Surface Nucleation-Mode Particles Increase During Intense Vertical Exchange Between the Atmospheric Boundary Layer and the Free Troposphere

Abstract

Vertical exchange between the atmospheric boundary layer (ABL) and the free troposphere (FT) is a key pathway for aerosol transport and can substantially influence surface air quality. In this study, an intense ABL-FT exchange event over Shanghai on 29 March 2024 was captured and analyzed using ground-based Doppler wind LiDAR, microwave radiometer, particle-size spectrometer measurements, and numerical diagnostics. Southwesterly warm-moist advection and daytime surface heating promoted the growth of a warm and moist boundary layer, while drier and more stably stratified air persisted above the ABL top. Under this thermodynamic background, the boundary-layer jet and associated dynamical disturbances favored vigorous ABL-FT exchange. During this period, vigorous vertical motions developed, with vertical velocities ranging from −1.5 to 2.0 m $s^{-1}$ and extending to nearly 3 km. The estimated ABL-FT air-mass exchange flux ranged from −0.97 to 1.46 kg $m^{-2}$ $s^{-1}$, with vertical motion accounting for 58.5% of the total exchange. During the exchange event, ultrafine-particle concentrations increased markedly at the surface, while particles smaller than 10 nm did not show a corresponding increase. These observations are more consistent with downward transport of particles likely present aloft, followed by growth within the boundary layer, than with a purely local surface-formation mechanism. This study highlights a possible linkage among dust intrusion, particle processes aloft, and ABL-FT mixing in shaping near-surface aerosol number concentrations, and advances understanding of vertical aerosol transport in megacity environments.