Biogeochemical regimes control marine aerosol emission of hydrogels in the Southwestern Pacific Ocean

Barthelmeß, Theresa; Sellegri, Karine; Pontiller, Benjamin; Scheidemann, Lindsay; Safi, Karl; Engel, Anja

doi:10.5194/egusphere-2026-1873

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https://doi.org/10.5194/egusphere-2026-1873

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10 Apr 2026

| 10 Apr 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Biogeochemical regimes control marine aerosol emission of hydrogels in the Southwestern Pacific Ocean

Theresa Barthelmeß, Karine Sellegri, Benjamin Pontiller, Lindsay Scheidemann, Karl Safi, and Anja Engel

Abstract. Ambient marine aerosols are frequently enriched in biogenic material. It was suggested that a critical fraction may be attributed to colloids and aggregates, which are composed of carbohydrates and proteins. Those hydrogels possess excellent cloud condensation and ice nucleation properties. Yet, most atmospheric measurements fail to detect marine hydrogels in aerosols directly, and the few studies which exist were conducted in the Northern hemisphere. Here, we present a comprehensive data set of carbohydrate and protein-enriched hydrogels in in sea spray aerosols (SSA) generated within representative regimes of the Southwestern Pacific Ocean. We relate the concentration of hydrogels in SSA to the occurrence of their precursors in surface seawater and other biogeochemical variables. The highest concentration (0.91 ± 0.72 × 10⁵ m^-3, corresponding to 4.1 ± 2.1 × 10³ particles ng^-1 Na⁺) and highest relative enrichment (5.3 ± 3.9 × 10⁵) of hydrogels in SSA (size range: 0.5–30 µm) was observed within the subtropical front, which is biologically most active. This was contrasted by subtropical waters, in which SSA concentration and enrichment decreased by one order of magnitude. Interestingly, the carbohydrate-to-protein ratio shifted with size in SSA hydrogels, while no such size-shift existed for marine samples. In comparison to their marine precursors, supermicron hydrogels in SSA were primarily composed of carbohydrates. Our results suggest that hydrogels may complement a considerable fraction of ambient marine aerosols, and significantly contribute to the atmospheric pool of cloud condensation and ice nuclei, in particular above the remote oceans of the Southern hemisphere.

Received: 01 Apr 2026 – Discussion started: 10 Apr 2026

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Theresa Barthelmeß, Karine Sellegri, Benjamin Pontiller, Lindsay Scheidemann, Karl Safi, and Anja Engel

