Variations in Arctic aerosol iron solubility in relation to leaching methodology, air mass characteristics, and seasonality

Marsay, Chris M.; Zhang, Ruifeng; Ebling, Alina M.; Morton, Peter L.; John, Seth G.; Landing, William M.; Buck, Clifton S.

doi:10.5194/egusphere-2026-1916

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https://doi.org/10.5194/egusphere-2026-1916

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28 Apr 2026

| 28 Apr 2026

Variations in Arctic aerosol iron solubility in relation to leaching methodology, air mass characteristics, and seasonality

Chris M. Marsay, Ruifeng Zhang, Alina M. Ebling, Peter L. Morton, Seth G. John, William M. Landing, and Clifton S. Buck

Abstract. Atmospheric deposition of the essential micronutrient, iron (Fe), can have an important influence on primary production and marine biogeochemistry. In the Arctic Ocean, the ongoing shift towards seasonal ice coverage means that summertime atmospheric deposition increasingly takes place direct to the surface ocean, rather than onto sea ice. As a result, atmospheric deposition of material emitted from natural and anthropogenic sources may become a more relevant Fe input to the region. As part of the U.S. GEOTRACES GN01 section, aerosols and precipitation samples were collected to quantify the atmospheric delivery of Fe and other trace elements to the Arctic Ocean. Aerosol Fe solubility was assessed using three different leaching approaches. The readily soluble fraction, determined by rapid exposure leaches with ultrapure water (UPW) and filtered seawater (SW) was low throughout GN01, averaging 0.7 % and 1.4 %, respectively. Solubility determined using a more aggressive acetic acid (HAc) leach as an upper limit estimate of post-deposition aerosol Fe bioavailability averaged 44 %. Comparison to Fe UPW-solubility data from winter (median 6.5 %) and springtime (median 1.9 %) aerosol samples collected during the MOSAiC expedition suggests a strong seasonality to Arctic aerosol Fe solubility, potentially associated with winter/springtime Arctic haze. Iron stable isotope analysis of GN01 total Fe (d⁵⁶Fe_Tot = +0.10 ± 0.13 ‰) and UPW-soluble Fe (d⁵⁶Fe_Sol = −0.17 ± 0.33 ‰) indicate the low summertime total Fe loading was dominated by mineral aerosols, albeit with anthropogenic contributions to the small soluble Fe fraction in some samples. Bulk deposition fluxes, calculated using the beryllium-7 method, were estimated at 0.8 ± 1.2 nmol m^-2 d^-1 UPW-soluble Fe, 1.8 ± 1.9 nmol m^-2 d^-1 SW-soluble Fe, and 46 ± 48 nmol m^-2 d^-1 HAc-soluble Fe, with the UPW-soluble Fe flux around an order of magnitude lower than that measured during the winter months.

Received: 03 Apr 2026 – Discussion started: 28 Apr 2026

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Chris M. Marsay, Ruifeng Zhang, Alina M. Ebling, Peter L. Morton, Seth G. John, William M. Landing, and Clifton S. Buck

Status: final response (author comments only)

RC1: 'Comment on egusphere-2026-1916', Anonymous Referee #1, 05 Jun 2026

This article combines together fe solubility measurements in the Arctic Ocean and, for the first time, explores the seasonal variability of both bulk deposition and relative solubility of fe transported and deposited via aerosol deposition. The Arctic provides a unique template for aerosol exploration given the year-round sea ice coverage that prevents complete deposition into the seawater where fe is likely to have the largest impact on biology. The authors do a very complete comparison with other aerosol studies in the Arctic from the same cruises they focus on (GEOTRACES GN01 and the MOSAiC expedition). Ultimately, they conclude that deposition of soluble Fe is more substantial in winter due in part to increased anthropogenic emissions despite lower atmospheric depositional rates. The authors also make some guesses of how this pattern may evolve or shift with projected reduced sea ice cover in the Arctic.
I enjoyed reading this manuscript and have no major comments or revisions for the authors, only a few questions and small technical corrections.
Minor comments:
Figure 5/Lines 461-462: I think the data shown in the FIgure 5 inset does not support the statement that no inverse pattern is noticeable within the Arctic data. I think there is a slight inverse pattern with the MOSAiC data. The Arctic data falls squarely within the range of previous data, particularly in the Atlantic datasets. I think you are trying to emphasize the very limited range in concentration compared to Atlantic/Pacific aerosols but I would rephrase this sentence.
Lines 519-520: repeated use of "resulting", sounds a bit messy.
Lines 609-612: Same thing, both sentences here start with "As a result". I would do a careful read through of the manuscript to make sure phrases like "as a result" and "suggests/suggesting" aren't over-used in quick succession.

