Insights into the real part of natural sea spray aerosol refractive index in the Pacific Ocean

Fan, Chengyi; He, Bishuo; Guo, Shuqi; Qiu, Jie; Zhao, Chunsheng

doi:https://doi.org/10.5194/acp-25-5761-2025

Articles | Volume 25, issue 11

https://doi.org/10.5194/acp-25-5761-2025

© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-25-5761-2025

© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 25, issue 11

Research article

|

11 Jun 2025

Research article |

| 11 Jun 2025

Insights into the real part of natural sea spray aerosol refractive index in the Pacific Ocean

Chengyi Fan, Bishuo He, Shuqi Guo, Jie Qiu, and Chunsheng Zhao

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Preprint (discussion started on 15 Jan 2025)

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RC1:
'Comment on egusphere-2024-3527', Anonymous Referee #1, 30 Jan 2025

Review of the paper “Insights into the real part of natural sea spray aerosol refractive index in the Pacific Ocean” by Fan et al.
The paper presents an interesting study of the real part of the refractive index of sea salt measured using an optical trap and proposes a linear fit of the data. The authors also extrapolate the fit outside the data range and provide some evidence of why that might be justifiable. The motivation of the work is good and the paper is mostly well written but some additional details need to be provided and the same clear caveats need to be explicitly mentioned before the paper can be published.
General Comments
1. The work provides the RRI of salt at a narrow range of specific wavelengths (around 650 nm). This important limitation should be evident from the beginning, so the authors should mention the wavelength in the abstract and at the end of the introduction. This issue is explicitly mentioned only at the very end of the paper while should be evident at the beginning already.
2. The size range generated by the nebulizer and the size range of trapping of the AOT should be discussed in more detail and critically compared to “real” sea salt particles.
3. The linear parameterizations provided by equations (1) and (2) are calculated from data at RH>60%. Is there a specific reason why data at lower RH were not directly measured? Is that because the assumption of sphericity must be satisfied to use Mie and only above an RH of 60% one can make that assumption confidently (the salt fully deliquesced and therefore spherical?)? This point should be clarified.
4. Perhaps I missed it, but I could not find what the authors mean by “artificial sea salt”. What is it? Later in the paper they mentioned that artificial sea salt is a good surrogate, but sodium chloride is not (lines 227-228), so artificial sea salt is not sodium chloride, what is it then?
Specific Comments
Line 34, page 2: Are these values a radiative effect? If so, where, at the surface, at the top of the atmosphere, for the atmosphere itself? Also, why is the range so large?
Lines 41 and 42, page 2. I understand how that can be a limitation of thin disk bulk measurements, potentially, but why is that a limitation in electrodynamic balances, that should provide similar measurement options to AOT?
Line 43, page 2: “fixed value” fixed with respect to what?
Lines 57-58, page 2: How similar does the process of generation by an atomizer resemble that of sea salt formation in the sea (from bubble bursts?). Might that affect the size and perhaps also the chemical composition of the aerosols produced?
Lines 86-87, page 3: Are there any concerns related to salt exclusion (brine) while freezing the sample? This detail should be at least briefly discussed.
Line 109, page 4: Again, how well does the nebulizer reproduce sea particle sizes similar to those found in marine air?
Line 122, page 5: How are the samples unfrozen and brought to 20 C before nebulization? Can the details of the tawing process and rate affect the end results?
Figure 3. I am not sure the term “stimulated” for the peaks is correct because it is still a spontaneous although narrow Raman peak, at least that one reaches optical densities that might result in nonlinear effects within the droplet, which might not be the case here.
Line 140, page 6: that’s the case for spherically-symmetric particles.
Line 141: Although the fitting method apparently is described in Preston and Reid (2013), it would be useful to provide a brief overview here.
Line 144, page 6: “this method can reach…” can in ideal conditions or did actually achieve this precision/accuracy in the specific work presented here? This also brings up the question, are these precisions or accuracies?
Line 155, page 7: It is not clear why 0.7% was chosen if the error is 1.3%. The criteria for this choice should be discussed.
Figure 4: Even though each of the AOT and E-AIM points seems to fall within the error bars of each other, there still seems to be a consistent positive bias in the AOT data. Are there any hypotheses of why that might be?
Lines 163-166: Similarly, is there a possible explanation for the positive bias of the parameterization by Cotterell?
Line 173: Is there any evaporation of the droplet happening over such a long period of time? If so, how is the corresponding change in the index of refraction and size accounted for? I would guess that one could calculate the index of refraction for different subsections of these 3 hours and compare for example, the results at the beginning, at a few intermediate times, and at the end to see if there is a consistent trend.
Figure 5 and corresponding discussion: the range of salinity seems very narrow from about 33.9 to 35.2 ‰, a change of just a little bit more than 1‰. Is it reasonable to extrapolate any conclusion (as in line 190) from such a narrow range? Also, what is the error in salinity?
Equation (1): The authors should provide the uncertainties on the slope (they do provide that for the intercept).
Line 202 (and then again on lines 211 and 225), please provide more digits (the first non-zero) for the p-values
Lines 203-206: The agreement is interesting and encouraging, but one should clearly caution that the intercept has been calculated from a fit of data only at the high end of the full RH range (>60%), so it is a significant extrapolation and cautions should be required. Isn’t deliquescence and efflorescence playing any role here (which I would expect to be non-linear)?
Figure 6: what is the symbol for artificial sea salt? Is the artificial sea salt represented by the light brownish dots indicated in the legend as “Sea salt”?
Lines 215-216: These are good lines of evidence, but I would not be so categoric about the validity of the fit.
Line 228: is the RRI of dry sodium chloride reported by Wang and Rood at 0% RH or 40% RH, if at 40% then, as discussed by the authors with regard to ammonium sulfate in lines 214-216 then would not be a fair comparison, and the comparison for published values of sodium chloride should be made at the same RH.
Line 234: is no the dry RRI, meaning at 0% or 40% RH?
Table 1: how is rd calculated?

