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

Abstract

Abstract. The formation of black carbon fractal aggregates (BCFAs) from combustion and subsequent ageing involves several stages resulting in modifications of particle size, morphology, and composition over time. To understand and quantify how each of these modifications influences the BC radiative forcing, the optical properties of BCFAs are modelled. Owing to the high computational time involved in numerical modelling, there are some gaps in terms of data coverage and knowledge regarding how optical properties of coated BCFAs vary over the range of different factors (size, shape, and composition). This investigation bridged those gaps by following a state-of-the-art description scheme of BCFAs based on morphology, composition, and wavelength. The BCFA optical properties were investigated as 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 are calculated using the multiple-sphere T-matrix (MSTM) method. For the first time, the modelled optical properties of BC are expressed in terms of mobility diameter (Dmob), making the results more relevant and relatable for ambient and laboratory BC studies. Amongst size, morphology, and composition, all the optical properties showed the highest variability with changing size. The cross sections varied from 0.0001 to 0.1 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 > 100 nm. Therefore, while using the simplified core–shell representation of BC in global models, the influence of morphology on radiative forcing estimations might not be adequately considered. The Ångström absorption exponent (AAE) varied from 1.06 up to 3.6 and increased with the fraction of organics (forganics). Measurement results of AAE ≫ 1 are often misinterpreted as biomass burning aerosol, it was observed that the AAE of purely black carbon particles can be ≫ 1 in the case of larger BC particles. The values of the absorption enhancement factor (Eλ) via coating were found to be between 1.01 and 3.28 in the visible spectrum. The Eλ was derived from Mie calculations for coated 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 tabulated results of the MSTM method. Spanning an extensive parameter space, the developed parameterization scheme showed promisingly high accuracy up to 98 % for the cross sections, 97 % for single-scattering albedos (SSAs), and 82 % for the asymmetry parameter (g).

Constraining the particle-scale diversity of black carbon light absorption using a unified framework

“…For ρ BC >1, MAC BC decreases with increased ρ BC , and the rate of decrease is dependent on R BC . The finding of decreased light absorption for ρ BC >1 is consistent with a recent study which also found decreased MAC BC with increasing aggregate size (Romshoo et al, 2021). Best-fit lines for the scaling of MAC BC as a function of R BC are shown as solid lines in Fig.…”

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

“…This study provides experimental support for previous theoretical work based on BC as fractal aggregates (e.g., Kahnert, 2010;Adaichi et al, 2010;Kahnert and Kanngießer, 2020;Smith and Grainger, 2014;Romshoo et al, 2021;Liu et al, 2019;Luo et al, 2018). Analysis of various modelling methods for BC particles showed that the selection of an appropriate size representation (polydisperse size method) and an appropriate morphological representation (aggregate morphology) could result in a more realistic prediction of BC's optical properties (s !…”

Siberian Arctic black carbon: gas flaring and wildfire impact

“…The AAE for the highly aged aerosols measured during the periods of low BC was lower than 1, reaching values as low as 0.2, which is addressed mostly to the aerosol size distribution (large particles) and internally mixed BC particles (Cappa et al, 2016). As shown by modeling studies (Virkkula, 2021), pure BC particles coated by non-absorbing coating can have AAE in the range from <1 to 1.7, depending also on the morphology of the fractal aggregates (Romshoo et al, 2021). The AAE becomes more reliable in periods of higher aerosol concentration levels in the cold period, when it ranged from 0.6 to 1.35.…”

“…Furthermore, chemical evolution after the emissions and atmospheric aging (i.e. aerosol mixing state, particle morphology and size distribution) additionally influence aerosol absorption, which can be noticed especially for the long-range transported air masses (Cappa et al, 2016;Saleh et al, 2013;Romshoo et al, 2021). (Forrister et al, 2015) have shown that BrC emitted from wildfires was highly unstable, with 6% of BrC remaining above background levels after two days.…”