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RC1:
'Comment on egusphere-2026-1873', Anonymous Referee #1, 15 Apr 2026 reply
Review for Barthelmeβ et al., submitted to ACP
The study “Biogeochemical regimes control marine aerosol emission of hydrogels in the Southwestern Pacific Ocean” by Barthelmeβ et al., submitted to ACP, presents measurements of hydrogels (TEP and CSP) using colorimetric staining assays and microscopic, size-resolved quantification in seawater from the Southwestern Pacific Ocean, as well as in nascent aerosols generated in a bubbling tank. The authors relate variations in gel properties to different biogeochemical regimes associated with distinct water masses and report enrichment factors during the transfer from seawater to aerosol phase. The study is motivated by the potential relevance of hydrogels for cloud formation and cloud properties.
Overall, I consider this study to be highly relevant for improving our understanding of aerosol–cloud interactions, particularly in the context of marine aerosols and their role in the climate system, with possible implications for future model parameterizations. I have no doubt that this manuscript has the potential to be suitable for publication after addressing the critics below.
However, in its current form, the manuscript suffers from substantial weaknesses in the presentation of results and discussion. These include considerable overselling of the findings, potential (mathematical or logical) errors in analysis and calculations, incomplete or misleading citation of certain information from the literature, a lack of clear structure, and, at times, a lack of rigor. These issues make it difficult to fully assess and appreciate the scientific value of this study. I strongly encourage the authors to invest additional effort into revising the manuscript to improve clarity, correctness, and overall quality of interpretation.
In the following, I outline my main concerns:
1.) Soot topic:
a) Clarification and Focus of the “Soot” Discussion: Your discussion of soot particles is currently not fully convincing and, in its present form, feels somewhat distracting because it appears repeatedly throughout the manuscript. As a result, it becomes difficult to identify your main findings. In my opinion, it would help to clarify your central messages. For example: Is your key result that SAW water masses are generally contaminated with soot, but that the corresponding aerosols are not significantly affected? If yes, consider structuring the paper clearly around this finding. If not, I would recommend substantially reducing the emphasis on the soot discussion.
b) Scientific Concerns About the Interpretation: I also have some concerns regarding the plausibility of your interpretation: You describe soot as hydrophobic (Line 439), which would suggest it remains in the surface microlayer. How likely is it, then, to find significant amounts at depths of 6 m? If soot sinks as part of aggregates (Line 440), one would expect the particles at that depth to be embedded in hydrogels rather than appearing as isolated small particles?
c) Identification of Black Particles as Soot: I do not find the current presentation of the black particles as soot (Section 2.3) entirely convincing. The supporting arguments are currently scattered across the manuscript (e.g., Line 438 mentioning potential origin from Australian bushfires). I suggest: Collecting and presenting all arguments for identifying these particles as soot in a more focused way, ideally within the Methods section. Using more cautious wording, such as: “Based on the available indications, we suggest that these small particles are likely soot.”
d) Handling of Contaminated Samples: It is important to clearly state how these filters were treated in your analysis. For example: Were the affected samples consequently excluded from further analysis because they interfere with hydrogel quantification? Or were they retained and marked (e.g., with asterisks) in the results?
e) Data Presentation and Figures: The current visual presentation is somewhat confusing. For example, in Figure 3: The plots are labelled as CSP and TEP, yet include black particle samples that you explicitly state are neither CSP nor TEP. I recommend either: Adjusting the figure title and labels to reflect what is actually shown, or modifying the visualization to avoid mixing different particle types in a misleading way. Additionally, have you considered improving the microscopic images of the contaminated samples by applying color filters (removing black) with the image software to reduce the visual impact of these black particles?
f) My personal recommendation: Overall, my suggestion would be: Briefly describe the observation and treatment of (potentially) soot-contaminated samples in a concise paragraph within the Methods section. If you wish to explore this topic further, consider moving the extended discussion to the supplementary material.
2. ) I found some of the phrasing somewhat strong and occasionally selective in how the literature is presented, which in places gives the impression of overstating the certainty or significance of the findings. For example, in Lines 13–14 you state that “these hydrogels possess excellent cloud condensation and ice nucleation properties,” whereas later (Line 37) this is phrased more cautiously as “hydrogels have been suggested to serve as cloud condensation…” This represents a substantial difference in level of certainty, and I would recommend aligning the wording more consistently with the actual state of the literature. Please check this for all statements throughout the manuscript.
3.) You emphasize the differing properties of the four marine regions throughout the manuscript; however, their characteristics are not sufficiently introduced. Referring the reader to Sellegri et al. (2023) & Barthelmeß et al. (2025) in Lines 83/84 & 273/274 for this information is not helpful, as the manuscript should be understandable on its own without requiring prior reading of additional papers. I recommend summarizing and directly comparing the key features of these four regions within your manuscript, for example in an additional table. This could be combined with Section 3.3, potentially moved to the beginning of the Results section. Such a restructuring would help the reader better understand why these regimes are distinct and why differences in aerosol hydrogels are to be expected.
4.) Enrichment calculations: Were your sodium measurements in aerosol particles size-resolved? Based on Table A2, this does not appear to be the case, as only a single sodium value per tank experiment is reported. If so, how were size-resolved enrichment factors for hydrogels (as presented in Table 1) calculated? Since sodium is known to be predominantly associated with the coarse mode and occurs at much lower concentrations in submicron particles, using size-resolved sodium data is important for obtaining accurate enrichment factors. If such size-resolved sodium measurements are not available, the only consistent approach would be to calculate enrichment factors for the entire size range as a bulk rather than for individual size classes. This also holds true for the hydrogel/Na+ ratios.
5.) Please elaborate on the potential consequences for your tank SSA results arising from the use of 6 m deep water, without considering the SML.
6.)You state that atmospheric measurements of TEP and CSP are scarce, and further describe TEP as polysaccharide-based and CSP as protein-based. However, to my knowledge there are several studies from different groups investigating polysaccharides and proteins in SSA. Integrating your findings more systematically, for example in terms of aerosol size distributions and the potential contribution of gel-like versus non-gel-like polysaccharides and proteins, could strengthen the contextualization of your study and better embed it within the existing literature.
7.) I was confused throughout the manuscript as to whether you are referring to results from seawater or aerosol samples. Please ensure consistent nomenclature (e.g. abbreviations, brackets, indices) or a clearer overall structure to make this distinction unambiguous. I may have misunderstood some aspects due to this lack of clarity. What exactly do you mean by “marine samples” (e.g. L22)? Aren’t all samples marine in this study?
8.) Abbreviations are introduced repeatedly throughout the manuscript, which makes the text harder to follow. It would greatly improve readability if all abbreviations were defined once (ideally at first occurrence) and then used consistently thereafter, at least within the main text. I recommend carefully reviewing the manuscript from beginning to end to ensure consistent usage. For example, the term “subtropical front (STF)” is introduced multiple times in slightly different forms (e.g., L85, L186, several figure captions, Table A4, L507, L516). A similar pattern occurs for “SSW” (e.g., Lines 97, 104, 185, 246) and other abbreviations such as Chla and STW.
9.) Since you measured size-resolved hydrogels, did you consider visualizing their relative size distribution in an additional figure and comparing it with the few existing studies, either from field observations or laboratory experiments? Also comparing them between seawater and aerosol could be interesting.
More specific comments:
L14–15 (&L67/68): “most atmospheric measurements fail to detect marine hydrogels” appears to be a very strong statement. Please rephrase it in a slightly more cautious way, e.g. in the sense that atmospheric measurements of marine hydrogels are still scarce and only limited studies exist.

