Seasonal variability and cloud-type effects on secondary organic aerosol formation during cloud events at a mountainous site in southeastern China

Abstract. Aerosol-cloud interactions exert substantial influences on atmospheric chemistry and regional climate, yet real-time characterization of chemical and microphysical evolution within cloud droplets remain limited. Here, two intensive campaigns were conducted at the high-altitude Shanghuang station in southeastern China during autumn 2023 and spring 2024, capturing distinct nocturnal orographic and long-persistence stratiform cloud events. Using a ground-based counterflow virtual impactor and an aerosol-cloud sampling inlet system along with integrated aerosol chemical speciation and cloud microphysical measurements, we resolved the composition of interstitial (INT), and residual (RES) particles during cloud events and ambient particles (AMB) under cloud-free conditions. Organic aerosols (OA) dominated particle mass across both seasons, while inorganic species (nitrate, sulfate, ammonium) exhibited high scavenging efficiencies (≥ 65–70 %) and strong enrichment in RES particles. Organic components showed seasonally contrasting partitioning patterns. Organics in INT particles exhibited a lower degree of oxidation during orographic clouds in 2023, whereas those in RES particles were more oxidized. In contrast, persistent clouds in 2024 displayed the opposite behaviour, reflecting shifts in aqueous-phase oxidation. Air-mass analysis further revealed pronounced source-dependent variability, with polluted westerly inflow leading to the highest particle loadings and most aged organic signatures. By linking chemistry with microphysics, we found that secondary organic aerosol formation preferentially occurs in smaller droplets, while OA from primary emissions is more efficiently incorporated into larger droplets through collision-coalescence. These results provide a quantitative, process-level understanding of seasonal and cloud-type controls on in-cloud chemical evolution, offering new constraints for representing aqueous-phase processing in atmospheric models.