Observed Evidence That Subsidence Process Stabilizes the Boundary Layer and Increases the Ground Concentration of Secondary Pollutants.

Subsidence in atmospheric boundary layer (ABL) appears to be rare and short‐lived, so it is very difficult to observe. In this paper, field experimental data from a large tethered balloon (1,900 m3) synchronously measuring meteorological parameters and air pollutants, a ground‐based aerosol lidar, a Doppler wind lidar, and some other ground observations were used to analyze the formation mechanism of subsidence and its influences on the variation of meteorological parameters and pollutant concentrations of the ABL. Results show that the occurrence of subsidence was closely correlated with the cold advection accompanied by cold front. Pollutants were horizontally transported to the observation site driven by southwest low‐level jet. Under the influence of subsidence behind the cold front and turbulent kinetic energy transported toward the surface, the vertical downward transport of pollutants aloft lasted for nearly 5 hr. The observation data of the tethered balloon suspending at 500 m show that the concentrations of PM2.5, NO3−, SO42−, and NH4+ increased simultaneously. O3 concentration in the lower atmosphere also increased. Subsidence had significant effects on the secondary inorganic aerosol concentrations but the concentration of organic aerosols was not affected. Subsidence led to a stronger inversion layer, further suppressing the vertical dispersion of pollutants. Both the stronger subsidence‐induced inversion layer and the downward transport of pollutants have led to an increase in the ground PM2.5 concentration. Our findings demonstrate the close connection between subsidence and air pollution process and may provide the scientific reference for air quality potential forecast. Plain Language Summary: Subsidence motion is an important kind of vertical exchange process and is usually known to exert great influences on the local air quality. The study on subsidence is poorly understood because of the less observation. Here, we utilized the multisource observation data of a tethered balloon platform and remote sensing measurements in Wangdu County located in the North China Plain to comprehensively study a subsidence case and investigate its impacts on the meteorological parameters and pollutants of the atmospheric boundary layer (ABL). We found that the subsidence located behind the cold front and the observation site has been controlled by cold advection. Subsidence stabilized the lower layer of the ABL and the inversion intensity was strengthened. Subsidence also contributed to the vertical transport of pollutants advected under the influence of low‐level jet. The vertical transport of pollutants had more significant effects on secondary inorganic aerosols, while the concentration of organics was generally not affected. Our study provides the observation evidence of the formation and maintenance of subsidence and highlights the impacts of subsidence to the variation of meteorological parameters and pollutant concentrations of the ABL. Key Points: Data from large tethered balloon (1,900 m3), aerosol lidar, and Doppler wind lidar observations were analyzedSubsidence led to the increase of surface inversion intensity and a reduction of the core of the low‐level jet (LLJ) and aggravated ground pollutionThe downdrafts and the negative vertical turbulent kinetic energy flux associated with LLJ enhanced the downward diffusion of pollutants [ABSTRACT FROM AUTHOR]