Optically effective complex refractive index of coated black carbon aerosols: from numerical aspects
Abstract. Aerosol complex refractive index (ACRI) is an important microphysical parameter used for the studies of modeling their radiative effects. With considerable uncertainties related to retrieval based on observations, a numerical study is a powerful method, if not the only one, to provide a better and more accurate understanding on retrieved optically effective ACRI of aged BC particles. Numerical investigations on the optically effective ACRI of polydisperse coated BC aggregates retrieved from their accurate scattering and absorption properties, which are calculated by the multiple-sphere T-matrix method (MSTM), without overall particle shape variations during retrieval, are carried out. The aim of this study is to evaluate the effects of aerosol microphysics, including shell/core Dp / Dc ratio, BC geometry, BC position inside coating, and size distribution, on retrieved optically effective ACRI of coated BC particles. At odds with expectations, retrieved optically effective ACRIs of coated BC particles in coarse mode, are not merely impacted by their chemical compositions and shell/core ratio, being highly complicated functions of particle microphysics. However, in accumulation mode, the coated BC optically effective ACRI is dominantly influenced by particle chemical compositions and shell/core ratio, although it shows slightly sensitive to BC geometry, BC position inside coating and particle size distribution. The popular volume weighted average (VWA) method and effective medium theory (EMT) provide acceptable ACRI results for coated BC in accumulation mode, and the resulting uncertainties in particle scattering and absorption are both less than approximately 10 %. For coarse coated BC, the VWA and EMT, nevertheless, produce dramatically higher imaginary parts than those of optically effective ACRIs, significantly overestimating particle absorption by a factor of nearly 2 for heavily coated BC with a large BC fractal dimension or BC close to coating boundary. Using the VWA could introduce significant overestimation in aged BC absorption analysis studies, and this may be one of the reasons why modelled aerosol optical depth is 20 % larger than observed, since it is widely employed in the state-of-the-art aerosol-climate models. We propose a simple new ACRI parameterization for fully coated BC with Dp / Dc ≥ 2.0 in coarse mode, which can serve as a guide for the improvement of ACRI of heavily coated BC, and its scattering and absorption errors are reduced by a factor of nearly 2 compared to the VWA. Our study indicates that a reliable estimate of the radiative effects of aged BC particles in coarse mode would require accounting for the optically effective ACRI, rather than the ACRI given by the VWA, in aerosol-climate models.