rradiocarbon (¹⁴C) isotope analysis reveals that fossil sources dominate SOA in both fine (95.8 %) and coarse (80.4 %) modes, while the small amount of biogenic SOA mostly existed 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 aromatic, C9 aromatic, styrene) and biogenic 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 VOCs precursors and key components, our study elucidates 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 the 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.