the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Measurement report: New insights into the mixing structures of black carbon on the eastern Tibetan Plateau: soot redistribution and fractal dimension enhancement by liquid‒liquid phase separation
Qi Yuan
Yuanyuan Wang
Yixin Chen
Siyao Yue
Jian Zhang
Yinxiao Zhang
Liang Xu
Dantong Liu
Pingqing Fu
Huiwang Gao
Abstract. Black carbon (BC, i.e., soot) absorbs radiation and contributes to glacier retreat over the Tibetan Plateau (TP). A lack of comprehensive understanding of the actual mixing state leads to large controversies in the climatic simulation of BC over the TP. In this study, ground-based sampling, electron microscopy analyses, and theoretical calculations were used to investigate the interactions among the liquid‒liquid phase separation (LLPS), soot redistribution in secondary particles, and fractal dimension (Df) of soot particles on the eastern rim of the TP. We found that more than half of the total analysed particles were soot-containing particles. One-third of soot-containing particles showed the LLPS phenomenon between organic matter and inorganic aerosols in individual particles, which further induced soot redistribution. The results show that a larger LLPS particle size, thicker organic coating, and smaller soot particles tended to drag soot from the sulfate core into the organic coating. The Df sequence is ranked as externally mixed soot (1.79 ± 0.09) < sulfate-coated soot (1.84 ± 0.07) < organic-coated soot (1.95 ± 0.06). We concluded that the soot redistribution process and high RH both promoted the morphological compaction of soot particles. This study indicates that soot-containing particles experienced consistent ageing processes that induced a more compact morphology and soot redistribution in the LLPS particles on the remote eastern rim of the TP. Understanding the microscopic changes in aged soot particles could further improve the current climate models and evaluations of BC’s radiative impacts on the eastern TP and similar remote air.
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Qi Yuan et al.
Status: final response (author comments only)
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RC1: 'Comment on acp-2022-831', Anonymous Referee #1, 09 Feb 2023
This manuscript by Yuan et al. reports detailed mixing states and shapes of soot particles mixed with organic matter and sulfate collected at the eastern Tibetan Plateau. They discussed liquid-liquid phase separation and redistribution of soot particles within particles. Mixing states and shapes of soot particles influence the optical properties of internally mixed particles and their radiation. Thus the results are important to the understanding of climate influence. My major concern is that it is probable that the mixing states and shapes that they measured could be influenced by both atmospheric processes and impaction on the substrates when collected. Therefore, I suggest more careful discussion of the influence of the changes on the filter should be provided. I also recommend having some discussion based on chemical and physical processes about liquid-liquid phase separation and soot redistribution.
Major comments
- The TEM images show mixing states after the particle collection on the substrates. Thus, changes in shapes and mixing states should be carefully discussed if they had changed in the air or on the substrate. Discussing the two-dimensional mixing states of particles on the substrate is acceptable. However, when discussing their mixing states in the atmosphere, the coating materials should cover the entire surface. When discussing the implication for the climate, the discussion should depend on their three-dimensional shape in the atmosphere. At least, the TEM images show that organic coatings cover the perimeter of the spread sulfate, which cannot be a realistic 3D shape in the atmosphere. The coating thickness in the TEM image may be different from that in the atmosphere as they spread over the substrate. Sulfates are also shrinking by losing water on the substrate and have some space with organic coatings (Fig. 3b). There are also some traces around the particles (Fig. 3b). As a result, the shapes and mixing states could have been different from their original or partially the same as the These points need to be clearly discussed in the paper.
- Although liquid-liquid phase separation and soot redistribution is interesting results, I suggest having more discussion based on chemical and physical processes. Why do they have such a process? What are the physical and chemical processes (e.g., the hygroscopicity of soot, surface tension, viscosity, etc.)? When did these processes occur? Some more discussion in Fig. 9 will be helpful in interpreting the results.
Specific comments
- Line 52 “This uncertainty in BC radiative forcing is largely” Are you discussing an uncertainty or “a large difference in several model studies” here? Is this uncertainty caused by only “the lensing effects of the coating”? I assume that different emission inventories are also the cause of large uncertainty.
- Line 121 “The equivalent circle diameter (ECD, d) and the equivalent volume diameter (EVD, D) were calculated according to the bearing area and bearing volume.” Is soot particle included in the plot? If so, EVD cannot be related to ECD because of its fractal shape.
- Line 151 “S4b). The sulfate core and OM-coating in secondary particles were identified as LLPS” Why? Please explain this reason.
- Line 157 “A laboratory study and field observations have shown that LLPS can drive soot in core–shell particles from inside inorganic aerosols to outer organic aerosols, which is called the soot redistribution phenomenon” Why does it happen? Please explain this soot redistribution phenomenon in more detail.
- Line 182 “Therefore, we can conclude that soot redistribution in secondary particles is a common phenomenon on Mt. Emei.” The results were obtained only from limited samples and periods. Therefore, it is difficult to have a general conclusion.
- Line 188 “Figure 5b shows that the entire particle size and coating thickness exhibited good correlations, suggesting that larger particles along with thicker OM-coatings can drive soot particles into the organics from the sulfate core due to LLPS.” I do not think the correlation suggests the latter sentence. There is a large gap between observation and the discussion.
