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

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. Plain Language Summary: Black carbon is the dominant light‐absorbing component of atmospheric aerosols, significantly affecting the global climate. In the atmospheric aging process, such as condensation and oxidation, fresh BC is gradually coated. The light absorption of BC is significantly associated with the amount of its coating material since the coating could act as "lens" to refract more light toward the BC core. However, this "lensing effect" for BC particles is poorly quantified, bringing uncertainties when evaluating the aerosol radiative forcing. In this study, we found there was a transitional behavior for the light absorption by BC particles which was related to the particle mixing states. When the non‐BC‐to‐BC mass ratio for particles is larger than 2, the coating materials are insufficient to encapsulate the BC core and cannot act as the coating "lens." Our study provides a step forward in understanding the global radiation effect of BC. Key Points: A transitional stage of the absorption enhancement (Eabs) during the evolution of black carbon (BC) was directly observed in the ambientBy tracking the evolution of fresh BC, the predicted Eabs based on the particle‐resolved model showed consistent transitional stageThe secondary inorganic species acted the most critical role in the coating materials to enhance the absorption of BC‐containing particles [ABSTRACT FROM AUTHOR]