Conclusions: I thought this discussion was pretty brief. I wonder if the authors could comment on whether Fe making it to open water may relieve potential emerging Fe limitation in the Eurasian sector (Rijkenberg et al., 2018) or whether larger scale shifts in atmospheric weather patterns (like the Arctic Oscillation Index) may exert a control on summer vs winter deposition.

Citation: https://doi.org/10.5194/egusphere-2026-1916-RC1
RC2: 'Comment on egusphere-2026-1916', Anonymous Referee #2, 15 Jun 2026

The manuscript entitled 'Variations in Arctic aerosol iron solubility in relation to leaching methodology, air mass characteristics, and seasonality' by Marsay et al., presents a comprehensive dataset on aerosol deposition as a significant source of iron to the Arctic Ocean. Given the ongoing changes in sea ice cover, a detailed analysis of aerosol iron solubility is essential for understanding the implications for marine productivity in the Arctic Ocean and, consequently, for regional climate change. Overall, this is a well-conceived study, and the authors appropriately highlight the importance of seasonal variations in Arctic aerosol iron solubility.
However, prior to further processing of this manuscript, I have a general comment concerning the interpretation of variations in Arctic aerosol iron solubility in relation to leaching methodology…, or actually the pH instead. It is well established that measurements of trace metal/element solubility are conducted using extraction in pure water, seawater, or a buffered acidic solution. I assume that the chemical properties of these solvents, particularly the final pH of each solution, exert a substantial influence on metal solubility (Mahowald et al., 2018; Wang et al., 2022).
Specific comments:
Line 126 (2.2 Aerosol sample processing): It would be useful to report the final pH measured in each leaching experiment, if available.
Line 220 (Figure 2 caption): Please include essential information regarding the fractional percentages, even if mentioned in line 229. I misunderstood when only reading the figure.
Line 239: While the statement may be true, it should be noted that greater variability in solubility seems to be observed between aerosols from different sources than between different leaching solutions (Mackey et al., 2015).
Lines 242-244: Please provide the specific numbers used for such comparisons.
Line 286 and lines 304-309: I suspect that pH is the key factor here. Was pH measured in each experiment?
Line 461: The statement is difficult to follow, even only considering the larger dataset. Are there any statistical data available to support that?
Line 510 (Figure 6 caption): It would be helpful to indicate the sample size within each boxplot (each season), e.g. jitter plot.
Refs:
Mahowald, N. M., D. S. Hamilton, K. R. M. Mackey, J. K. Moore, A. R. Baker, R. A. Scanza, and Y. Zhang (2018), Aerosol trace metal leaching and impacts on marine microorganisms, Nat Commun, 9(1), 2614.
Mackey, K. R. M., Chien, C.-T. C.-T., Post, A. F., Saito, M. A. & Paytan, A. Rapid and gradual modes of aerosol trace metal dissolution in seawater. Front. Microbiol. 5,1–11 (2015).
Wang, G., et al. (2022), Quantitative Decomposition of Influencing Factors to Aerosol pH Variation over the Coasts of the South China Sea, East China Sea, and Bohai Sea, Environmental Science & Technology Letters, 9(10), 815-821.

Citation: https://doi.org/10.5194/egusphere-2026-1916-RC2
RC3: 'Comment on egusphere-2026-1916', Anonymous Referee #3, 16 Jun 2026

Review of egusphere-2026-1916 “Variations in Arctic aerosol iron solubility in relation to leaching methodology, air mass characteristics, and seasonality” by Marsay et al.

Summary:

This article presents iron concentrations and isotopic compositions in aerosols, either as bulk values or as an estimate of the soluble or labile fraction of aerosol iron. It also reports trace element concentrations in wet deposition samples.

The approach taken in this article is interesting and original. It benefits greatly from the first author's participation in several sampling campaigns in the Arctic Ocean across different seasons. Temporal comparisons within a single region are uncommon and particularly relevant in an area undergoing rapid change.

However, a major problem affects this article. The isotopic data obtained after UPW and SW leaching are presented with error bars that are largely underestimated. The reported measurement uncertainty corresponds only to the internal error of the mass spectrometer, and does not account for the fact that the blank represents a very significant fraction of the samples (blank/sample ratio of 58% on average for UPW and 60% for SW). I think that the uncertainties are at least 1‰, meaning the values are indistinguishable from one another (Fig. 6).

In my view, as they stand, the isotopic compositions of the UPW and SW leaches are not publishable. These data, and the interpretations derived from them, must be either removed or substantially revised to reflect the true measurement uncertainty. A detailed explanation is provided below.

Major scientific comment: These samples show a blank/sample ratio of 58% (UPW) and 60% (SW) on average (line 217). This is not surprising given the small amounts of iron that can be leached by UPW or SW. The iron concentrations of these samples were corrected accordingly, as mentioned on line 193. However, there is no mention of any blank correction applied to the isotopic compositions. The uncertainty associated with the isotopic compositions reported by the authors is that of the internal error (lines 205–206), which is not representative.

