the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Feb 2025
A critical review of the use of iron isotopes in atmospheric aerosol research
Abstract. Deposition of atmospheric aerosols is recognized as a major source of iron (Fe) to the surface oceans, where it acts as a key micronutrient for primary productivity and metabolic functions of marine microbes. Initially, natural desert dust was thought to be the main source of aerosol Fe, albeit largely insoluble; however, in the last few decades, the role of anthropogenic and wildfire sources in providing soluble Fe to aerosols has been increasingly recognized. The stable isotope ratio of Fe (δ56Fe) has emerged as a potential tracer for discriminating and quantifying sources of aerosol Fe. In this review, we examine the state of the field for using δ56Fe as an aerosol source tracer, and constraints on endmember signatures. We begin with an overview of the methodology of δ56Fe analysis for aerosol samples. We then describe knowledge of δ56Fe endmember signatures of different source materials, and review existing knowledge of the δ56Fe signature of ambient aerosols collected from around the globe, and how these measurements can be used to enhance atmospheric Fe deposition modelling. We also examine the various chemical processing mechanisms which might influence δ56Fe source signatures of aerosol Fe during its transport in the atmosphere. This review paper is concluded with a perspective on the state of the field and a call for future work. Overall, we find aerosol δ56Fe to be a promising tracer, but highlight that greater constraints on both source endmembers and processing mechanisms are needed to fully utilize this tracer.
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CC1: 'Comment on egusphere-2025-474', Capucine Camin, 22 Apr 2025
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Great manuscript! I would like to inform you about this work: https://doi.org/10.5194/egusphere-2024-3777 (the lastest version is in the discussion section)
Citation: https://doi.org/10.5194/egusphere-2025-474-CC1 -
CC2: 'Reply on CC1', Tim Conway, 24 Apr 2025
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That manuscript looks great! We will be sure to add it.
Have you seen our recent work here? May be useful for your manuscript also if it is not in final form:
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024GL113877
Tim
Citation: https://doi.org/10.5194/egusphere-2025-474-CC2 -
CC3: 'Reply on CC2', Capucine Camin, 28 Apr 2025
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Thank you very much! In fact, this article is very interesting, but unfortunately we submitted it before publication and did not see it during the review process. It is no longer possible to change the manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-474-CC3
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CC3: 'Reply on CC2', Capucine Camin, 28 Apr 2025
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CC2: 'Reply on CC1', Tim Conway, 24 Apr 2025
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RC1: 'Comment on egusphere-2025-474', Anonymous Referee #1, 30 Apr 2025
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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 PM2.5, 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.
Citation: https://doi.org/10.5194/egusphere-2025-474-RC1
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