Anthropogenic Aerosol Impacts on CCN Activity and Thermodynamic Structure at the Top of the Continental Boundary Layer: A Wintertime Case Study in China

Aerosol radiative forcing regulates global climate through both direct and indirect effects, yet it remains highly uncertain. Most aerosol observations are collected near the surface, offering limited constraints on aerosol properties at the top of the planetary boundary layer (PBL) - a critical region that directly interacts with clouds and modulates PBL stability. The insufficient characterization of aerosols at the PBL top contributes to persistent model uncertainties in radiative forcing estimates. Here, we present in situ measurements of aerosol physical and chemical properties on 28-29 November 2023 at a mountain-top site in eastern China (1143 m a.s.l.) near the top of the polluted continental boundary layer. The station resided within the mixed layer during daytime and above it during nighttime and early morning, consistent with typical continental boundary-layer evolution. Back-trajectory analysis identified three major aerosol sources: local new particle formation, entrainment from the free troposphere, and long-range transport of near-surface anthropogenic emissions. The transported anthropogenic emissions were found to be the dominant aerosol source at the PBL top. These emissions significantly increased aerosol mass, number concentration, and particle size, and enhanced hygroscopicity, thereby strengthening aerosol cloud condensation nuclei activity in this region. The associated increases in aerosol absorption and scattering potentially contributed to the observed weaker turbulence and higher humidity near the PBL top, producing conditions favorable for low-cloud formation. Observations of aerosol properties at the top of the boundary layer are essential to better constrain models and reduce uncertainties in aerosol radiative forcing.