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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-836
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
04 Oct 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
The Absorption Ångström Exponent of black carbon: from numerical aspects
Chao Liu1,2, Chul Eddy Chul3, and Yan Yin1,2 1Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, China
2Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, School of Atmospheric Physics, Nanjing University of Information Science & Technology, Nanjing 210044, China
3Desert Research Institute, Reno, 89512, USA
Abstract. The Absorption Ångström Exponent (AAE) is an important aerosol optical parameter used for aerosol characterization and apportionment studies. The AAE of black carbon (BC) is widely accepted to be 1.0, although observational estimates give a quite wide range of 0.6~1.1. With considerable uncertainties related to observations, a numerical study is a powerful method, if not the only one, to provide a better and more accurate understanding on BC AAE. This study calculates BC AAE using realistic particle geometries based on fractal aggregate and an accurate numerical optical model (namely the Multiple-Sphere T-Matrix method). At odds with the expectations, BC AAE is not 1.0, even when BC is assumed to have small sizes and a wavelength independent refractive index. With a wavelength independent refractive index, the AAE of fresh BC is approximately 1.05, and is quite insensitive to particle size distribution. BC AAE goes lower when BC particles are aged (compact structures or coated by other scattering materials). For coated BC, we prescribed the coating thickness distribution based on a published experimental study, where smaller BC cores were shown to develop thicker coating than bigger BC cores. Both Compact and Coated BC the AAE ranges, at realistic particle sizes. For both Compact and Coated BC, the AAE is highly sensitive to particle size distribution, ranging from approximately 0.8 to 1.0 for relatively large BC with wavelength-independent refractive index. When the refractive index is allowed to vary with wavelength, a feature with observational backing, the BC AAE shows a much wider range. We propose that the presented results herein serve as a comprehensive guide for the response of BC AAE to BC size, refractive index, and geometry.

Citation: Liu, C., Chul, C. E., and Yin, Y.: The Absorption Ångström Exponent of black carbon: from numerical aspects, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-836, in review, 2017.
Chao Liu et al.
Chao Liu et al.
Chao Liu et al.

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Short summary
The Absorption Ångström Exponent (AAE) of black carbon (BC) is widely accepted to be 1.0, although observational estimates give a quite wide range of 0.6~1.1. This study investigates BC AAE numerically using realistic particle properties and accurate numerical models. The significantly influence of BC microphysical properties on BC AAE is revealed by simple linear formulas, and the widely accepted BC AAE value of 1.0 is not right for even small BC with wavelength-independent refractive index.
The Absorption Ångström Exponent (AAE) of black carbon (BC) is widely accepted to be 1.0,...
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