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 process-resolved characterizations 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 nocturnal orographic and long-persistence stratiform cloud events. Using two complementary cloud-droplet sampling systems, the ground-based counterflow virtual impactor and a custom-designed aerosol-cloud sampling inlet system, along with aerosol chemical speciation and cloud microphysical measurements, we resolved the composition of interstitial (INT), residual (RES) and ambient (AMB) particles. 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, with differences between INT and RES particles indicating variability in aqueous-phase processing. Air-mass analysis further revealed pronounced source-dependent variability, with polluted westerly inflow leading to the highest particle loadings and most aged organic signatures. Linking chemistry with microphysics, we found that secondary organic aerosol (SOA) formation is favored in smaller droplets, whereas primary organic aerosol is preferentially incorporated into larger droplets through collision-coalescence. The contrasting evolution of oxygento-carbon ratio in RES and INT particles as a function of OA/ΔCO indicates distinct oxidation pathways for activated and non-activated aerosols. These results demonstrate that droplet size and cloud dynamics jointly regulate aerosol processing and that in-cloud oxidation pathways differ between particle types. This study provides process-level constraints for improving the representation of aqueous-phase SOA formation and aerosol-cloud interactions in atmospheric models, particularly in regions influenced by complex terrain and variable cloud regimes.