Direct Observation of the Transitional Stage of Mixing-State-Related Absorption Enhancement for Atmospheric Black Carbon

Abstract

The addition of coating to the black carbon (BC) enhances its absorption as more light is focused by the coating “lens.” The absorption enhancement factor (Eabs) of BC is difficult to quantify due to an inadequate representation of its mixing structure and the interaction with radiation. Here, by tracking the evolution of the fresh BC particles in the ambient, we found a transitional stage of the particle Eabs with the non-BC-to-BC mass ratio (RBC) at ∼2, below which there were insufficient coating materials to encapsulate the BC core and the absorption enhancement was not significant (∼14%). When the RBC >∼ 2, obvious absorption enhancement occurred as the BC cores were fully covered. Secondary inorganic species played the most critical role in the coating materials to enhance the “lensing effect.” We suppose the particle-resolved core-shell Mie model can be applied in the Eabs prediction for most cases.