Abstract in general: Please introduce the terms TEP and CSP in the abstract, as these are central to the manuscript. Since you use staining-based approaches, these are also the appropriate and standard terms. Replacing them with “carbohydrate- and protein-enriched hydrogels” without proper introduction to their chemical relationship feels potentially misleading.

L16-17: Please be more cautious with the use of the term “representative regimes.” First, as far as I understand, every region has its own specific characteristics; therefore, please specify for which parameters or aspects you consider them representative. Second, your sample size is quite limited (which is of course common for this type of field measurements), but it should be acknowledged that the dataset may not fully capture seasonal and meteorological variability required for a complete sense of representativeness. In this context, the term “representative” appears potentially misleading, as it could imply that these regions are comprehensively characterized. Furthermore, it feels somewhat controversial that within this relatively small sampling area you already define four distinct regimes with apparently very different characteristics, this also raises the question of what exactly is meant by “representative” in this context.

L24–25: “…and significantly contribute to the atmospheric pool of cloud condensation and ice nuclei.” This is a very strong statement for the abstract, given that neither cloud condensation nuclei nor ice nuclei were measured in this study, nor was any related modelling performed. Please consider rephrasing this statement more cautiously or removing it altogether.

Line 41, Line 457, Line 485: In these sections, enzymes and proteins appear to be used interchangeably, which appears confusing. While enzymes are indeed a specific class of proteins, they are typically discussed in the context of their catalytic function to initiate certain chemical reaction. However, their catalytic role does not seem to have any relevance for your train of thoughts. It would be helpful to clarify which term is intended in each case. Please chose for one of the two.

L44/45: ‘Proteins […] represent persistent ice nuclei’…. this statement is surprising in its current form, as protein-based ice nuclei are generally considered among the most fragile types of ice nuclei rather than persistent ones. Furthermore, could you rephrase this sentence to clearly specify which marine proteins found in the SSA are meant, and in what sense they act as particularly efficient or persistent ice nuclei?

L46: “enabling ice cloud formation”… consider rephrasing to “enabling ice formation in clouds.”

L48/49: Please add for which region and which temperature range this information holds true

L71/72: ‘in SSA or ambient aerosols’…. please choose one term only.

L72: “…and none over the Southern Hemisphere…” This statement is certainly correct. However, as I understand it, your study did not include ambient atmospheric measurements of hydrogels in the Southern Hemisphere, but rather tank experiments using water collected from the Southern Hemisphere. Tank studies are certainly valuable; however, it is important to clearly distinguish between laboratory experiments and ambient atmospheric observations so that readers are not misled. In its current form, this statement appears potentially misleading and should be rephrased accordingly.

