Shielding effect of brown organic coating on black carbon aerosols

The light absorption of black carbon (BC) particles is influenced by their mixing structures and coating compositions. Liquid-liquid phase separation (LLPS) is an important microscopic phenomenon which can separate organic and inorganic phases and redistribute BC from the inorganic core (Icore) to organic coating (Ocoating). This study combines transmission electron microscopy and a 3D modeling method—Electron-Microscope-To-BC-Simulation (EMBS) to investigate how microphysical properties, such as coating compositions, Ocoating thickness, and BC position, influence the light absorption of BC particles. We found that the position of BC significantly influences its light absorption. The light absorption of BC centering in Icore is stronger below 600 nm than BC in the Ocoating. When Ocoating is considered as BrC, it reduces the light absorption of BC within Icore and Ocoating by 1.8% and 49.8%, respectively, at 350 nm due to the shielding effect, which blocks more photons from reaching the BC core. However, when accounting for the intrinsic light absorption of BrC and BC, the contribution of BrC shielding effect to individual BC particle is merely –3.0%±1.6%. The result indicates that the primary role of BrC coating still keeps light absorption rather than shielding effect in the LLPS particles. This study highlights that brown organic coating and mixing structure of BC should be comprehensively considered pertaining to optical absorption of BC-containing particles in atmospheric models.