This paper provides a comprehensive summary of iron isotopes in aerosols, including analytical methods. Each paper is carefully read and explained, but I believe further discussion is necessary to integrate the findings more cohesively.

For example, a more comprehensive discussion could be achieved by taking into account previous data, such as (i) differences in δ⁵⁶Fe endmembers among different papers and (ii) the extent to which isotope fractionation caused by chemical processes in the atmosphere actually alters the δ⁵⁶Fe composition of soluble iron in aerosols.

Additionally, I suggest reconsidering the structure of the manuscript. For example, some of the discussions in each subsection in Section 3 are only summaries of the other subsections or continuation of explanations; it would be more effective to create a separate discussion section to provide a more comprehensive synthesis.

Also, the same references appear several times in one paragraph, making it difficult to read, and it would be better to reduce such redundancies.

Specific points are described below:

• L. 111: It is unclear what “marine source materials” means.
• L. 136: Please spell out the “IRMM” at first mention.
• Section 2.2: I suggest adding information on the required Fe amount for TIMS measurement and the purification method for MC-ICP-MS analysis.
• L. 179: Please include the procedural blank value.
• L. 188-191: I recommend moving this paragraph to another subsection within Section 2 and providing further explanation regarding aerosol sample processing, including sampling, acid digestion, extraction procedure, and so on. Otherwise, the explanation seems a bit abrupt.
• Section 3.2: It would be more effective to summarize previous aerosol δ⁵⁶Fe results in a figure.
• Table S1 and S2: Please add more information such as:

            (1) A simple explanation of the sample (e.g. whether it was collected near specific sources or in urban/suburban areas);

            (2) Soluble Fe extraction method (if available);

            (3) If it is a certified reference material, the name of CRM.

• L. 259: Biomass burning is not mentioned in Section 3.2.3.
• L. 262: This sentence could be omitted.
• L. 302~: An explanation of Strategies I and II is necessary here.
• L. 313~: I found it unclear that Strategy I is isotopically lighter than Strategy II; it rather appears that Strategy II yields lower δ⁵⁶Fe values. This might be due to δ⁵⁶Fe being shown as a range. Please consider a more effective way for a clearer explanation. In addition, the reasons for the different δ⁵⁶Fe values between Strategies I and II should be included.
• L. 324~: This paragraph may be unnecessary.
• L. 330: I found the title "anthropogenic aerosols" unclear; the relationships among Sections 3.2.2, 3.2.3, and 3.3 were not obvious. I understood that 3.2.3 describes materials that can be atmospheric anthropogenic aerosols. Therefore, I suggest changing the title to something like “Materials that can be emitted as aerosol particles” or simply “fly ash and road dust”. It would also be beneficial to review the structure of 3.2.2 and 3.2.3 to determine which discussion should be in the same section or not. Combining the two sections into one would be another option.
• L. 353~: This paragraph may not be necessary or could be simplified.
• L. 380~: As mentioned earlier, this section would be better placed in Section 4 or another discussion section.
• L. 406: The meaning of "total" should be clarified, such as by stating “total (acid digested).”
• L. 409: It may be more effective to organize sections based on a different perspective (e.g., sampling location (land or ocean) or particle size separation) rather than publication year.
• L. 425: PM should be spelled out.
• L. 529: The endmember values should be mentioned.
• L. 534: Please clarify whether there is a difference between "fine (<2.5 μm) particles" and PM_2.5, and unify the terminology.
• L. 545: -1.87 and +0.28 should be rounded to -1.9 and +0.3, aligned with “-0.5 to +0.4‰”
• L. 580: As mentioned above, please reconsider the position of this section, and reduce the number of references cited within the same paragraph for better readability.
• L. 626: Mead et al. (2013), Kurisu et al. (2016), and Li et al. (2022) report δ⁵⁶Fe values of coal fly ash but do not claim that the heavy δ⁵⁶Fe in aerosols originates from coal fly ash. Thus, this citation may be inappropriate. It should also be cited that heavy δ⁵⁶Fe may result from isotopic fractionation in the atmosphere (Labatut et al., 2014; Camin et al., 2024).
• L. 642: “IMPACT” model should be spelled out.
• L. 646: Kurisu et al. (2021) suggests the underestimation of mineral dust in coarse particles by the model as well as the inappropriate endmember value of anthropogenic Fe.
• L. 692: Please verify whether the fractionation factor cited here is appropriate. Skulan et al. (2002) suggest the kinetic isotope fractionation factor of hematite precipitation as +1.32‰, meaning the dissolved phase becomes heavier. They also suggest that there is no clear fractionation during the dissolution experiment with 0.9 mol/L HCl.
• L. 700: This part may be unnecessary. Since the conditions of each experiment are different in terms of mineralogy, temperature, solvent, equilibrium/kinetic, etc., it is understandable that the degree of fractionation varies.
• L. 745: The title of this section should be revised, as it is not a discussion in response to the previous section, but simply describes an experiment under real atmospheric conditions.
• L. 755: Consider explaining the dissolution rate (%) rather than dissolution time, as it would facilitate comparisons with other studies.
• L. 784: Could you clarify what kind of experimental measurements you expect? I believe this is an important perspective that should be addressed.
• L. 798-800: Based on the discussion in Section 3.5, if isotopic fractionation of Fe aerosols in the atmosphere is driven by chemical processes, a relationship between solubility and isotopic fractionation (difference between total and soluble) should exist—i.e., lower solubility corresponds to larger isotope fractionation. I am curious whether such a relationship can be identified from the compiled data. This discussion should address whether the low δ⁵⁶Fe in soluble Fe is due to different sources or chemical processes.

