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Discussion papers
https://doi.org/10.5194/acp-2020-224
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-224
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 26 Mar 2020

Submitted as: research article | 26 Mar 2020

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This preprint is currently under review for the journal ACP.

The absorption Angstrom exponent of black carbon with brown coatings: effects of aerosol microphysics and parameterization

Xiaolin Zhang1,2,3, Mao Mao1,2, and Shihao Tang3 Xiaolin Zhang et al.
  • 1Key Laboratory of Meteorological Disaster, Ministry of Education, Nanjing University of Information Science & Technology, Nanjing, 210044, China
  • 2School of Atmospheric Physics, Nanjing University of Information Science & Technology, Nanjing, 210044, China
  • 3National Satellite Meteorological Center, China Meteorological Administration, Beijing, 100081, China

Abstract. Aerosol absorption Angstrom exponent (AAE) is a crucial optical parameter for their apportionment and characterization. Due to considerable inconsistences associated with observations, a numerical research is a powerful means to give better understanding of the AAE of aged BC aerosols. Numerical studies of the AAE of polydisperse BC aggregates with brown coatings using the exact multiple-sphere T-matrix method (MSTM) are performed. The objective of the study is to thoroughly assess the AAE of coated BC influenced by their observation-based detailed microphysics and then provide a new AAE parameterization for application. At odds with our expectations, BC coated by thin brown carbon with more large particles can have an AAE smaller than 1.0, indicating that BC aerosols internally mixing with brown carbon can even show lower AAE than pure BC particles. The AAE of BC with brown coatings is highly sensitive to absorbing volume fraction of coating, coated volume fraction of BC, shell / core ratio, and particle size distribution with a wide variation, whereas the impacts of BC geometry and BC position within coating are trivial. The AAE of BC with brown coatings can be larger than 3.0, if there are more small coated BC particles, heavy coating, or more brown carbon. However, the AAE of BC with non-absorbing coating shows weakly sensitive to particle microphysics with values around 1.0 (i.e., 0.7–1.4), suggesting the substantial role of absorbing volume fraction of coating in the AAE determination. With more realistic BC geometries, our study also indicates that occurrence of brown carbon may be made confidently unless AAE > 1.4. In addition, we present a parameterization of the AAE of coated BC with a size distribution on the basis of numerical results, which can act as a guide for the AAE response to absorbing volume fraction of coating, coated volume fraction of BC, and shell / core ratio. Our findings can improve the understanding and application of the AAE of BC with brown coatings.

Xiaolin Zhang et al.

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Short summary
The understanding on the absorption Angstrom exponent (AAE) of aged BC particles affected by their microphysics is numerically evaluated. With the key sensitive microphysical parameters, a simple parameterization of the AAE of coated BC with a size distribution is then proposed. It is found that BC coated by thin brown carbon with more large particles can show an AAE smaller than pure BC particles. Our findings can improve the understanding and application of the AAE of BC with brown coatings.
The understanding on the absorption Angstrom exponent (AAE) of aged BC particles affected by...
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