Citation: https://doi.org/10.5194/egusphere-2024-3527-RC1
- AC1: 'Reply on RC1', Chunsheng Zhao, 04 Mar 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3527/egusphere-2024-3527-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-3527-AC1
RC2:
'Comment on egusphere-2024-3527', Anonymous Referee #3, 05 Feb 2025
Fan et al. show a method to quantify the real part of natural sea spray aerosol refractive index and develop a linear regression relation to estimate the real part of the refractive index under different relative humidity. Overall, this study is well designed and has potential important impacts on reducing uncertainties of aerosol climate effects. However, there are some points I cannot fully support. Moreover, some critical information is missing. I have my comments below. Thus, I recommend major revision before considering it for publication.
General comments:
The authors claim that this method can measure particle sizes. However, the discussion of particle sizes is missing. This is critical for calculating the refractive index. Please add a discussion about how you measure particle sizes and what sizes of particles you used in your refractive index calculation. Also, what is the upper and lower limit of particle sizes your optical tweezer can measure?

It should be noted that the particles used in this manuscript are crystals, which have delinquency points. Since the significant fraction of these particles should be NaCl, I expect the delinquency point to be around 75% RH at 293 K. Therefore, it raises two questions. First, the particle should not be spherical below the delinquency point, which will lead to uncertainties by using the Mie simulation. Second, the linear correlation between n and RH in this manuscript is impractical from the RH range below the delinquency point. In addition, the delinquency point also depends on the temperature. Therefore, the proposed correlation might not be useful in the real world.

It will be useful to provide a chemical composition of particles analyzed in this study, which is important for understanding these particles' hygroscopicity and refractive index. Considering the relation between RH and scattering, the hygroscopic growth of SSA, which is chemical composition dependent, is important. Please discuss how the different chemical compositions can affect the derived relation.

How representative is the size of particles measured in this study compared to the real-world SSAs? Moreover, it is also unclear to me how you generate SSA particles. The authors mentioned they used an atomizer. Which type did you use? The atomizer is not suitable for generating coarse-mode particles. Typically, scientists use nebulizers to generate coarse-mode particles.

Specific comments:
The introduction section misses the discussion of the nephelometer, a well-established instrument to measure scattering coefficients.

Section 2.2: As mentioned in L56, this is a real-time measurement. How many particles/droplets can you measure every set, and how long does it take to measure 1 set sample?

L112: What is the smallest size the CCD camera can measure?

L150: What is the E-AIM IV model? Please provide some basic information and the input parameters you used in the SI.

L165: What is the “inherent overestimation in the parametrization” here?

L188: Are you ignoring the organics? If yes, please state the potential uncertainty in your results.

L235 eq 4: Please explain in the text what V_i, r_d and r_w are.

L243: How did you calculate AOD?
Citation: https://doi.org/10.5194/egusphere-2024-3527-RC2
- AC2: 'Reply on RC2', Chunsheng Zhao, 04 Mar 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3527/egusphere-2024-3527-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-3527-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Chunsheng Zhao on behalf of the Authors (04 Mar 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (07 Mar 2025) by Stefania Gilardoni

RR by Anonymous Referee #1 (22 Mar 2025)

RR by Anonymous Referee #3 (23 Mar 2025)

ED: Publish as is (24 Mar 2025) by Stefania Gilardoni

AR by Chunsheng Zhao on behalf of the Authors (24 Mar 2025) Author's response Manuscript

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Short summary

Marine aerosols play a critical role in weather and climate, and their real part of the refractive index (RRI) is a key factor in their radiative effects. We present a study of RRI measurements using optical tweezer technology and find that the calculated results of RRI using the traditional method disagree with the measurements. A parameterization of the RRI and relative humidity relationship is proposed, and it will improve the radiation calculation in numerical models.

Insights into the real part of natural sea spray aerosol refractive index in the Pacific Ocean

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