Advanced modeling of the absorption enhancement of black carbon particles in chamber experiments by considering the morphology and coating thickness

Abstract

Measurements studies have shown that the absorption of radiation by black carbon (BC) increases as the particles age. However, there are significant discrepancies between the measured and modeled absorption enhancement (E_abs), largely due to the simplifications used in modeling the mixing states and shape diversities. We took advantage of chamber experiments on BC aging and developed an efficient method to resolve the particle shape based on the relationship between the coating fraction (ΔD_ve/D_ve,0) and fractal dimension (D_f), which can also reflect the variations of D_f during the whole BC aging process. BC with externally and partly mixed states (0 ⩽ ΔD_ve/D_ve,0 ⩽ 0.5) can be considered to be uniformly distributed with the D_f values of 1.8–2.1, whereas the D_f values are constrained in the range 2.2–2.8 for fully mixed states (ΔD_ve/D_ve,0 > 0.5). The morphological parameters (i.e., the effective density and the dynamic shape factor) were compared with the measured values to verify the simulated morphology. The simulated mean deviations of morphological parameters were smaller than 8% for the method resolving the particle shape. By applying a realistic shape and refractive index, the mass absorption cross for fully mixed states can be improved by 11% compared with a simplified core–shell model. Based on our understanding of the influence of D_f and ΔD_ve/D_ve,0 on E_abs, we propose a two-stage calibration equation to correct the E_abs values estimated by the core–shell model, which reduces the simulation error in the Mie calculation by 6%–14%.