An observational study of the boundary-layer entrainment and impact of aerosol radiative effect under aerosol-polluted conditions

Abstract

Aerosols exert a profound impact on the atmospheric boundary layer (ABL) structures and entrainment processes. However, observational analysis of entrainment under heavily polluted condition is rarely documented due to insufficient measurements. In this study, vertical profiles of potential temperature, water vapor, and particulate matter with aerodynamic diameter smaller than 2.5 μm (PM2.5) measured by unmanned aerial vehicle (UAV), LiDAR-measured extinction coefficients, as well as surface observations are presented to investigate the impact of aerosol radiative effect on entrainment processes under heavy aerosol pollution conditions. Results show that aerosol radiative effect reduces surface heat fluxes and influences entrainment substantially. Observations indicate that the column-like plume rise structure is the instigator accounting for the entrainment processes observed during daytime. A well-established entrainment zone identified in the nighttime (e.g., 02:00 local standard time, LST) is mainly due to strong wind shear generating turbulence. The parameters describing entrainment processes are largely modified by aerosol radiative effect. Specifically, the jumps of scalars on the heavy pollution day are much larger than that on the clean day. Entrainment zone thickness enlarges substantially as PM2.5 concentrations increase. The bin-averaged ratios of entrainment to surface heat fluxes are close to a constant (about 0.21) but show a large standard deviation with aerosol pollution conditions. The dimensionless entrainment velocity does not follow the −1 power-law relationship with Richardson number for the heavily polluted conditions, which is consistent with the previous large-eddy simulation (LES) results. This observational study provides valuable evidence in support of high-resolution modeling studies of entrainment with heavily-polluted aerosol conditions.