Atmospheric heating rate due to black carbon aerosols: Uncertainties and impact factors

Abstract This study investigates the impacts of black carbon (BC) properties (vertical concentration, shape, size, and mixing state) and atmospheric variables (cloud and aerosol loading, surface albedo, and solar zenith angle) on BC radiative effects, especially vertical distributions of heating rate due to BC absorption. BC and aerosol observations from aircraft in situ measurements, lidar, and the Aerosol Robotic Network (AERONET) are used to constrain their properties. The library for radiative transfer (Libradtran) model is used to calculate BC radiative forcing (RF). BC optical properties are obtained from numerical modeling with aggregate or spherical structures and different size distributions. By modifying optical properties, different BC geometries and size distributions result in uncertainties on RF and heating rate less than 30%, while the uncertainty with different BC mixing states is as large as ~80%. Vertical distribution of BC concentrations explain relative differences in RF and heating rate in the atmosphere by less than 10%, but can induce different heating rate vertical profiles, thus different planetary boundary layer (PBL) stabilities. Due to the significant influence of cloudy and aerosol conditions on incident solar radiation, atmospheric conditions play an important role in determining BC heating rate. Meanwhile, the effects of surface albedo and solar zenith angle on heating rate are more significantly on the bottom. Taking the above factors into account, we introduce an empirical approximation of BC heating rate to estimate its influence on atmosphere. With the simple formula, the BC heating rate for a particular atmospheric layer can be approximated with the vertical condition generally known, and this can be further applied to determine whether BC promotes or suppresses PBL development. Considering the importance of BC vertical concentration on its heating rate, we suggest that light-absorbing aerosols, and their vertical distributions must be better measured and modeled, to improve the understanding of their radiative effects and interaction with PBL.