- Line 202 “The results suggest that the coarser particles following the thicker Omcoatings captured more soot particles in the OM-coating during the redistribution process” Why can it be concluded that it happened “during the redistribution process”? Can they simply be coagulated in the atmosphere, not “during the redistribution process”?
- Line 204 “direct in situ evidence” I do not think it is direct and in situ evidence. They are obtained from the observation of filter samples.
- Line 205 “soot size” Is soot size provided? Fig. 4a shows that soot has a narrow size distribution. Which data should I see?
- Line 210 “The average Df of externally mixed soot on Mt. Emei was 1.79±0.09 (Table 1), which was slightly higher than that on the southeastern TP (1.75±0.08) (Yuan et al., 2019), suggesting that the sources of soot particles in the eastern TP atmosphere were more complex” First, I do not understand the interpretation of “more complex.” Second, values 1.79±0.09 and 1.75±0.08 essentially have no difference.
- Line 216 “The sulfate-coated soot and organic-coated soot particles had a higher CV (0.87 and 0.87, respectively), higher RN (0.41 and 0.42, respectively) and lower AR (1.61 and 1.61, respectively) than those of externally mixed soot (avg. CV=0.81, avg. RN=0.38, and avg. AR=1.63).” Interestingly, the sulfate-coated and organic-coated soot particles had nearly the same morphological parameters. Are they contradict the discussion of their fractal dimension in line 220? I do not see “a significant increase in fractal dimension” (line 220) when considering their error range and the plot in Fig 8a. The difference can be within an uncertainty range.
- Figure 7a. Please add a unit for the x-axis (nm).
- Figure 9. I suggest having more discussion in Fig. 9. What do (>90%) and (>70%) mean? At high RH, I guess sulfates deliquesced and had a much larger size. I suggest adding how the liquid-liquid separation and soot redistribution occur in this figure.
Citation: https://doi.org/10.5194/acp-2022-831-RC1 -
RC2: 'Comment on acp-2022-831', Anonymous Referee #2, 13 Mar 2023
The authors provide a measurement report about how black carbon (BC) is distributed within aged mixed organic/inorganic aerosol particles collected on the eastern Tibetan Plateau mountain site in July 2016. The used ground based collection on TEM grids and TEM and AFM to obtain size, mixing state and morphology. Basically, they confirm their previous result, Zhang et al. (2022), that liquid-liquid phase separation redistributes BC to the organic coatings for a wide range of relative humidities. In addition to their previous work, they deduced the fractal dimension (Df) of the BC and see a ranking with decreasing Df from externally mixed BC to sulfate coated BC to organic coated BC.
As the morphology of BC in internally mixed aged aerosol is clearly important for analyzing its radiative impact, II feel this measurement report should be published as it reconfirms previous work measured at different sites. However, I ask the authors to take the following comments/suggestions into account for a revised manuscript.
The reader would benefit, if the connection to their previous work (Zhang et al., 2022) would be made stronger throughout the whole manuscript. For example, it remains unclear to me whether there is a significant difference in the ratio between organic coating thickness and BC size as a threshold above which the BC redistributes to the organic coating between the present study and that of Zhang et al. (2022). There, the authors came up with a ratio of 0.24, now they state this ratio is 0.2. My feeling is there is no significant difference between these thresholds (as they are somewhat arbitrary), but the authors need to discuss this.
My other concern is the significance of the differences they observe in the fractal dimension between the different morphologies. I can see that the difference between externally mixed BC and internally mixed BC in Df is significant. I doubt that the small differences the authors see between sulfate coated BC and organic coated BC are significant. The authors need to explain in detail their uncertainty analysis for the values they provide in Table 1. While they state “The standard error for Df was calculated from the uncertainty in the mean-square fit considering the uncertainty in N and dp.”, the details remain unclear to the reader. In addition, they do not comment on that Df is higher at elevated RH for sulfate coated BC compared to organic coated BC while it is the opposite at lower RH.
Technical comment:
Line 149: I suggest citing here some of the relevant lab studies, in particular also the cryo TEM work of the Freedman group as well. In particular, she showed that there is a size dependence on LLPS (e.g. Altlaf et al., 2016).
References:
Altlaf et al.: “Role of nucleation mechanism on the size dependent morphology of organic aerosol”, Chem. Commun., (2016), 52, 9220.
Zhang et al.: “Liquid-liquid phase separation reduces radiative absorption by aged black carbon aerosols”, COMMUNICATIONS EARTH & ENVIRONMENT | (2022) 3:128 | https://doi.org/10.1038/s43247-022-00462-1
Citation: https://doi.org/10.5194/acp-2022-831-RC2
Qi Yuan et al.
Data sets
Measurement report: New insights into the mixing structures of black carbon on the eastern Tibetan Plateau: soot redistribution and fractal dimension enhancement by liquid‒liquid phase separation. figshare. Dataset. Yuan, Qi https://doi.org/10.6084/m9.figshare.21988439
Qi Yuan et al.
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