Various formation pathways of size-resolved secondary organic aerosols at a coastal site in Eastern China

Secondary organic aerosol (SOA) exists across a broad particle size range plays a crucial role in regional air pollution and climate, yet its formation mechanisms under varying meteorological conditions remain poorly understood. In this study, size-segregated aerosol samples (0.056–18 μm) were collected in Jiaxing, a coastal city in eastern China. Samples were categorized into land breeze (LP), sea breeze (SP), and dust event periods (DP). Oxygenated organic aerosols (OOAs) were identified as the dominant component of organic matters across all periods (65.9–81.6 %). Radiocarbon (¹⁴C) analysis revealed that fossil-derived OOA exhibited a dominant contribution to total OOC (78.1 %). High correlations of OOA and O₃ in both fine and coarse modes indicated the important role of gas-phase oxidation across all particle sizes, while a specific association between fine-mode SOA and Fe ions suggested an additional aqueous-phase contribution in the fine mode. During the LP, the relative short transport distance of land air masses resulted in lower oxidation states for both fine and coarse particles, leading to the lowest O/C values. In contrast, during the SP, increased aerosol liquid water enhanced SOA formation, elevating O/C in coarse particles. During the DP, extensive multi-generational oxidation occurred during long-range transport, yielding higher fine-mode O/C. The coarse-mode OOA was positively correlated with Cl⁻ and Ca²⁺, implying heterogeneous formation related to soil dusts. This study highlights the complexity of SOA formation pathways through multiple air mass types and different particle sizes, underscoring the necessity to incorporate various formation pathways into atmospheric models for accurate simulations and policy guidance.