Abstract
It is documented that some unreasonable low real parts of aerosol complex refractive index (ACRI) are retrieved from measured particle optical properties, and whether the optically effective ACRI has a physically-based sense is still an open question. Numerical determination of the optically effective ACRI from accurately calculated scattering and absorption properties of polydisperse coated BC particles by the core-shell Mie, representing some internal-mixed aged BC particles under polluted urban environments, is performed with a Mie theory based data analysis scheme. After getting rid of retrieval uncertainties (such as altered shapes), the look-up tables without limiting ACRI ranges based on known particle compositions could theoretically result in some unreasonable extreme real parts of optically effective ACRI of coarse coated BC with some shell/core ratios. Based on limited look-up tables with aerosol compositions considered, the retrieved imaginary parts of optically effective ACRIs of coarse coated BC, showing smooth as a function of shell/core ratio, are significantly lower than those approximated by the volume weighted average (VWA) method by a factor of nearly 3. Although the VWA shows acceptable performances for coated BC in the Aitken or accumulation modes, it could overestimate the absorption of coarse coated BC by a factor of ~2 for large shell/core ratio. This may be one of the reasons why modelled aerosol optical depth is 20% larger than observed. Our study indicates that the optically effective ACRI of internal-mixed BC particles does have a physically-based meaning like real ACRI, unless limited ACRI look-up tables considering aerosol compositions are applied for retrieval. We suggest that the optically effective ACRI, rather than the ACRI given by the VWA, should be considered for coarse internal-mixed particles in the state-of-the-art aerosol-climate models.
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