This review is amazingly thorough and much needed by the community as a summary of Fe isotope values as they relate to aerosols. The supplementary tables are invaluable as comprehensive summaries of published Fe isotope values. I am very excited to see this paper published.

There is some work, however, that needs to be done to make the manuscript a more cohesive unit. Terms need to be summarized the first time they are used (this is especially true for operational definitions such as “soluble”), and word choice needs to be used more consistently (for example, desert dust and UCC should not be used interchangeable/have different definitions in different sections). Further, I suggest re-organizing the manuscript as right now it reads as separately written sections that are sometimes redundant and do not lead the reader to the comprehensive takeaways for different processes.

Right now, I feel like this paper is a very detailed review of all applicable literature, but I think it could be made stronger by streamlining the details and adding an introductory/summary as things relate to aerosols for all sections. This will allow the relevance and summary of the literature to be clear to readers.

Possible new, simplified outline (would require reorganizing current paragraphs into the correct section:

1. Introduction
   1. Fe isotopes
   2. Fe isotope analysis
2. Aerosol Endmembers
   1. Natural aerosols
   2. Anthropogenic aerosols

1. Fractionation processes

1. Fe isotope aerosol studies
   1. Ambient aerosols
   2. Marine source attribution
   3. Models
2. Future Work

Specific line comments/questions:

L23-40: Abstract would need to be reorganized if the suggestion of changed manuscript outline is selected

L45: Define dissolved, also consider using the abbreviation dFe throughout the paper

L48-49: Define HNLC

L50: Define the multiple ways you use the word soluble in this paper, i.e. size fraction and chemical leaching method

L73: The , after troposphere makes it seem like it’s part of the list that follows

L82: At this point, it is difficult to understand what you mean by ‘ambient aerosols’. I would explicitly define it. This is an example also of where using desert dust and UCC consistently and making the definitions clear is important.

L95: Define enrichment factors and add a sentence for how they are used before you mention that there isn’t a consensus

L104: Single particle analysis through what methods?

L116-121: An example of a section that can be moved to the modeling section. Also, it was unclear, how they determined coal combustion as a major anthropogenic source. Seems like a key takeaway.

L137-139: Add information about how IRMM-014 is running out and what standard the field is moving to.

L148: I would argue that 1-3permille is a more realistic range across transects.

L150: short -> shorter

L172: Choose either SE, 1sigma, and 2sigma error for consistent notation throughout the paper.

L174-187: Talking in ng of Fe is not common for many of the target readers of this article. I would consider explaining how you get these numbers from concentration to make sure readers are on the same page.

L174: Add “of Fe” after a minimum mass

L186: Define “such precision”

L191: Add sources

L198: This is an example of a section that is out of place in my opinion. It comes right after you finish explaining that you won’t discuss in-depth marine studies. It also references fractionation processes in the surface that you have yet to discuss in the manuscript. Also, it is unclear how you call things “dust” when you later explicitly state that the surface has to be ignored through fractionation.

L203: Different precision range than earlier in the manuscript.

L220: Confusing that you call it 2 component mixing model when there are more than two sources.

L226: I recommend adding more sources that discuss the fractionation process at the surface ocean.

L247: This would probably make more sense being moved to the modeling section.

L256: More background information on how aerosols are shown to be a mixture of natural and anthropogenic sources and choose the words that fit into each category that you will consistently use throughout.

L275-279: Missing + before heavier Fe isotope values

L276: Define ATD

L288: Calculate an actual average or state something like, “for natural dust contributions, we can assume an endmember equal to that of UCC (+0.09 permille). Also, I think it is important to mention in the natural aerosol section that not all natural aerosols are from deserts.

L353-261: This is a paragraph that I think would be a better introduction to the anthropogenic aerosol section

L368: If you are using leach data, talk about how the leaches are proven to fractionate or not fractionate Fe and the sources that support this.

L381-391: I think this paragraph would be a better introduction to an endmembers section.

L399: Define “fine” and “coarse” and let the reader know if these definitions change from study to study.

L409: If you keep this outline, why is there a 2016 cutoff?

L411: Define TSP

L458: These are the lightest isotope values included in the paper. Might be worth positing why it makes sense they are so light (or why it is surprising)

L480-501: This is an example location where I think this information would flow better in the applicable endmember section.

L519-579: These sections are examples of where it feels like there was a different author.  They are a lot more detailed/lend a full paragraph to one study, compared to elsewhere in the manuscript. I think it would help the reader to streamline this and draw connections between the studies as applicable.

L644: Discuss why different endmembers were used in this study.

L644: I understand why you have included this section, as aerosols are greatly chemically altered before deposition, however this has not been explained to the readers. If you leave these sections, make sure to include examples of how this is applicable to aerosol studies and the main takeaways aerosol people should know from this information.

L764: I really liked the perspectives suggestion, I would just suggest that you make It even more strongly stated that these are the recommended steps that Fe isotope aerosol research needs to take. You say it, but stronger language would make it stand out. Also make sure to have some sort of conclusion at the end and that the manuscript doesn’t just end at the end of the last suggestion.

Fig 1: Not everything in this figure is discussed in the paper (i.e. SO2, NOx, VOC, DMS, CO2). Either include them in the fractionation process section or remove them from the figure.

Fig 4: I think this figure would be even stronger if there was some indication on the figure itself of the concentration differences between the two seasons.  

Supp Table S1&S2: Make sure to define your size fractions