Review of Hu et al.: Abundant water-soluble calcium coatings on fine Asian dust particles

The authors present a single-particle-scale study showing that calcium in freshly emitted Asian mineral dust is more soluble and reactive than commonly assumed. Using laboratory-generated dust from the Taklimakan and Gobi deserts and CCSEM analysis before and after water dialysis, they show that a significant fraction of Ca-containing particles carries water-soluble calcium, which the authors state occurs largely as nanometer-scale calcite and gypsum coatings on insoluble mineral cores. These coatings are claimed to dissolve rapidly without significantly altering particle size distributions, accounting for 20-40 % of the Ca mass in dust particles. The results could imply that Asian dust provides a faster and more effective source of alkalinity for atmospheric acid neutralization and ocean buffering than represented in many current models.

I think that the topic is interesting and might indeed have relevance for modeling the dust cycle in particular for East Asia, where the Ca fraction in the dust in particularly high.

It seems, however, that there are some additional explanations, clarifications and corrections or modifications of the conclusions required.

Major concerns:

• Low sample number – the authors base their conclusions on 4 samples from different deserts covering more than 500,000 km². As deserts can be highly variable on a sub-km scale, the precise location of sampling might be more relevant for the results than the desert where the sample was taken. Also, the sampling itself is largely unexplained.
• Unclear methodology description – there are some important details missing from the method description, and there are also some apparent mistakes in the classification tables.
• Images shown don’t support the mentioned observations – coatings are frequently mentioned but never shown. Also, a volume change of the particles is presented in the data but never shown in an image. If the analysis is really based on these images, there seem to be logical gaps regarding the volume and mass determination.
• General implications – Conclusions based on the lab-generated dust are not necessarily valid for ambient dust. Also, it remains unclear whether the observed dissolution state is a final one. At least estimates for the dissolution kinetics or a time series of dissolution should be done. The pH of the solvent significantly impacts the kinetics but is higher than what one would expect for cloud water. As a result, it is not clear, how relevant the results are for the atmosphere.

Details

L99: How was the location of sampling selected? Please give more details on the sampling, as it might have severe impact on the results (e.g. compare the compositional data for different sediment types here doi: 10.5194/acp-23-15815-2023).

L109: The 0.2 µm is the pore size, porosity would be e.g. a fraction of pores vs. solid volume

L117: How thick was the coating? From the images in Figure 6, it seems to be quite thick.

L120: two … spectrometers

L122: parameter… compositions - check for singular / plural and make consistent.

L123: … provided reproducible sizing … how was the reproducibility checked?

L123: What type of quantification was used for obtaining the elemental composition from the spectra? Was there any type of correction (ZAF or similar) applied? Was there a quantification threshold applied? The information is relevant in view of the small relative contributions used for classification. In L180 there is mentioned a ZAF-correction, but not which.

L129: Figure S2 is illegible in the PDF (resolution). The referred paper of Zhang & Iwasaka used TEM grids for the analysis, here we have polycarbonate filters. Can you please provide more evidence that the particle location remained stable (e.g. some higher resolution detail images before / after).

Also, how do you make sure that the state you observe after dialysis is a final state, i.e. everything soluble is dissolved, and not just an arbitrary intermediate state of dissolution? You refer to the problem in L336-338.

L150: The classification criteria in table S2 require a bit more explanation. Are all elements not in the list for a certain category required to be zero or undetected? If not, there would be overlaps e.g. between the Ca-C-O category and for example the Ca-S-O category (imagine 98.4% O, 0.8% S, 0.8% Ca).

Or are the criteria evaluated from top to bottom, using the first match?

Why is O included e.g. for Si-O and Al-Si-O, but not for Na-S-O?

Why is N included in Al-Si-O-Na? Why is C included in Al-Si-O-Ca?

A part is given in Wt%, another in At%. For what reason?

There are two categories with the same name (Al-Si-O-K), but slightly differing criteria.

L155: How was the volume calculated, which assumptions were made?

L160: How was the total mass of elemental Ca quantified? From the EDX spectra? Using oxide assumptions, or what?

