the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 May 2024
Technical note: Numerical quantification of the mixing states of partially-coated black carbon based on the single-particle soot photometer: Implication for global radiative forcing
Abstract. In this work, we have performed a series of numerical investigations on the mixing states of partially-coated black carbon (BC) based on the single-particle soot photometer (SP2). First, we calculated the scattering signal returned from partially-coated BC based on the SP2 measurement, and then the mixing states were determined using Mie theory, where the difference between the determined and "true" mixing states can represent the uncertainties of the SP2 measurement. We found that the SP2 measurement can provide good estimates for small, heavily coated BCs and shows better performance for fully coated BCs. However, the microphysical properties of BCs have a significant impact on the accuracy of the SP2 measurement; sometimes deviations of about -22 % to 28 % were observed for the determined particle-to-core size ratio (Dp/Dc). When considering a size distribution, the error in the effective radius is generally within about -17 % to 8.8 %. We also investigated the effects of Mie-based models using the SP2 determined and volume-mean Dp/Dc on the radiative effects of partially-coated BC. We found that both Mie models based on the volume mean and SP2 determined mixing states overestimate the absorption enhancement (Eabs) and direct radiative forcing (DRF) of BC. The Mie model based on the SP2 measurement does not necessarily provide worse estimates of radiative properties, although some errors occur in the determination of the mixing states, since the fraction of the coated core (F) in the particle scale is an important factor affecting Eabs and DRF. Sometimes the inaccurate measurements of the mixing states by SP2 would offset the influence of F. Moreover, our results based on the Mie model considering F can significantly improve the estimates for the absorption and DRF of partially-coated BC, although the morphology also has some influence. Therefore, we suggest adding a parameter F to model the radiative effect of BC in climate modeling even when a Mie-based model is used.
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RC1: 'Review of egusphere-2024-1155', Anonymous Referee #1, 18 Jun 2024
Review of egusphere-2024-1155 by Jie Luo, Miao Hu, et al.
This manuscript evaluates the accuracy of core-shell Mie theory in comparison with a computationally accurate MSTM model, for fractal (Df=1.8) and compact (Df=2.6) aggregates with a varied coated fraction F. The authors have not directly addressed the fact that coatings will cause soot aggregates to become compact. Instead, they have cleverly included the no-restructuring and full-restructuring cases in their varying of F (the fraction of aggregate inside the coating droplet). This results in a test data set that includes the more realistic case where only the coated part of the aggregate is restructured.
Unfortunately, the manuscript has some major issues. It does not accurately acknowledge the existing literature to an extent that I have to recommend rejection for more than one reason. To fix these issues, the manuscript requires a complete rewrite, with new title, new figures, and reframing (in terms of the literature context). Therefore, it only makes sense to re-submit a new manuscript in the future.
First, the authors have presented their work as "SP2 vesus truth" which is completely misleading. The SP2 measures scattering cross sections (line 113). The SP2 does not assume a core-shell configuration. It is SP2 users who have analyzed SP2 data using this assumption, but it is not a feature of the instrument. For example, Liu et al. (2017 http://dx.doi.org/10.1038/ngeo2901) showed that improved SP2 analysis models can be used successfully. The authors must therefore rewrite their manuscript as "core-shell vs MSTM". With this new scope, it will become obvious that the manuscript must be completely rewritten to cite and quantitatively discuss the many papers which have already discussed this topic. It needs to be clear why this manuscript should be published since many core-shell vs MSTM (or DDA) papers exist.
Second, the fact that the relative position of the coating and core has a huge influence on absorption properties was also shown in many other studies. For example, by Fuller et al. (1999) and multiple others. This manuscript's approach to studying this topic might be a worthwhile contribution to the existing literature. But the authors have to show this. They have to cite and discuss those previous manuscripts if they believe their contribution adds to them. The authors' observation that partially coated aggregates can be treated as a sum of coated and uncoated particles was interesting and should be explored. Again, this would require completely rewriting the manuscript. Also, this would require the authors to study values of F from 0.0 to 1.0, and not only to 0.3. There is no reason to stop at 0.3.
Finally, the use of a global chemical transport model in the present study to illustrate the effects of the soot MAC introduced an unnecessary and distracting complexity. No parameters except MAC were varied in the model. Therefore, simply plot MAC. Using an extremely complex model to demonstrate a minor point (influence of F) does not provide scientific insight.
Citation: https://doi.org/10.5194/egusphere-2024-1155-RC1 -
RC2: 'Comment on egusphere-2024-1155', Anonymous Referee #2, 24 Jun 2024
This manuscript quantifies the uncertainty of the mixing state and absorption of partially-coated BC based on SP2 with numerical simulation, and points out that quantification of the mixing states of partially-coated black carbon based on the single-particle soot, as well as its impacts on BC absorption and radiative forcing. Based on the simulation, the authors suggest adding a parameter F to model the radiative effect of BC in climate modeling. In general, the manuscript is not rigorously organized and the figures are not clear. The following issues should be taken into consideration for improvement.
Major comments:
1. The manuscript highlights the importance of partially-coated BC, and there is large uncertainty when assuming BC is fully coated. The title is “Numerical quantification of the mixing states of partially-coated black carbon based on the single-particle soot photometer”, but there is no measurement data from SP2. In fact, it is just the difference between Mie theory and MSTM. This problem is common in current regional or climate model, not just for SP2. Zhang et al (2018) did similar work on numerical simulation of partially-coated BC absorption. In the introduction part, the progress of studying on partially-coated BC should be summarized and the novelty of this study should be pointed out.
Zhang, X., Mao, M., Yin, Y., and Wang, B.: Numerical investigation on absorption enhancement of black carbon aerosols partially coated with nonabsorbing organics, Journal of Geophysical Research: Atmospheres, 123, 1297–1308, https://doi.org/https://doi.org/10.1002/2017JD027833, 2018.
2. As stated in the manuscript, the model calculated MACBC is inconsistent with the measured MACBC, and most models underestimate MACBC based on the measured mass density and refractive index, why did the authors choose a MACBC of 7.5 m2g−1 in this study? Both fluffy and compact BC aggregates were considered in this study, and MACBC also varies for both BC shapes. How does MACBC affect the calculated absorption and radiative forcing?
3. In the fourth paragraph, it is mentioned twice that “BC is often partially-coated”. If it was written carefully, I think the authors want to emphasize the importance of partially-coated BC. However, the exact fraction of partially-coated BC in the real atmosphere is more convincing than current expression. In addition, the fraction of partially-coated BC in the real atmosphere also impacts on the uncertainties of climate model, so the uncertainty of climate model is not reliable when consider partially-coated BC alone.
4. The authors propose to consider not only the effects of mixing states (Dp/Dc) but also the effects of the proportion of the coated BC core (F) in climate model. So how to determine F value in climate model? Any suggestions?
Minor comments:
1. Lines 39: please add the corresponding references that point out that that simplifying the microphysical properties of BC aerosols can lead to inaccurate determination of mixing states.
2. Line 75: the second “of” should be changed as “and”.
3. The font size varies a lot in different figures. For figure 2, 5, 6, 7,10, 11 and 12, the font size should be enlarged.
4. In figure 2, 5 and 10, it is hard to figure out the results, because there are too many legends and the colors are hard to distinguish. Please replot and improve the quality.
5. What is the difference between “BC” and “BCs”? Why do the authors use these two expressions?
Citation: https://doi.org/10.5194/egusphere-2024-1155-RC2
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