the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Measurements and Calculations of Enhanced Side/Back Scattering of Visible Radiation by Black Carbon Aggregates
Carynelisa Haspel
Cuiqi Zhang
Martin Johann Wolf
Daniel James Cziczo
Maor Sela
Abstract. Aerosol particles have both natural and anthropogenic origins and are ubiquitous in the atmosphere. One particularly important type is carbonaceous aerosol, including a specific subset, often termed ‘elemental carbon’ chemically or ‘black carbon’ (BC) radiatively. Carbonaceous aerosol particles have implications for atmospheric chemistry, human health, and climate both directly and via their ability to act as site of cloud droplet or ice crystal formation. Laboratory experiments and theory are needed to better understand these particles, specifically their radiative impact. We present here laboratory measurements of side/back scattering of visible radiation by analogues of atmospheric BC aggregates obtained using a depolarizing optical particle counter and accompanying theoretical calculations of scattering by compact and fractal theoretical BC aggregates. We show that with random-orientation, the theoretical calculations reproduce the qualitative behavior of the measurements but are unable to reproduce the highest values of the linear depolarization ratio; we are only able to obtain high values of the linear depolarization ratio using fixed orientation. Thus, we suggest that it is possible that models of scattering by BC aggregates that employ the random orientation assumption/option may underpredict the linear depolarization ratio of actual BC aggregates. Both our measurements and our theoretical calculations point to the possibility that bare (uncoated) BC aggregates, as opposed to the aged/coated BC or soot that was investigated in previous studies, can exhibit higher backscattering linear depolarization than previously assumed.
Carynelisa Haspel et al.
Status: open (extended)
-
RC1: 'Comment on egusphere-2023-556', Anonymous Referee #1, 15 May 2023
reply
General comments:
The authors of this study examined the depolarization ratio of pure black carbon fractal aggregates. Based on previous experimental data, the depolarization ratio (DPR) has been modeled and the model has been compared to the measured DPR obtained through SPIN measurements. It has been demonstrated that using random orientation in T-matrix models results in an underestimation of the DPR. The purpose of this article is to highlight the importance of using fixed orientation when modeling the optical properties of BC fractal aggregates. The findings of this study are original in that they extend the previous work done by Sela and Haspel (2021), and there is a novelty to the analysis. I would like to provide some comments on the paper, and once these comments have been addressed, I recommend it for publication.
Specific comments:
- 1. The authors begin their introduction by describing previous research. In my opinion, however, it is missing context about why we need to investigate the DPR of soot particles and how they might be relevant to remote sensing or radiative forcing estimates.
- It would be better to move some information, in particular the mathematical equations in Section 2.2, to the Appendix about theoretical methods in order to make the publication easier to read.
- The difference between eq. 8 and eq. 9 is not clear. The optical efficiency can be calculated as the ratio between the optical cross-section and the geometric cross-section. You have taken the ratio of two geometric cross-sections in equation 8.
- In the results of the total scattered intensity over theta-sca (for example, Figs. 4, 6, 7 You may also so on), the authors have pointed out the differences in soot characteristics using terms such as SC_300_35, etc. There is an overwhelming amount of terminology used in the caption of these results, which makes it difficult for the reader to understand them. In each figure, it is recommended to indicate the point of difference within the panels. For examples in Fig. 4, the difference between the panels is the change in the mobility diameter. A description of the corresponding mobility diameter can be included in each panel by the authors. And specify the other common properties as a single line in the caption. Similarly, in the discussion, the authors can specify the relevant parameter that they are discussing in each section (rather than using long technical terms). The second part of this comment is a choice of style. The authors can choose to remain with their style.
- The authors have discussed the difference between the modelled DPR and the measured DPR at the end of section 3.2. This was done by comparing the DPR from Table 2 with those from Table 4. It is an important result. Ideally, the differences should be visualized as a 1:1 corelation with a linear regression fit.
- The results, particularly the tables about optical properties, should be moved to the Appendix or Supplementary sections.
- In Table 9, the authors provide the range of optical properties for different realisations of pure BC aggregates. It would be preferable if the range was expressed as a percentage change. The study by Romshoo et al., 2021 (Fig. 4) has also demonstrated that there is variability in the optical properties of pure BCFAs at 660 nm for 30 realisations at different mobility diameters and fractal dimensions. What is the comparison between your results and theirs? Romshoo, B., Müller, T., Pfeifer, S., Saturno, J., Nowak, A., Ciupek, K., Quincey, P., and Wiedensohler, A.: Optical properties of coated black carbon aggregates: numerical simulations, radiative forcing estimates, and size-resolved parameterization scheme, Atmos. Chem. Phys., 21, 12989–13010, https://doi.org/10.5194/acp-21-12989-2021, 2021.
- Similarly to comment 7, it would be more beneficial to have the range of optical properties and the DPR expressed as a percent change for random vs fixed orientation. It is easier for others to use or cite this information.
- The authors mentioned the atmospheric implications in the conclusion, but no discussion/result has been provided. Would it be possible for the authors to explain how the DPR range (random vs fixed) affects the calculation of lidar backscattering? The sensitivity study can be demonstrated using a simple analytical equation, is there one?
- The SPIN measures at 135 degrees, which is why the authors were able to compare the modelling and measurements of DPR for a single wavelength. Would it be possible for the authors to mention in the discussion whether there are other instruments that can be used to measure other angles or all angles? Is it possible to improve the understanding of this subject by using such instruments?
Technical corrections:
- Line 53 – “vein” do the authors mean “way”
- Line 92 – Section 2.1 instead of “section 1”
- Line 41-44 – sometimes it is better to write two sentences instead of one long sentence
- Captions of figures – the entire text does not need to be in Bold
- At the end of the manuscript, in the Appendix, please provide the list of terms/abbreviations
Citation: https://doi.org/10.5194/egusphere-2023-556-RC1
Carynelisa Haspel et al.
Carynelisa Haspel et al.
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 124 | 28 | 6 | 158 | 2 | 2 |
- HTML: 124
- PDF: 28
- XML: 6
- Total: 158
- BibTeX: 2
- EndNote: 2
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1