In Situ Observations Reveal That the South Asian Summer Monsoon Weakens Aerosol Cloud Activation Over the Southern Tibetan Plateau

Abstract The South Asian Summer Monsoon (SASM) strongly influences aerosol and moisture transport over the Tibetan Plateau (TP), shaping its environment and climate. However, its impact on aerosol activation remains unclear due to limited observations. Here, in situ measurements at Mount Qomolangma Station during the 2024 SASM reveal weak aerosol activation capacity over the southern TP, with hygroscopicity parameter values mostly below 0.1 and daily‐mean cloud condensation nuclei (CCN) concentrations at 0.2% supersaturation ranging from 5 to 282 cm −3 . Notably, CCN concentration, activation fraction, and hygroscopicity all exhibit significant negative correlations with SASM intensity, suggesting that stronger monsoonal transport reduces aerosol activation as clouds. This weakening is attributed to orographic lifting of monsoon air masses, which enhances cloud formation, precipitation, and wet scavenging, depleting CCN during upslope transport before the airmass reaches Qomolangma. These findings have important implications for understanding cloud formation and aerosol–cloud interactions over this climatically sensitive region.