L82: “Surface seawater samples”?

L89: The tank was constructed from “stainless steel.” In Lines 347 and 363, you argue that waters in the region of your study are depleted in iron, which influences (inhibits) microbial growth. Would corrosion under saline conditions potentially lead to an increase in dissolved iron within the tank water? If so, could this have altered the biogeochemical and microbial conditions during the experiments through the release of iron ions?

L90 ‘….,which is equivalent to a water depth of 10 cm”. This statement is not clear to me. Does this refer to a water level of 10 cm in the tank (i.e., the filling height)?

L91: ‘The residence time of the water in the chamber was ~4 min.’ If the water remains in the tank for such a short time while the ship continues along its cruise track, it seems likely that a single tank aerosol sample could be influenced by water masses with varying biogeochemical properties over the ~24-hour sampling period. Did you collect multiple seawater samples to account for this variability? If not, how did you ensure that the single corresponding seawater sample is representative of the water that actually generated the aerosols?

Section 2.2: Please add a suitable reference for each measurement parameter, where the full measurement protocol is described in detail. Furthermore, please clarify where the sodium measurements are described in the methods section.

L101: Did you measure free or combined amino acids?

L107: Could you clarify why 10 mL of Milli-Q water was filtered for the blank, while 30-80 mL were filtered for the field samples? This difference raises the question of whether the blank and samples are directly comparable.

L114-120 This section might be more appropriately placed in Section 2.1 (Sampling)

L121–124: For aerosol filters, a “dual-staining approach” was used due to the limited availability of filters. Please elaborate on how this methodological difference between seawater and aerosol filter processing may have introduced artefacts and potentially affected the comparability of the resulting data.

L141: ‘In comparison to hydrogels in SSW,” …. reads strange in combination with the rest of the sentence. Is it possible to eliminate this part?

L147: Consider rephrasing the section title “Data processing and Statistics” or similar.

L149–155: I appreciate that statistical tests were performed to assess the suitability of the microscopic data. However, it remains unclear what conclusions can be drawn from the results regarding normality and homogeneity. A brief clarification (ideally in one sentence) would be helpful. In addition, since the data appear to violate these assumptions in several cases, it would be important to explain what consequences were drawn from this (e.g., data interpretation, change in measurement protocol,…). How does this affect the reliability of your results?

L155: Please specify what is meant by “certain parameters”.

L161: “If the EF is smaller than 1,…..”I think this statement typically applies to enrichment in the SML. For aerosol enrichment, this interpretation does not appear directly appropriate.

L191: How do you expect heavy rainfall to have impacted seawater at 6 m depth? Was the upper water column strongly diluted? Do you still consider the assumption of approximately 10 g Na⁺/L (Line 163) to be valid for your subsequent enrichment calculations? Please clarify and streamline how relevant this observation is for the discussion.

Table 1: Do these enrichment factors refer specifically to CSP, TEP, or to hydrogels in general?

L229: “SSA hydrogel abundance and area varied greatly along the cruise track”… this statement is potentially misleading, as the study is based on tank experiments rather than in situ atmospheric measurements along the cruise track.

Figure 6: Add significance to correlation plots. Could you make it visually more obvious if the parameters were measured in the seawater or in the aerosols?

L340: “…high amino acid degradation indices…” sounds interesting but in the current form not entirely clear. Could you briefly elaborate and clarify what this implies?

L380-385: Please reconsider whether the extended repetition of results from Lines 253–255 is necessary here, or if this section could be condensed.

L386-394 This section appears rather long and partially contradictory. It could potentially be shortened to something like:’ Aller et al. (2017) reported TEP and CSP as 1–100% of SSA particle mass using a colorimetric approach, and although methods slightly differ, their SSA concentrations (2–14 μg XG and BSA equivalents m-3) are broadly consistent with our hydrogel area in SSA when translated into equivalent concentrations using the SSW-derived relationship.’?

L403-404: For which size range were the enrichment factors calculated?

