Preprints
https://doi.org/10.5194/acp-2020-1290
https://doi.org/10.5194/acp-2020-1290

  15 Jan 2021

15 Jan 2021

Review status: a revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.

Radiative properties of coated black carbon aggregates: numerical simulations and radiative forcing estimates

Baseerat Romshoo1, Thomas Müller1, Sascha Pfeifer1, Jorge Saturno2, Andreas Nowak2, Krzysztof Ciupek3, Paul Quincey3, and Alfred Wiedensohler1 Baseerat Romshoo et al.
  • 1Leibniz Institute for Tropospheric Research, 04318, Leipzig, Germany
  • 2PTB Physikalisch-Technische Bundesanstalt, 38116, Braunschweig, Germany
  • 3Environment Department, National Physical Laboratory (NPL), Teddington, TW11 0LW, UK

Abstract. The formation of black carbon fractal aggregates (BCFAs) from combustion and subsequent aging 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 radiative 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 radiative 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 BCFAs radiative 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 radiative properties are calculated using the multiple sphere T-matrix (MSTM) method. Amongst size, morphology, and composition, all the radiative properties showed the highest variability with changing size. The cross-sections varied from 0.0001 μm2 to 0.1 μm2 for BCFA Dmob ranging from 24 nm to 810 nm. After size or Dmob, the absorption cross-section (Cabs) and BC mass absorption cross-section (MACBC) showed the highest sensitivity towards composition or forganics, whereas the asymmetry parameter (g) showed higher dependence on morphology, which is represented by Df. The Ångstrom absorption exponent varied from 1.06 up to 3.6 and increases with the fraction of organics (forganics). The values of the absorption enhancement factor (Eλ) were found 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 towards modifications in morphology and composition. The black carbon radiative forcing ΔFTOA (Wm−2) decreases up to 61 % as the BCFA becomes more compact in morphology. Whereas, there is a decrease of > 50 % in ΔFTOA as the organic content of the particle increase up to 90 %. Based on our results, which showed a significant effect of coating and morphology on the BC radiative properties, a parametrization scheme for radiative 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 over an extensive parameter space, the developed parametrization scheme showed promisingly high accuracy up to 98 % for the cross-sections, 97 % for single scattering albedos (SSA), and 82 % for asymmetry parameter (g).

Baseerat Romshoo et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2020-1290', Anonymous Referee #1, 14 Feb 2021
    • AC1: 'Reply on RC1', baseerat romshoo, 19 May 2021
  • RC2: 'Comment on acp-2020-1290', Anonymous Referee #2, 08 Apr 2021
    • AC2: 'Reply on RC2', baseerat romshoo, 19 May 2021

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2020-1290', Anonymous Referee #1, 14 Feb 2021
    • AC1: 'Reply on RC1', baseerat romshoo, 19 May 2021
  • RC2: 'Comment on acp-2020-1290', Anonymous Referee #2, 08 Apr 2021
    • AC2: 'Reply on RC2', baseerat romshoo, 19 May 2021

Baseerat Romshoo et al.

Baseerat Romshoo et al.

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Short summary
The modifications in radiative properties of black carbon (BC) due to aging are presented and quantified in this study using a state-of-the-art description scheme of BC fractal aggregates. It is shown that the relative change in BC radiative forcing can be larger than 50 % as a function of changing fractal dimension and organic content. A comprehensive parameterization scheme for coated BC radiative properties is developed with applications for modeling, ambient, and laboratory-based BC studies.
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