Nonlinear Enhancement of Radiative Absorption by Black Carbon in Response to Particle Mixing Structure

Black carbon (BC) strongly absorbs solar radiation, contributing to global warming. Absorption enhancement of BC particles is difficult to quantify due to an inadequate representation of their complex morphology and mixing structures, as well as interaction with radiation. Here, we apply a 3D method accounting for detailed BC mixing structures to predict the absorption enhancement of individual BC particles (Eabs) and the total BC particle population (Eabs, bulk). The diverse range of mixing structures in individual BC particles leads to variable Eabs that could hardly be predicted by empirical approximations. We find that the volume proportion of the BC embedded in coating (F) determines Eabs when the particle to BC core diameter ratio (Dp/Dc) is larger than 2.0. Our findings reveal the potential mechanism behind the differences in observed and modeled Eabs, bulk. The framework builds a bridge connecting the microscopic mixing structure of individual BC particle with Eabs, bulk. Plain Language Summary Absorption by black carbon (BC) in the atmosphere strongly affects radiative balance and global climate. The large discrepancies in observed and modeled BC absorption enhancements raise a hot debate. Through applying a new 3D shape model based on electron microscope observations, we propose a new framework that estimates BC absorption enhancement through accounting for mixing structure diversity in individual particles. Our results reveal that the diverse range of mixing structures in individual particles in ambient air leads to complex absorption enhancement that could hardly be predicted by the empirical approximation. The bulk absorption enhancements based on diverse mixing structures provide an explanation for the globally disparate results from laboratory and field observations. The new framework linking microphysical structures to bulk BC optical properties can be used to improve assessment of climate impact. Key Points The impact on absorption enhancement by embedded fraction is significantly enhanced when BC particle to core diameter ratio >2.0 The embedded fraction and coating thickness in aged BC particles can lead to significant differences in bulk absorption enhancement A new bridge is built to connect the microscopic mixing structure of individual aged BC particles with bulk absorption enhancement of BC