The impacts of residual layer on the vertical distributions of pollutant particulate matter: combining large tethered balloon and remote sensing observations

The combination of in situ observations from a large tethered balloon and remote sensing instruments (aerosol lidar and Doppler wind lidar) enabled the evolution of the residual layer (RL) to be observed during an intensive vertical detection experiment of the planetary boundary layer (PBL) conducted during December (Dec) 2018 in Wangdu County, China. This paper focused on the important role played by the RL in the variations of the vertical distributions of pollutant particulates. The results of the present analysis revealed the following. (1) A considerable proportion of pollutant particulates remained suspended in the RL (e.g., the nitrate concentration reached 30 µg m^-3) in the nocturnal boundary layer (NBL). Multilayer pollutant structures appeared often, partly because of the existence of the RL. Because pollutants were still stored in the RL and the shallow surface inversion layer, the aerosol lidar-calculated PBL height was closer to the top of the RL before midnight in the NBL; after midnight, the PBL height was more consistent with the top of the surface inversion layer. (2) As the convective mixing layer gradually became established after sunrise the following day, the pollutants stored in the nocturnal RL of the preceding night were entrained downward into the mixing layer. The early morning PM_2.5 concentration near 700 m in the RL on Dec 20 decreased obviously compared with the concentration at 13:34 on Dec 20 at the same height (ranging from 30 µg m^-3 to 5 µg m^-3). The nitrate concentration also decreased significantly in the RL, but its concentration increased to 12 µg m^-3 in the mixing layer. Near-surface PM_2.5 diffused upward more easily due to strong vertical mixing during the daytime, causing reductions in the surface concentration. The mixing layer heights in Wangdu County were estimated to be 600 m in the winter, and various emitted pollutant particulates eventually became well-mixed within the mixing layer. The daytime mixing layer heights were consistent with the PBL heights calculated by aerosol lidar representing the pollutant accumulation depth. (3) The RL was characterized by a Richardson number (Ri) below the threshold value of 0.25, revealing that turbulence still existed within the RL.