L167: I assume v is the flow rate? Missing.

L181: If you classify particles with Ca > 0.5%, how do you use a 1.0% error margin?

L187: They are termed ‘mixed …’ in the S2 table. Keep it consistent.

L192: Table S3: What is the +/- variation in the table? Standard deviation?

L217: Wouldn’t one assume that the cloud water pH is generally lower than 6.4? E.g., references in doi: 10.1029/2019GL082067

L225: I don’t understand what is meant by the decrease of the total number. What is the reference, from where the number decreased, and why is this specific to the single particle perspective (is there another in the paper?)?

L227: Which elements were regarded for comparing the mobility? I could imagine that Cl under the measurement conditions is more mobile.

L227: I don’t understand the logic behind the conclusion, that a stronger decrease in Ca-containing particle number compared to all dust particles points to a mixture. Wouldn’t it be the other way around? If all particles were mixtures, the number would decrease similarly for both (or remain constant).

L231: How do we know about gravel surfaces in both deserts, if we’re looking at the results of artificially generated dust? In a desert, there might be a Ca-dominated crust, which would probably have a considerable impact on the emissions process and is probably different for a topsoil, which was collected with a shovel.

L235: I understand from the method part, that the particle volume was estimated from the particle shape as seen in the electron microscope. However, for all particles shown in Figure 6 and S6, the electron images before and after dialysis are nearly identical. So, how can a mass loss be quantified?

Figure 1: The mass loss per particle doesn’t seem to be significant. Please comment on that.

How can on the one hand for GB-Sand the mass of Ca per particle decrease, but at the other hand the volume increase?

L237: Why does a different Ca particle mass indicate a different mineralogical composition? It should primarily come from a different particle size.

L239: For GB-Gobi the IQD gets larger. The min-max distance might well depend on the number of particles – the more you have to analyze, the higher the probability of extreme values. Please do some statistical checks on the significance.

L263: I don’t see any evidence for a coating. These can be simply internally mixed aggregates, which are quite common in desert soils.

Figure 2: Please restrict the figure labels to significant digits corresponding to the measurement error.

L273: The form factor, as you define it, is calculated using the perimeter squared, so it is extremely sensitive to that value. That means that single pixel differences of the segmentation map have a strong impact on the result. Looking at your Figure 6, the images taken after dialysis are softer than before, i.e. they have less well-defined edges. As both Ca and non-Ca particles show a decrease in form factor, did you make sure that the change in image quality is not the reason for that?

L274: The soluble Ca fractions you show in the images are not smooth.

L282: Where is Ca in albite?

L293: Combining the thickness of the coating and the observation of no morphological change – could it be that the coating just preserves the shape of the particle and the particle itself inside it dissolved? I have a hard time imagining that particles are on one hand claimed to be partly dissolved (medium Ca-particles, Figure 2), but on the other hand should not show any morphological change.

L297: Again, missing evidence for a coating.

L297: One would expect that the majority of compound here is crystalline (i.e. has a crystal lattice, is a mineral).

L299: If you have high resolution images, can you show the mentioned coating? It should also be visible on the EDX mapping.

Figure 6: The authors claim a coating of Ca substances here. However, the images demonstrate that Ca is in the center of the particle in all shown cases. While it might be on the backside, I don’t see any evidence of a coating. In the third row of images, also the Si seems to be gone, though it is difficult to compare the left and right mappings, as apparently the intensity-to-color scaling is different.

Spectra are illegible.

L312-358: This is an introductory paragraph and literature review. The content should be mainly moved into the introduction (and maybe shortened), only results from the present work and directly connected papers should be discussed in a results section.

L327: A trajectory analysis commonly related, in atmospheric science, to an air parcel transport history. I think an aerosol transport model is meant here.

L327: I don’t see the logical contrast to the previous sentence here.

L329-330: What is the connection to weathering here?

L331-338: That’s interesting in general, but I don’t see how it relates to this paper directly.

L353-355: I’ve seen no evidence for that, on the contrary. That would also need to be supported with calculations of dissolution kinetics or direct evidence of coatings, e.g. in mappings.