PNNL

Abstract

Understanding the activation potential of aerosols as cloud condensation nuclei (CCN) is essential in climate and hydrological cycle studies, yet our knowledge of their mechanism remains limited. In this study, we conducted field studies 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 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, especially during the formation of orographic cloud. The two observatories, Waliguan Baseline Observatory (WLG) and Qilian Shan Station of Glaciology and Ecologic Environment (LHG), are situated on the eastern and western edge of the QLM, respectively, representing the nearest and farthest areas from anthropogenic activities in this region. Notably, the average NCCN was approximately 2–3 higher at WLG compared to LHG (120.5–504.5 vs. 233.7–1407.0 at SS of 0.2%–1.0%). This pattern is consistent with that of sub-micro aerosol loading at two locations (PM1: 2.7 ± 1.3 µg m–3 vs. 9.1 ± 5.3 µg m–3). Distinct diurnal variations in aerosol and NCCN at both sites were observed which were related to the variations on local mountain-valley breeze and aerosol sources. The chemical compositions of aerosols were primarily dominated by sulfate and organic aerosol (OA) at these two sites (totally accounting more than 65% in mass). Due to the more pristine location, more aged OA and higher contribution of sulfate were observed at LHG. A very high hygroscopicity parameter of PM1 calculated using chemical composition was observed at these two sites (0.46 ± 0.03 at LHG vs. 0.36 ± 0.04 at WLG). High aerosol loading episodes impacted by anthropogenic emission were observed at two sites with about 2 factors higher in NCCN than other periods. Exploration on high-loading case studies at each site, we found than the CCN activity was dominated by aerosol size, but the ratio of NCCN between 0.2% and 1.0% was increased during particle growth and oxidization under the aqueous processes of orographic cloud, suggesting the importance of chemical properties on the CCN activity in this region. Clouse investigation assuming both internal external mixing state with bulk chemical composition of aerosol present a 20%–100% over-prediction than measured CCN concentration at different SS level. These findings collectively underscore the significant impact of anthropogenic air plumes on CCN concentrations in the QLM and their potential influence on precipitation patterns, especially during summertime when natural aerosol concentration was limited.