If the authors still wish to present these data by correcting the isotopic compositions of the UPW and SW leaches, they should:

- Report the blank/sample ratios for all samples, at least in a supplementary table;

- Provide an isotopic composition of the blank with a realistic confidence interval, justifying their choice;

- Correct the isotopic compositions of the samples for the isotopic composition of the blank, and account for the error propagation arising from the isotopic composition and concentration of the blank;

- Revisit all interpretations based on these data, taking their uncertainty into account (likely larger than 1‰).

Regarding the HAC leaching, the blank/sample ratio is probably lower (this still needs to be reported), but it may remain significant which is the case whenever the blank accounts for more than roughly 5 percent of the sample. Indeed, there is generally an uncertainty associated with the isotopic composition of the blank, and in the absence of proper characterization, the hypothesis that this uncertainty is on the order of 1‰ cannot be excluded. If the blank is significant, the same approach as for the UPW and SW leaching should be applied.

I suggest that the authors also discuss the potential isotopic fractionation induced by these leaching steps.

Other scientific comments:

Line 28. It is unclear whether the UPW-soluble Fe flux refers only to the summer or to the annual average.

Lines 45-47. I would suggest placing more emphasis on the importance of iron (Fe) for marine life (a nutrient, sometimes a limiting factor in marine primary production, etc.).

Lines 76-78. This sentence is hard to follow. Starting with a definition of the Arctic haze and its seasonality would probably help the reader.

Lines 99-101. Figure 1, b). It is difficult to understand what Ppt05, Ppt06, Ppt07 and Ppt08 are, even though Table 1 helps. I was looking, for instance, for an explanation of the black square. I would suggest describing this in the caption: Ppt05 and Ppt07 are cutoff bottle samples, while Ppt06 and Ppt08 are N-con samples, and they are geographically very close, which explains why they are hard to distinguish on the figure.

Line 104. How was this uncertainty on the filtered air taken into account for aer01 (Table 1, Marsay et al., 2018a)? Could this uncertainty be propagated through all the subsequent concentration and flux calculations, in order to avoid any partial conclusions? Please check that the conclusions drawn for aer01 are consistent across all possible cases, i.e., a filtered volume of 444.3, 559.7 or 675.1 m⁻³.

Line 131. It might help the reader to specify that the collected aerosols are "bulk", meaning that they include both the wet and dry fractions of an aerosol, and that different protocols were then applied to assess the total, soluble or labile fraction of a bulk aerosol. Alternatively, the definition of a bulk aerosol could simply be given around line 90.

Line 131. Are the filters acid-cleaned too?

Line 135. At which temperature?

Line 155. Given the large UPW rinsing volumes, did you measure the iron blanks of the UPW? If so, could you report the Fe concentration in the UPW, or at least provide the values in a supplementary table?

Line 157. Why this specific protocol of 100 mL in a single step?

Line 250. It could be useful to also draw the lower error bar in Figure 2. Currently only the positive error is displayed, which makes it difficult to see whether Total/HAc and UPW/SW differ at the 95% level. Switching from a bar to a point value might help. Moreover, I cannot find the information on the error bar for concentration - is it 2sd or rsd (line 195)? Please make this clearer and consistent in the methods section.

Line 254. Is the volume uncertainty of aer01 taken into account in the solubility calculation?

Line 267. Yes, even though the lithogenic fraction is conventionally considered refractory, it can be dissolved in seawater (Rousseau et al., 2015) and during atmospheric transport (references should be available).

Line 272. The intercomparison has been well described.

Line 297. Consequently, for future cruises it could be interesting to compare the efficiency of the 1×100 mL and 3×50 mL protocols.

Line 331. If Ppt13 is removed, does this still hold?

Line 351. This is an open question (no need to answer it in the article): do you think there is a way to estimate this "overestimate" due to the unknown Fe content of refractory particles?

Line 390. The -1.6‰ to -4.7‰ range is partial. Wang et al. (2022) published a review of iron isotope signatures in aerosols. Signatures heavier than -1.6‰ can also be attributed to anthropogenic aerosols, including positive ones (Flament et al., 2008). Kurisu et al. (2016) and Kurisu & Takahashi (2019) reported values close to the one observed for aer01. This is mentioned later in the article, but you could also briefly note that a mixing between anthropogenic and crustal aerosols can lead to such a slightly negative value.

Lines 398 and 400. Camin et al. (2025) published complementary positive signatures in the equatorial Pacific and discussed isotopic fractionation during transport.

