Seasonal regime shifts in atmospheric aging of vehicular water-soluble organic aerosols: evidence from functionalization transformation indices and molecular conformational modeling

Abstract. Vehicular water-soluble organic aerosols (WSOA) critically impact urban air quality and climate, yet their molecular-level responses to seasonal aging remain unclear. This study integrates FT‑ICR MS with a three‑tier framework that includes core molecular screening, functionalization transformation index quantification, and three‑dimensional conformational modeling to decipher seasonal evolution of tunnel WSOA. Summer core molecules are dominated by saturated, low‑nitrogen aliphatics and lipids (88.5 %), whereas winter cores shift to unsaturated, high‑nitrogen aromatics and CRAMs (93.4 %). Functionalization transformation index diagnostics shows summer hydroxylation dominates (HI = 33,471), while winter exhibits net hydroxyl loss (HI = –2,787) and positive carbonylation (CNI = 989), suggesting a regime shift from •OH‑dominated photochemistry in summer to NO₃•/O₃‑enhanced chemistry in winter. Conformational modeling revealed opposite diurnal reorganizations, where summer encapsulation enriched hydrogen bond acceptors (HBA) by 48 yet reduced solvent-accessible polar surface area (PSA) by 790 Ų, whereas winter hydration depleted HBA by 78 yet expanded PSA by 1,638 Ų. Molecular conjugation, rather than bulk polarity, primarily governs chromophore activity. This framework moves beyond elemental composition to three‑dimensional structural interpretation, providing transferable, experimentally constrained parameters for chemical transport models, refining predictions of brown carbon light absorption properties, and offering a mechanistic basis for season‑specific air pollution mitigation strategies.