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. The formation of black carbon fractal aggregates (BCFAs) fromcombustion and subsequent ageing involves several stages resulting inmodifications of particle size, morphology, and composition over time. Tounderstand and quantify how each of these modifications influences the BCradiative forcing, the optical properties of BCFAs are modelled. Owing tothe high computational time involved in numerical modelling, there are somegaps in terms of data coverage and knowledge regarding how opticalproperties of coated BCFAs vary over the range of different factors (size,shape, and composition). This investigation bridged those gaps by followinga state-of-the-art description scheme of BCFAs based on morphology,composition, and wavelength. The BCFA optical properties were investigatedas a function of the radius of the primary particle (ao), fractal dimension (Df), fraction of organics (forganics), wavelength (λ), and mobility diameter (Dmob). The optical properties arecalculated using the multiple-sphere T-matrix (MSTM) method. For the firsttime, the modelled optical properties of BC are expressed in terms ofmobility diameter (Dmob), making the results more relevant andrelatable for ambient and laboratory BC studies. Amongst size, morphology,and composition, all the optical properties showed the highest variabilitywith changing size. The cross sections varied from 0.0001 to0.1 µm2 for BCFA Dmob ranging from 24 to 810 nm. It has been shown that MACBC and single-scattering albedo (SSA) are sensitive to morphology, especially for larger particles with Dmob &gt; 100 nm. Therefore, while usingthe simplified core–shell representation of BC in global models, theinfluence of morphology on radiative forcing estimations might not beadequately considered. The Ångström absorption exponent (AAE) varied from1.06 up to 3.6 and increased with the fraction of organics(forganics). Measurement results of AAE ≫ 1 areoften misinterpreted as biomass burning aerosol, it was observed that the AAE of purely black carbon particles can be ≫ 1 inthe case of larger BC particles. The values of the absorption enhancementfactor (Eλ) via coating were found to be between 1.01 and 3.28 in thevisible spectrum. The Eλ was derived from Mie calculations forcoated volume equivalent spheres and from MSTM for coated BCFAs. Mie-calculated enhancement factors were found to be larger by a factor of 1.1 to 1.5 than their corresponding values calculated from the MSTM method. It is shown that radiative forcings are highly sensitive to modifications in morphology and composition. The black carbon radiative forcing ΔFTOA (W m−2) decreases up to 61 % as the BCFA becomes more compact, indicating that global model calculations should account for changes in morphology. A decrease of more than 50 % in ΔFTOA was observed as the organic content of the particle increased up to 90 %. The changes in the ageing factors (composition and morphology) in tandem result in an overall decrease in the ΔFTOA. A parameterization scheme for optical properties of BC fractal aggregates was developed, which is applicable for modelling, ambient, and laboratory-based BC studies. The parameterization scheme for the cross sections (extinction, absorption, and scattering), single-scattering albedo (SSA), and asymmetry parameter (g) of pure and coated BCFAs as a function of Dmob were derived from tabulatedresults of the MSTM method. Spanning an extensive parameter space, thedeveloped parameterization scheme showed promisingly high accuracy up to98 % for the cross sections, 97 % for single-scattering albedos (SSAs), and 82 % for the asymmetry parameter (g).

Optical properties of coated black carbon aggregates: numerical simulations, radiative forcing estimates, and size-resolved parameterization scheme