Comment on egusphere-2022-573

Abstract

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 parametrization 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. In the first step, the modeling techniques were examined for bare BC particles, and in the second step, for BC particles with organic material. A total of six morphological representations of BC particles were compared, three each for spherical and fractal aggregate morphologies. The BC fractal aggregate is 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 discrepancy between modelled and measured particle light absorption coefficient σabs to 10 %, for particles with volume mean mobility diameters between 60–160 nm. However, for particles larger than 100 nm, the Absorption Ångström Exponent (AAE) calculated by using a spherical morphology was more consistent with the measured value. 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 (mre and mim), the fractal dimension (Df), and the primary particle radius (app) of an aggregate were investigated. The modelled optical properties of BC are well aligned with laboratory-measured values when the following assumptions are used in the fractal aggregate representation: mre between 1.6 to 2; mim between 0.50 to 1; Df from 1.7 to 1.9, and app between 10 to 14 nm. Overall, this study provides experimental support for emphasizing the use of an appropriate size representation (polydisperse size method) and an appropriate morphological representation (aggregate morphology) for optical modelling and parametrization scheme development of BC.