Constructing Shapes and Mixing Structures of Black Carbon Particles With Applications to Optical Calculations

Black carbon (BC) aerosols strongly absorb the solar radiation, affecting the regional and global climate through direct and indirect radiative forcing. The optical properties of BC are critical factors to estimate their radiative forcing. However, the optical absorption of BC is still under controversy partially due to the weakness in quantifying their complex morphology and mixing structures. Although a Discrete Dipole Approximation (DDA) can calculate optical properties of fine particles with arbitrary shapes, an appropriate definition of realistic BC shape models for optical simulation is essentially required. Here we present a novel Electron‐Microscope‐to‐BC‐Simulation (EMBS) tool to construct realistic BC shape models with various morphology and mixing structures for optical calculation using DDA. The optical properties of BC particles with different particle morphology, coating thickness, and embedded fraction (F) are estimated based on electron microscope. We find that absorption enhancement (Eabs) of the realistic irregular model is larger than that of the present commonly used spherical model (i.e., BC aggregate with spherical coating). The BC core morphology greatly influences Eabs of the embedded BC particles with irregular coating when the volume‐equivalent‐diameter ratio of particle to core (Dp/Dc) is larger than 1.8. The F significantly influences Eabs of BC particles, suggesting that the mixing structure between coating and core is an important factor to determine the optical absorption of aged BC particles. The study highlights that the BC core morphology, coating shape, coating thickness, and mixing structures influence their optical properties and should be considered as important variables in climate models. Plain Language Summary Black carbon (BC) is one of the most important aerosol impacting the regional and global climate change due to its strong optical absorption in the atmosphere. However, it is challenging to precisely estimate the BC optical properties using instruments and numerical models because of its complex morphology and mixing structures in individual particles. This study for the first time presents a novel tool (named as EMBS) to construct realistic 3D shape models based on real BC particles from electron microscopic images for their optical calculation. We found that the commonly used spherical shape models may underestimate the absorption enhancement (Eabs) of BC particles. The mixing structures between BC and non‐B