Measurement Report: Molecular Dynamics of Organic Aerosol in the Wintertime East China Sea Driven by East Asian Outflows

Abstract. Organic aerosol (OA) represents a significant component in the marine atmosphere, yet its chemical composition and sources remain poorly constrained owing to its relatively sparse observations in coastal regions. This study investigates molecular characteristics and evolution of OA by high-resolution mass spectrometry during winter over the East China Sea. The detected thousands of organic compounds exhibited low O/C ratios (0–0.6) but broad H/C distributions. Over 95 % (ESI+) and 60 % (ESI-) of detected molecules were highly sensitive to air mass origins, revealing the dynamic interplay between marine and anthropogenic emissions. The CHON subgroup dominated the ESI+ mode (55 – 58 %), while the dominant subgroups in the ESI- mode varied with atmospheric processes. The CHON- subgroup fraction surged substantially during pollution episodes, particularly for nitro-aromatics. The CHOS- subgroup was more sensitive to aqueous-phase reactions and facilitated by elevated SO₂/SO₄²⁻ levels, leading to the formation of organosulfate (O/S ≥ 4). Marine biomarkers persisted across all periods, indicating pronounced resilience of marine emissions even in winter. The bulk volatility increased during polluted periods, particularly for CHON- and CHN+ subgroups, driven by primary emissions and suppressed oxidation. This study demonstrates that continental outflows, atmospheric secondary processes, and persistent marine emissions collectively complicated OA molecular characteristics over the East China Sea.