This paper presents the spatial distribution of iron isotopes in aerosols over the equatorial Pacific, a region with limited previous data. The consistently higher isotope ratios compared to the crustal average are noteworthy and represent significant findings for understanding iron cycling in the ocean using isotopic approaches.

However, the evidence provided to attribute these high isotope ratios to the partial dissolution of aerosols seems insufficient to fully confirm this explanation. While the authors suggest partial dissolution as a possibility through the elimination of other potential sources, the influence of anthropogenic sources (e.g. fly ash) or sea spray cannot be definitively ruled out based on the current results.

To strengthen the conclusion, it would be preferable to include supporting data and further discussions, such as comparisons with EF of other elements, fractional Fe solubility (which are expected to be low), microscopic analyses, etc. Otherwise, the interpretation should remain more cautious, suggesting partial dissolution as one of several possible explanations.

Furthermore, there are several grammatical errors and expressions that are not scientifically appropriate. I recommend thoroughly revising the manuscript to ensure clarity and proper scientific writing.

Specific comments

Abstract

・L. 25: If “EUCFe” is abbreviation, please define it.

・L. 26: “-0.16” this “-“ is a hyphen, not a minus sign. It should be corrected.

1. Introduction

・L. 77: The conjunction “Therefore” is inappropriate in this context.

2. Sampling locations and methods

・L. 95-96: Please add references if there are any papers previously published regarding Fe studies.

・L. 97: Provide details on the sampler (e.g., model, manufacturer) if available.

・L. 104: “L.min^-1” should be written as “L min^-1” .  Please check the submission guidance.

・L. 125: Replace “a second time” with “twice”.

・L. 125: PFe should be defined as it appears for the first time here.

・L. 127, 130: The company name (Thermo) should be included.

・L. 133: Does this blank contain contamination from the sampling filter? If not, please add the information on them, since filters are usually the largest contamination source.

・L. 136: Why was the error verified with suspended particle samples? I expect that there was not enough aerosol sample for duplicate analyses, but you should mention them.

・L. 142: The δ⁵⁶Fe value for any reference materials should be reported here.

・L. 144: Why were river water samples used? Aerosol or sediment reference materials would be more appropriate for validating sample processing and analysis.

・L. 147: This explanation is unclear. Does this mean that blanks for elements other than Fe were not measured and were instead estimated based on Fe blank? Please measure other element blanks as well. The assumption that other elements follow crustal composition is not always valid (e.g., Zn is prone to contaminate).

・Figure 2: It is still complicated. Consider showing trajectories for each area in separate panels or using different colors for distinct areas.

3. Results

・L. 199: How about conducting a forward trajectory to confirm no volcanic emissions affected the sample?

・L. 212: I didn’t understand the meaning of “and by extension of sea spray.” Also, please add references here.

・Figure 3 (L. 248): Correct 0,07 to 0.07

4. Discussion

・The possibility of contamination from the ship's exhaust should also be addressed. For instance, please demonstrate that the concentrations of specific tracers (e.g., vanadium) are not high and show no correlation with δ⁵⁶Fe results.

・L. 259~271: Include the [EI_SW-ref]/[Na_SW-ref] value and discuss the potential impact of the sea surface microlayer (SML), which is enriched with bioactive trace metals (Tovar-Sanchez et al., 2014) and can be a source of Fe in the open ocean.

・L. 275-281. I understand that the δ⁵⁶Fe of volcanic materials don’t explain the high δ⁵⁶Fe values in the aerosols, but here you should explain that there was no impact of volcanic activities based on Fe concentrations (as you mentioned in the result),  back/forward trajectories, or other tracers if available.

・L. 283: Although Mead et al. (2013) implicated the low δ⁵⁶Fe originated from biomass burning (due to the low δ⁵⁶Fe of higher plant), Kurisu and Takahashi (2019) suggested that δ⁵⁶Fe of biomass burning is not negative, due to the influence of suspended soil. Thus, biomass burning cannot necessarily yields negative δ⁵⁶Fe vlaues. 

・L. 288-294: It is unclear from the trajectory why A238 alone suggests potential anthropogenic influence. Please present enrichment factors (EFs) for Pb or Zn as evidence for anthropogenic impact.

