Impact of Anthropogenic Aerosol Transport on Cloud Condensation Nuclei Activity During Summertime in Qilian Mountain, in the Northern Tibetan Plateau.

In this study, we conducted field campaigns at two mountain‐top observatories on the Qilian Mountains (QLM) in the northeastern Tibetan Plateau of China, which plays a vital role in sustaining water resources for the downstream arid and semi‐arid regions. The two observatories, Waliguan Baseline Observatory (WLG) and Laohugou station (LHG), are situated on the eastern and western edge of the QLM, respectively. The objectives of this study were to examine the properties of atmospheric aerosols, CCN concentrations (NCCN) at varying supersaturation levels (SS = 0.2%–1.0%), and the hygroscopic nature of aerosols in the QLM, especially during the formation of orographic cloud. Notably, the average aerosol concentration and NCCN was approximately 2–3 factors higher at WLG compared to LHG. The chemical compositions of aerosols were primarily dominated by sulfate and organic aerosol (OA) at these two sites. A very high hygroscopicity parameter of aerosol calculated using chemical composition was observed at these two sites (0.37 ± 0.03 at LHG vs. 0.30 ± 0.04 at WLG). Enhanced aerosol loading episodes impacted by anthropogenic emission were observed at these two sites. Exploration on high‐loading cases at each site, we found that the CCN activity was dominated by aerosol size, but the chemical processes of aerosol during the formation of orographic cloud could also be important in CCN activity, especially for low SS conditions. These findings collectively underscore the significant impact of anthropogenic air plumes on CCN concentrations in the QLM and their potential influence on precipitation patterns. Plain Language Summary: The Qilian Mountains (QLM), situated in the northeastern Tibetan Plateau of China, have been constructed as a national park, dedicated to protecting its fragile ecological environment. Renowned for its "wet island" effect, the mountains serve as a crucial water resource for the downstream residents in northwest China. Orographic clouds, a primary source of precipitation in these mountains, heavily rely on the presence of cloud condensation nuclei (CCN), typically dominated by natural mineral dust due to its arid and semi‐arid regions around. In recent years, rapid urbanization and heavy human activities around has introduced a significant influx of anthropogenic aerosols. However, the impact of anthropogenic aerosol on the orographic cloud is unclear, especially during summer season when natural mineral dust emissions are limited. To unravel this mystery, we conducted campaigns at two mountain‐top observatories to explore the impact of anthropogenic aerosol transport from low elevation regions during summer season to mountain areas. Our findings highlight the substantial impact of anthropogenic aerosols on CCN concentrations, which offer valuable insights into the understanding the interaction of aerosol and cloud, and potential impact on precipitation in this mountain region. Key Points: Two mountain‐top studies were explored to characterize aerosol and CCN on the QLM in the northeastern TP of ChinaAn overall low NCCN values were observed in these mountain areas, but the concentration at the east was 2–3 times higher than the westElevated NCCN occurred at these two sites when anthropogenic pollutants were transported upslope [ABSTRACT FROM AUTHOR]