Molecular evolution of oxygenated organic molecules in a cloud-influenced forested mountain environment

Abstract. Atmospheric oxygenated organic molecules (OOMs) link the oxidation of volatile organic compound to secondary organic aerosol formation, yet their molecular evolution in cloud-influenced forested mountain environments remains poorly constrained. Here, gas-phase OOMs were measured using an iodide-adduct chemical ionization mass spectrometer at a high-altitude forested station in southeastern China during autumn 2023. Among the 1503 identified OOMs, isoprene-derived OOMs were primarily controlled by daytime OH-initiated oxidation, whereas monoterpene-derived OOMs were driven by coupled OH and NO₃​ chemistry. Positive matrix factorization resolved nine distinct OOM factors, revealing that biogenic oxidation dominated OOM production during warm periods (mean temperature ~20 °C), accounting for 68–79 % of the total OOMs, whereas anthropogenic and regional transport factors became prominent during cooler periods (below 15 °C), accounting for up to 63 % of the total budget. Cloud processes exerted a pronounced influence on OOM composition through competitive wet scavenging and multiphase chemistry. During cloud events, isoprene-derived and monoterpene-derived organic nitrates (MT-ONs) decreased by 75 % and 46 %, respectively, while the relative contribution of MT-ONs increased from 23 % to 40 %, indicating distinct cloud-processing mechanisms and hydrolysis pathways. Furthermore, the contribution of sulfur-containing OOMs increased substantially, highlighting the importance of aqueous-phase formation pathways. These results demonstrate that cloud processing influences the composition and sources of OOMs in forested mountain atmospheres and should be considered when evaluating the atmospheric fate of biogenic oxidation products.