・L. 313-331: While EF > 10 typically indicates a strong influence from non-crustal sources, even EF = 2 suggests a significant contribution (50%) from other sources, potentially altering δ⁵⁶Fe. At least, EF value of A266 (4.94) should be discussed. Also, address why A238 also yields EF close to 1 in spite of the possible impact of anthropogenic components.

・L.353-362: Discuss the dissolution mechanisms (e.g., proton-promoted, ligand-promoted, or reductive ligand-promoted) for each reference and identify the most likely mechanism for this study.

・L. 376: Verify whether +0.23 should be －0.23.

・L. 377: Why did you choose －1.1‰ as a fractionation factor? Maters et al. (2022) suggested －1.8‰ as a fractionation factor, which might be applicable here. In this case, 1-f should be lower.

・L. 385-398: Did you measure Fe solubility in your samples? If the dissolution and separation occurred in the atmosphere, the solubility of these samples should be low. Also, discuss the fate of the dissolved phase—whether it remains in the atmosphere as a separate particle or is removed via wet deposition. Explain why the residual signal (low δ⁵⁶Fe) is not observed.

・L. 389: The value “52%” is obtained from size-separated samples and is not directly comparable.

・Table 4. Correct 0,14 to 0.14 for all entries.

Review of Manuscript Egusphere-2024-3777

Title: Iron isotopes reveal significant aerosol dissolution over the Pacific Ocean

by: Capucine Camin, François Lacan, Catherine Pradoux, Marie Labatut, Anne Johansen and James W. Murray.

General Comment: This paper deals with a crucial question in ocean biogeochemistry: the limited stock of bioaccessible iron (Fe) for primary production in a large part of the global ocean.  Far from epicontinental seas, Fe-bearing aerosols deposition represents a dominant part of Fe supply essential for primary production. In this context, Fe isotopic signatures have emerged as a sensitive tool for identifying and distinguishing soluble Fe sources that feed the ocean, by using isotopic fingerprinting methods. But tracing the sources that control the amount of atmospheric Fe reaching the oceans is extremely tricky because of the isotopic fractionation that happens within cloud waters, during the partial dissolution of aerosols.  The aim of this noteworthy work is to present valuable data addressing this topic over a region suffering from a lack of records on this issue. However, a number of points need to be clarified before publication and I therefore recommend a revision of the manuscript

Specific Comments:

Lines 69-70: I don't think it's fair to say that studies focusing on Fe isotopes in the marine environment are rare. The authors cite a number of them, but others relevant works can be added:

- Chen, T., Li, W., Guo, B., Liu, R., Li, G., Zhao, L., and Ji, J.: Reactive iron isotope signatures of the East Asian dust particles: Implications for iron cycling in the deep North Pacific, Chem. Geol., 531, 119342, https://doi.org/10.1016/j.chemgeo.2019.119342, 2020.

- Fitzsimmons, J. N. and Conway, T. M.: Novel Insights into Marine Iron Biogeochemistry from Iron Isotopes, Annu. Rev. Mar. Sci., 15, 383–406, https://doi.org/10.1146/annurev-marine-032822-103431, 2023.

- König, D., Conway, T. M., Hamilton, D. S., and Tagliabue, A.: Surface Ocean Biogeochemistry Regulates the Impact of Anthropogenic Aerosol Fe Deposition on the Cycling of Iron and Iron Isotopes in the North Pacific, Geophys. Res. Lett., 49, e2022GL098016, https://doi.org/10.1029/2022GL098016, 2022.

Lines 72-73: I think the reader would appreciate having in the text the key values of Fe isotope signatures reported by Wang et al. (2022) for natural and anthropogenic aerosol sources. Please, add these data.

Lines 99-100: Please specify the HNO₃ grade used for membranes cleaning.

Lines 103-105: The variations in air flow during particles sampling (from 8 L.min^-1 for some samples to 28 L.min^-1 for the others) may cause isokineticity differences that could influence the size range of collected aerosols. Please calculate and report the air speed at the inlet in the two cases, to estimate the extent to which particle sizes may be affected.

