Enhanced Bottom-of-the-Atmosphere Cooling and Atmosphere Heating Efficiency by Mixed-Type Aerosols: A Classification Based on Aerosol Nonsphericity

Abstract

The current understanding of the climate effects of mixed-type aerosols is an open question. The optical and radiative properties of the anthropogenic, mixed-type, and dust aerosols were studied using simultaneous observations of a sun photometer and a depolarization lidar over the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL), northwestern China. The aerosol radiative effect was calculated using the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model and was in good agreement with the Aerosol Robotic Network (AERONET) product. The anthropogenic, mixed-type, and dust aerosols were identified mainly based on the lidar-measured depolarization ratio, which was supported by the airmass back trajectories. The mixed-type aerosols exhibit lower (higher) extinctions below (above) 1.5 km above the ground, indicating anthropogenic pollution from the atmospheric boundary layer and dust aerosols above. The dust aerosols exhibit the highest absolute radiative effect because of the highest aerosol loading. However, the mixed-type aerosols are effective in both scattering and absorbing solar radiation, leading to the highest cooling efficiency at the bottom of the atmosphere (BOA), 7.4% and 6.5% higher than those of the anthropogenic and dust aerosols, respectively. The mixed-type aerosols exhibit the highest warming efficiency in the atmosphere (ATM), 20.8% and 28.2% higher than the anthropogenic and dust aerosols, respectively. The mixed-type aerosols also show the lowest cooling efficiency at the top of the atmosphere (TOA). The results suggest the necessity of carefully characterizing the mixed-type aerosols in atmospheric numerical models to more precisely assess the energy budget of the Earth–atmosphere system.