Formation and Evolution of Secondary Aerosols at a Mountain Site in Southeastern China: Impacts of Dust Transport and Cloud Processes

Abstract Secondary aerosols are a major component of atmospheric particulate matter, but their formation pathways under different conditions in high mountains remain incompletely understood, impeding accurate assessment of their environmental and climatic impacts. In this study, the physicochemical properties and formation mechanisms of secondary aerosols in cloud‐free, cloud interstitial, and cloud residual particles are investigated at a mountain site in southeastern China. The results show that elevated concentrations of NO 3 − and more‐oxidized oxygenated organic aerosol (MO‐OOA) in cloud‐free particles primarily result from aqueous‐phase reactions along the dust transport. Aqueous‐phase processes during cloud events promoted the formation of NO 3 − , SO 4 2− , NH 4 + and MO‐OOA, with cloud residual particles exhibited 1.2–1.6 times higher growth rates than cloud interstitial particles. Lower fractions of C 4 H 7 + , C 4 H 9 + , C 5 H 6 O + , and C 3 H 7 O 3 + were found in cloud residual particles, suggesting that cloud droplets could facilitate the aging of these organics. Additionally, higher liquid water content increases the O/C ratio in secondary organic aerosols (SOA) by 0.1–0.15, and higher O 3 concentrations facilitate oxidation in cloud droplets via hydroxyl radicals (•OH) production in the acidic cloud water. SOA in cloud residual particles was enriched with carbonyl functional groups (>C=O), suggesting the presence of oxidized acids or esters, which contrasted with less‐oxidized aldehydes/ketones in cloud‐free and cloud interstitial particles. These findings highlight the combined influence of long‐range transport and cloud processing on secondary aerosol chemistry in high mountain region in southeastern China and provide critical insights for improving aerosol‐climate models in topographically complex regions.