Figure 2 (L. 161-164): The color code used to differentiate the trajectories of air masses is, in my opinion, unclear. Please, use a color code referring to the areas from which air masses originate.

Table 2: I don’t understand why a given element (e.g., Ba, V and Rb) presents different detection limits for some concentrations? The analytical detection limit refers to the variance of the analytical blank of a measured element and then for an optimized protocol, applied to all samples, for the considered element. Thank you for clarifying this point.

Lines 371-382: If Δ⁵⁶Fe_{(solution – particle)} in Eq. 4 represents the initial fractionation step (i.e., for a fractional Fe solubility close to zero), the value -1.8‰ (Fig.4 in Maters et al., 2022) seems more relevant.

Lines 412-413: The separation by shattering of the leached fraction of particles, enriched in light Fe (⁵⁴Fe), from the residual solid phase, containing heavy isotopes (⁵⁶Fe and ⁵⁷Fe) in larger proportions, is a central point to consider, to explain the observed isotopic composition of collected samples (except A 238). As evidenced by many authors (e.g., Kurisu et al., 2021; 2024; Mead et al., 2013; Wei et al., 2024), negative δ⁵⁶Fe values are, most of the time, measured in the fine fraction of aerosols (PM₁ or, at least, PM_2.5). There are convincing evidences (ex. Conway et al., 2019) liking these observations to a strong influence of non-crustal sources (anthropogenic combustion sources for example). This is particularly true in multi-influenced oceanic environments (see Kurisu et al., 2021). In the present study, Fe enrichment factors (relative to Ti) suggest the dominance of crustal Fe (except for the A266 sample, which should be carefully re-examined). We cannot therefore consider that we are in a multi-influenced oceanic environment. Then, in my opinion, if the collected particles display only δ⁵⁶Fe values greater than the UCC mean isotopic composition (δ⁵⁶Fe = + 0.07‰), it’s because fine particles generated by shattering and enriched in light Fe are not (or very poorly) collected in the sampling conditions of the EUCFe cruise (only particles > 1 μm are collected). These “secondary” particles, produced by shattering, are undoubtedly present in the fine fraction of aerosols collected in multi-influenced oceanic environments, but not as a major component, by comparison with pyrogenic particles produced by high-temperature (industry, traffic, coal combustion, biomass burning) processes.

Technical Comments

• Sometimes hyphens are used instead of minus signs ( L.32; L. 71; Table 3; L. 230; L. 255; etc..). Please check.
• Line 376: a coma should be deleted between “observed” and “values”

References cited:

Conway, T.M., Hamilton, D.S., Shelley, R.U., Aguilar-Islas, A.M., Landing, W.M., Mahowald, N.M., John, S.G., 2019. Nat. Commun., 10, 2628. https://doi.org/10.1038/s41467-019-10457-w

Kurisu, M., Sakata, K., Uematsu, M., Ito, A., Takahashi, Y., 2021. Atmos. Chem. Phys., 21, 16027–16050. https://doi.org/10.5194/acp-21-16027-2021

Kurisu, M., Sakata, K., Nishioka, J., Obata, H., Conway, T. M., Hunt, H. R., Sieber, M., Suzuki, K., Kashiwabara, T., Kubo, S., Takada, M., and Takahashi, Y, Geochim. Cosmochim. Acta, 378, 168–185, https://doi.org/10.1016/j.gca.2024.06.009, 2024.

Maters, E.C., D. S. Mulholland, P. Flament, J. de Jong,, N. Mattielli, K. Deboudt, G. Dhont and E. Bychkov, 2022. Chemosphere, 299, 134472 https://doi.org/10.1016/j.chemosphere.2022.134472

Mead, C., Herckes, P., Majestic, B.J., Anbar, A.D., 2013. Geophys. Res. Lett., 40, 5722–5727. https://doi.org/10.1002/2013GL057713

T. Wei, Z. Dong , C. Zong, X. Liu, S. Kang, Y. Yan and J. Ren, 2024. Earth-Science Reviews, 258, 104943. https://doi.org/10.1016/j.earscirev.2024.104943