Preprints
https://doi.org/10.5194/acp-2022-542
https://doi.org/10.5194/acp-2022-542
 
02 Sep 2022
02 Sep 2022
Status: this preprint is currently under review for the journal ACP.

Variations of the density of ambient black carbon retrieved by a new method: importance to CCN prediction

Jingye Ren1,2, Fang Zhang2, Lu Chen1, and Jieyao Liu1 Jingye Ren et al.
  • 1College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, China
  • 2School of Civil and Environmental Engineering, Harbin Institute of Technology (Shenzhen), 518055 Shenzhen, China

Abstract. The effective density of black carbon (BC) is a crucial factor relevant to its morphology and mixing state that would add uncertainty in evaluating its climate effect. Here, we develop a new method to retrieve the effective density of ambient BC combining field observations with the Köhler theory. The uncertainty of the new retrieval method was evaluated within ±30 %, which is primarily caused by assumptions of the hygroscopic parameter of organics and the fraction of primary organic aerosols in non-hygroscopic or hygroscopic mode. Using the new method, we obtain that the ambient BC density during the campaign varies widely from 0.14 to 2.1 g cm−3, with a campaign mean density of 1.11±0.54 g cm−3 for internally-mixed BC that accounts for 79±18 % of total BC particles. The retrieved values fall within the range of typical density of internally-mixed BC reported in the literatures. We further examined the sensitivity of cloud condensation nuclei (CCN) number concentrations (NCCN) prediction to variations of BC density, showing an uncertainty of −28 %~11 % in calculating NCCN at supersaturations of 0.2 % and 0.4 % by varying the BC density within the retrieved ranges. We also find that the NCCN is more sensitive to the variations of BC density when it is <1.0 g cm−3, illustrating a necessity of accounting for such effect closer to source regions where the BC particles are mostly freshly emitted. The CCN closure achieves when introducing the retrieved real-time BC density and mixing state. This study provides a unique way of utilizing field measurements to infer ambient BC density and highlights the importance of applying varying BC density values in models when predicting CCN and assessing its relevant climate effect.

Jingye Ren et al.

Status: open (until 14 Oct 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-542', Anonymous Referee #1, 22 Sep 2022 reply
  • RC2: 'Comment on acp-2022-542', Anonymous Referee #2, 26 Sep 2022 reply

Jingye Ren et al.

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Short summary
The effective density of BC is closely linked to its morphology and mixing state and could cause uncertainty in evaluating CCN activity. We develop a new method to retrieve the mixing state and effective density of BC in urban atmosphere. We find that the mean retried density of internal-mixed BC was lower than the value assuming void-free spherical structures. Our study suggests the importance of accounting for varying BC density in models when assessing its climate effect in urban atmosphere.
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