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Discussion papers
https://doi.org/10.5194/acp-2019-244
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-244
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 20 May 2019

Research article | 20 May 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Mixing characteristics of refractory black carbon aerosols determined by a tandem CPMA-SP2 system at an urban site in Beijing

Hang Liu1,2, Xiaole Pan1, Dantong Liu3, Xiaoyong Liu1,4, Xueshun Chen1, Yu Tian1, Yele Sun1,2,4, Pingqing Fu5, and Zifa Wang1,2,4 Hang Liu et al.
  • 1State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029, China
  • 2University of Chinese Academy of Sciences, Beijing, 100049, China
  • 3Department of Atmospheric Sciences, School of Earth Sciences, Zhejiang University, Hangzhou, Zhejiang, 310027, China
  • 4Center for Excellence in Regional Atmospheric Environment, Chinese Academy of Science, Xiamen, 361021, China
  • 5Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China

Abstract. Black carbon aerosols play an important role in climate change by absorbing solar radiation and degrading visibility. In this study, the mixing state of refractory black carbon (rBC) at an urban site in Beijing was studied with a single particle soot photometer (SP2), as well as a tandem observation system with a centrifugal particle mass analyzer (CPMA) and a differential mobility analyzer (DMA), in early summer of 2018. The results demonstrated that the mass-equivalent size distribution of rBC exhibited an approximately lognormal distribution with a mass median diameter (MMD) of 171.2 nm. When the site experienced prevailing southerly winds, the MMD of rBC increased notably by 19 %. During the observational period, the ratio of the diameter of rBC-containing particles (Dp) to the rBC core (Dc) was 1.20 on average for Dc = 180 nm, indicating that the majority of rBC particles were thinly coated. The Dp / Dc value exhibited a clear diurnal pattern, with a maximum at 1400 LST and an enhancing rate of 0.013/h; higher Ox conditions increased the coating enhancing rate. Bare rBC particles were primarily in a fractal structure with a mass fractal dimension (Dfm) of 2.35, with limited variation during both clean and pollution periods, indicating significant impacts from on-road vehicle emissions. The morphology of rBC-containing particles vairied with aging processes. The mixing state of rBC particles could be indicated by the mass ratio of non-refractory matter to rBC (MR). In the present study, rBC-containing particles were primarily found in an external fractal structure when MR < 1.5 and changed to a core-shell structure when MR > 6, at which the measured scattering cross section of rBC-containing particles was consistent with that based on the Mie-scattering simulation. We found only 9 % of the rBC-containing particles were in core-shell structures on clean days with a particle mass of 10 fg, and the number fraction of core-shell structures increased considerably to 32 % on pollution days. Considering the morphology change, the absorption enhancement (Eabs) was 11.7 % higher based on core-shell structures. This study highlights the combined effects of morphology and coating thickness on the Eabs of rBC-containing particles, which will be helpful for determining the climatic effects of BC.

Hang Liu et al.
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Short summary
The bare black carbon (BC) was in a fractal structure. With coating thickness increasing, BC changed from a fractal structure to a core-shell structure. In the ambient atmosphere, plenty of BC particles were not in a perfect core-shell structure. This study brought attention to the combined effects of morphology and coating thickness on the absorption enhancement of BC-containing particles, which would be helpful for determining the climatic effects of BC.
The bare black carbon (BC) was in a fractal structure. With coating thickness increasing, BC...
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