Importance of size representation and morphology in modelling optical properties of black carbon: comparison between laboratory measurements and model simulations

Black carbon (BC) from incomplete combustion of biomass or fossil fuels is the strongest absorbing aerosol component in the atmosphere. Optical properties of BC are essential in climate models for quantification of their impact on radiative forcing. The global climate models, however, consider BC to be spherical particles, which causes uncertainties in their optical properties. Based on this, an increasing number of model-based studies provide databases and parameterization schemes for the optical properties of BC, using more realistic fractal aggregate morphologies. In this study, the reliability of the different modelling techniques of BC was investigated by comparing them to laboratory measurements. The modelling techniques were examined for bare BC particles in the first step and for BC particles with organic material in the second step. A total of six morphological representations of BC particles were compared, three each for spherical and fractal aggregate morphologies. In general, the aggregate representation performed well for modelling the particle light absorption coefficient inline-formulaσ_abs, single-scattering albedo SSA, and mass absorption cross-section MACinline-formula_BC for laboratory-generated BC particles with volume mean mobility diameters inline-formulad_p,V larger than 100 nm. However, for modelling Ångström absorption exponent AAE, it was difficult to suggest a method due to size dependence, although the spherical assumption was in better agreement in some cases. The BC fractal aggregates are usually modelled using monodispersed particles, since their optical simulations are computationally expensive. In such studies, the modelled optical properties showed a 25 % uncertainty in using the monodisperse size method. It is shown that using the polydisperse size distribution in combination with fractal aggregate morphology reduces the uncertainty in measured inline-formulaσ_abs to 10 % for particles with inline-formulad_p,V between 60–160 nm.

Furthermore, the sensitivities of the BC optical properties to the various model input parameters such as the real and imaginary parts of the refractive index (inline-formulam_re and inline-formulam_im), the fractal dimension (inline-formulaD_f), and the primary particle radius (inline-formulaa_pp) of an aggregate were investigated. When the BC particle is small and rather fresh, the change in the inline-formulaD_f had relatively little effect on the optical properties. There was, however, a significant relationship between inline-formulaa_pp and the particle light scattering, which increased by a factor of up to 6 with increasing total particle size. The modelled optical properties of BC are well aligned with laboratory-measured values when the following assumptions are used in the fractal aggregate representation: inline-formulam_re between 1.6 and 2, inline-formulam_im between 0.50 and 1, inline-formulaD_f from 1.7 to 1.9, and inline-formulaa_pp between 10 and 14 nm. Overall, this study provides experimental support for emphasizing the importance of an appropriate size representation (polydisperse size method) and an appropriate morphological representation for optical modelling and parameterization scheme development of BC.