Abstract View
Modeling the Size-Dependent of Mass Absorption Cross-Section of Black Carbon Aggregates
FENGSHAN LIU, Joel Corbin, Prem Lobo, Gregory Smallwood, National Research Council Canada
Abstract Number: 589
Working Group: Aerosol Physics
Abstract
The mass absorption cross-section (MAC) of black carbon (BC) particles is an important parameter to convert the measured BC aerosol absorption coefficient to BC mass concentration. MAC has often been considered an intrinsic property of BC particles. Several recent ex-situ measurements of MAC of BC aerosols sampled from different combustion sources showed that the MAC displays a fairly strong increasing trend with increasing particle mobility diameter (dm) from about 50 nm to 200 nm by about 30 to 40% and then reaches a plateau at larger dm. On the other hand, the modeled MAC of BC aggregates displays very weak size dependence over the dm range of 50 to 200 nm, regardless of the solution methods (Mie, Rayleigh-Debye-Gans theory, and numerically accurate generalized Mie-solution method), particle morphologies (sphere, fractal aggregates formed by monodisperse or polydisperse spherical monomers in point-contact or with overlap), and refractive index (RI) relevant to mature soot. Therefore, the current theoretical methods and BC particle morphology models fail to reproduce the observed size-dependent BC MAC over the dm range of 50 to 200 nm.
The potential influence of size-dependent refractive index (RI) of BC particles was modeled by using the model of Kelesidis and Pratsinis (2019). This model predicts a rapid evolution of BC RI from nucleation soot to mature soot with increasing dm and there is only small variation in RI over the dm range of 50 to 200 nm, failing to predict the observed size-dependent BC MAC.
A modified size-dependent RI model was proposed to reproduce the strong size-dependent BC MAC. However, the mechanism for the strong size-dependent RI over the mobility diameter range of 50 to 200 nm remains unclear.