Accounting for the effects of non-ideal minor structures on the optical properties of black carbon aerosols

Abstract

<p><strong>Abstract.</strong> Black carbon (BC) aerosol is the strongest sunlight-absorbing aerosol, and its optical properties are fundamental to radiative forcing estimations and retrievals of its size and concentration. During incomplete combustion, BC particles exist as aggregate structures with small monomers, which are widely represented by the idealized fractal aggregate model formed by monodisperse spherical monomers in point-contact. In reality, BC particles possess complex and non-ideal minor structures besides the overall aggregate structure, altering its optical properties in unforeseen ways. This study introduces a parameter <q>volume variation</q> to quantify and unify different minor structures, and develops an empirical relation to account for their effects on BC optical properties from those of ideal aggregates. Minor structures considered are the polydispersity of monomer size, irregularity and coating of individual monomer, and necking and overlapping among monomers. The discrete dipole approximation is used to calculate the optical properties of aggregates with these minor structures. Minor structures result in scattering cross section enhancement slightly more than that of absorption cross section, and their effects become weaker with the increase of wavelength. Their effects on the angular-dependent phase matrix as well as asymmetry factor are negligible. Our results suggest that a correction ratio of 1.05 is necessary to account for the mass/volume normalized absorption and scattering of non-ideal aggregates in comparison to ideal ones. In other words, minor structures tend to enhance the BC mass absorption and scattering by 5 %, which also applies to aggregates with multiple minor structures. The simulations of optical properties of non-ideal aggregates are greatly simplified, because they can be directly obtained from those of the corresponding ideal aggregates. We expect this generalized correction to find wide use for modeling realistic BC aggregates due to the simplicity involved in generating ideal fractal aggregates, and it is of great value for not only interpretation of measurements but also practical modeling that requires large amount of simulations.</p>