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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 29 Jul 2019

Submitted as: research article | 29 Jul 2019

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

Effective densities of soot particles and their relationships with the mixing state at an urban site of the Beijing mega-city in the winter of 2018

Hang Liu1,2, Xiaole Pan1, Yu Wu3, Dawei Wang1, Yu Tian1,2, Xiaoyong Liu1,4, Lu Lei1,2, 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
  • 3State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, No. 20 Datun Road, Beijing 100101, 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. The effective density (ρeff) of refractory black carbon (rBC) is a key parameter relevant to their mixing state that imposes great uncertainty when evaluating the direct radiation forcing effect. In this study, a novel tandem DMA-CPMA-SP2 system was used to investigate the relationship between the effective density (ρeff) and the mixing state of rBC particles during the winter of 2018 in the Beijing mega-city. During the experiment, aerosols with a known mobility diameter (Dmob) and known ρeff values (0.8, 1.0, 1.2, 1.4, 1.6, and 1.8 g/cm3) were selected and measured by the SP2 to obtain their corresponding mixing states. The results showed that the ρeff well represented the morphological variation in rBC-containing particles. The rBC-containing particles changed from an irregular and loose structure to a compact spherical structure with the increase in ρeff. A ρeff value of 1.4 g/cm3 was the morphological transition point. The morphology and ρeff value of the rBC-containing particles were intrinsically related to the mass ratio of non-refractory matter to rBC (MR). As the ρeff values of the rBC-containing particles increased from 0.8 to 1.8 g/cm3, the MR of the rBC-containing particles significantly increased from 2 up to 6–8, indicating that atmospheric aging processes were likely to lead to the reconstruction of more compact and regular particle shapes. During the observation period, the ρeff of most rBC-containing particles was lower than the morphology transition point independent of the pollution conditions, suggesting that the major rBC-containing particles did not have a spherical structure. Simulation based on an aggregate model considering the morphological information of the particles demonstrated that absorption enhancement of rBC-containing particles could be overestimated by ~ 17 % by using a core-shell model. This study highlights the strong dependence of the morphology of ambient rBC-containing particles on ρeff and will be helpful for elucidating the micro physical characteristics of rBC and reducing uncertainty in the evaluation of rBC climate effects and health risks.

Hang Liu et al.
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Status: final response (author comments only)
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Hang Liu et al.
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Short summary
The relationship among the effective density, rBC's coating thickness and rBC's morphology was investigated. rBC with larger effective density adopted a more regular shape due to more coating thickness. The effective density distribution of ambient rBC was also measured. From the information of effective density, the ambient rBC mainly adopt an irregular shape which can cause large uncertainties in the rBC's optical properties.
The relationship among the effective density, rBC's coating thickness and rBC's morphology was...