Line 408. The Fe/Al and Fe/Ti elemental ratios presented in Marsay et al. (2018) are compared to the global crustal value from Taylor and McLennan (1995). Crustal values can show considerable heterogeneity, so comparing them to a more regional ratio could be more accurate, or at least, it would be worth checking that the regional ratio does not differ significantly from the one given in Taylor and McLennan (1995).

Lines 428-430. This is an interesting result.

Line 458. Maters et al. (2022) observed this process as well (Fig. 1 of their article).

Line 509. The statistical approach is interesting and strengthens the conclusions of the article.

Technical comments

Lines 42-44. I recommend choosing between 'photic' and 'euphotic' and using the same word throughout to avoid losing the reader. You could also define it quickly.

Lines 45-47. This sentence is hard to follow, maybe separate it in two parts.

Line 130. For clarity purpose, to precise that ‘digests’ means total, e.g. ‘digests (total) …’.

Line 163. Please define the acronym Hac.

Line 354. A space is missing between ‘(2021)’ and ‘discussed’.

Line 575. Marsay et al. is repeated twice.
References

Camin, C., Lacan, F., Pradoux, C., Labatut, M., Johansen, A., and Murray, J. W.: Iron isotopes suggest significant aerosol dissolution over the Pacific Ocean, Atmos Chem Phys, 25, 8213–8228, https://doi.org/10.5194/acp-25-8213-2025, 2025.

Flament, P., Mattielli, N., Aimoz, L., Choël, M., Deboudt, K., Jong, J. de, Rimetz-Planchon, J., and Weis, D.: Iron isotopic fractionation in industrial emissions and urban aerosols, Chemosphere, 73, 1793–1798, https://doi.org/10.1016/j.chemosphere.2008.08.042, 2008.

Kurisu, M. and Takahashi, Y.: Testing Iron Stable Isotope Ratios as a Signature of Biomass Burning, Atmosphere-Basel, 10, 76, https://doi.org/10.3390/atmos10020076, 2019.

Kurisu, M., Sakata, K., Miyamoto, C., Takaku, Y., Iizuka, T., and Takahashi, Y.: Variation of Iron Isotope Ratios in Anthropogenic Materials Emitted through Combustion Processes, Chem Lett, 45, 970–972, https://doi.org/10.1246/cl.160451, 2016.

Marsay, C. M., Lam, P. J., Heller, M. I., Lee, J.-M., and John, S. G.: Distribution and isotopic signature of ligand-leachable particulate iron along the GEOTRACES GP16 East Pacific Zonal Transect, Mar Chem, 201, 198–211, https://doi.org/10.1016/j.marchem.2017.07.003, 2018.

Maters, E. C., Mulholland, D. S., Flament, P., de Jong, J., Mattielli, N., Deboudt, K., Dhont, G., and Bychkov, E.: Laboratory study of iron isotope fractionation during dissolution of mineral dust and industrial ash in simulated cloud water, Chemosphere, 299, 134472, https://doi.org/10.1016/j.chemosphere.2022.134472, 2022.

Rousseau, T. C. C., Sonke, J. E., Chmeleff, J., van Beek, P., Souhaut, M., Boaventura, G., Seyler, P., and Jeandel, C.: Rapid neodymium release to marine waters from lithogenic sediments in the Amazon estuary, Nat Commun, 6, 7592, https://doi.org/10.1038/ncomms8592, 2015.

Taylor, S. R. and McLennan, S. M.: The geochemical evolution of the continental crust, Reviews of Geophysics, 33, 241–265, https://doi.org/10.1029/95RG00262, 1995.

Wang, Y., Wu, L., Hu, W., Li, W., Shi, Z., Harrison, R. M., and Fu, P.: Stable iron isotopic composition of atmospheric aerosols: An overview, npj Clim Atmos Sci, 5, 1–13, https://doi.org/10.1038/s41612-022-00299-7, 2022.

Citation: https://doi.org/10.5194/egusphere-2026-1916-RC3

Chris M. Marsay, Ruifeng Zhang, Alina M. Ebling, Peter L. Morton, Seth G. John, William M. Landing, and Clifton S. Buck

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https://doi.org/10.5194/egusphere-2026-1916-supplement

Chris M. Marsay, Ruifeng Zhang, Alina M. Ebling, Peter L. Morton, Seth G. John, William M. Landing, and Clifton S. Buck

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Short summary

Solubility of the micronutrient, iron, was measured in Arctic aerosols collected during GEOTRACES GN01 (summer 2015). Solubility is compared across three different methods and used to estimate the deposition flux of bioavailable iron to the Arctic Ocean. Variations in solubility are described in terms of air mass characteristics. Combining this dataset with winter/spring aerosol iron measurements from the MOSAiC project, we assess the seasonality of aerosol iron solubility in the Arctic.


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