Measurement report: Molecular-level insights into the seasonal formation of organosulfates in a mountain atmosphere

Abstract. Organosulfates (OSs) are important components of secondary organic aerosol, yet their molecular composition and formation processes in mountain atmospheres remain poorly constrained. In this study, PM_2.5 samples collected during summer and winter at a high-altitude background site at Wuyi Mountain, southeastern China, were analyzed using high-performance liquid chromatography coupled with orbitrap mass spectrometry (HPLC-Orbitrap MS) to examine how seasonal changes in precursor availability and atmospheric processing influence OS molecular composition. Clear seasonal differences were observed in the molecular composition and structural characteristics. Summer OSs were enriched in more highly oxygenated species (with OS_c >-1 accounting for 72 % of the total OS signal abundance), whereas winter OSs were characterized by larger (C₅–C₂₃), less oxygenated (O₄–O₇), and more unsaturated molecules, reflecting more diverse precursor influences. Among 35 structurally identified OSs, isoprene- and monoterpene-derived species were enriched in summer, while aliphatic-like OSs and monoterpene-derived nitrooxy-OSs were more abundant in winter, indicating seasonal variations in precursor availability and the relative importance of atmospheric processing. Mechanistic analysis suggested that summer OS formation was more closely associated with sulfate-rich and highly acidic conditions favorable for multiphase processing, whereas wintertime OS formation reflected greater influences from elevated atmospheric oxidants and nitrogen-related chemistry. Regional transport further contributed to wintertime OS composition by increasing anthropogenic precursor inputs and promoting atmospheric aging. These findings demonstrate that seasonal OS variability in mountain environments reflects the interplay between precursor availability and atmospheric processing, providing field-based molecular evidence for the coupling of biogenic and anthropogenic influences in secondary organic aerosol formation and evolution.