L420 “bulk seawater” or “underlying seawater”? The SML is also seawater, so the terminology should be used more precisely.😉

L445–446: Did you account for the conversion between (spherical) radius and diameter? As I understand, hydrogels were characterized in your study using diameters, whereas the discussion appears to refer to radii from the cited reference. Please check this for consistency. Additionally, consider specifying a size range for the “film fragments”.

L471: This section is titled “4.3.1 ….”, but there is no section 4.3.2. Should it be 4.4 instead?

Figure 7 is intended to summarize the hypothesized hydrogel formation and aerosol composition. While I find the visual design appealing, I am currently unable to extract clear mechanistic information on the underlying processes from this figure. It is not clear how the different regions (STF, Mix, SAW, STW) are represented and how they relate to biogeochemical seawater processes and aerosols. Furthermore, it is difficult to distinguish whether these shapes represent bacteria, algae, or gel structures. In addition, the color scheme is somewhat confusing, as bright blue appears to represent both TEP and the air–sea interface (SML?), while dark blue represents CSP, the nanolayer (?) and underlying water. What do we learn from the representation of the sun and the sun light in the water column? I recommend revising the figure to improve clarity and ensure that it can be understood more intuitively, ideally without requiring extensive explanation in the caption. Furthermore, I suggest considering the revised version of Figure 7 as a potential Table of Content figure rather than a main figure, as it appears more conceptual than data-driven.

I am aware of a recent preprint by Freney et al. (https://doi.org/10.5194/egusphere-2026-87), which appears to be based on the same campaign(?) and the same tank experiments(?). While the two studies clearly address different aspects, there seems to be some overlap in the underlying data and context. It may be beneficial for the authors to consider cross-referencing this work where appropriate, as it could help to strengthen their own conclusions.

Tables A1 and A2 (Longitude, Latitude, Air Volume, etc.): replace commas “,” with “.”

Table A2: specify the start and end times of aerosol sampling (including exact hours). Add “+” to “Na+”. Replace “Filtration time” with “Sampling time”.

Table A4 (where is Table A3?): ensure the entire table is fully visible on the page.

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Citation: https://doi.org/10.5194/egusphere-2026-1873-RC1
RC2: 'Comment on egusphere-2026-1873', Anonymous Referee #2, 23 Apr 2026 reply

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-1873/egusphere-2026-1873-RC2-supplement.pdf
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Citation: https://doi.org/10.5194/egusphere-2026-1873-RC2
RC3:
'Comment on egusphere-2026-1873', Anonymous Referee #3, 01 May 2026 reply
Barthelmeß et al. presents a novel, comprehensive investigation into the emission of hydrogels in sea spray aerosols (SSA) across four distinct biogeochemical regimes in the Southwestern Pacific Ocean. The study used a plunging-jet bubble chamber to simulate natural SSA generation, and quantified hydrogels in both surface seawater (SSW) and the resulting SSA. Overall, the manuscript is well presented and the finding is important for better understanding of hydrogel contents in aerosols and their impacts on cloud condensation and ice nuclei formation. However, I think the manuscript needs further revision before considering for publication.
I am not sure if that is my pdf problem, I saw that most of the figures in the manuscript are not clear. Make sure all the figures are clear enough to read.

Abstract, Line 16: Typo for “in in”.

Line 83: What are the physicochemical and biological properties of the seawater taken form the four marine regions? Do the properties change when introducing to the bubble chamber? The information should be introduced in the text.

Line 92: It is better to briefly include the information of aerosol size distribution in the text or in the SI.

Line 141: Why the sampling time varies? Will this have effects on the chemical analysis? As can be seen in the Table A2, the Na in SSA is n/a when the filtering time is less than 20h.

L160: What is the particle size of aerosol used for the enrichment factors calculation?

Line 186: There are only two samples for the subtropical waters (STW), with such a few sample, how will it affects the statistics or reliability of the results with these samples?

Line 439: The authors suggest soot may have interfered with bubble scavenging in the SAW but was not visible on aerosol filters. Could this have contributed to the observed variability in SSA concentrations?

Figure 7: The figure is quite confusing and I have problem of understanding the logical flow of the key message without going through the figure caption and main text. The coloring is hard to follow, for examples, what does yellow circle and in the seawater represent? While for the size of pie charts, does it mean anything here? It needs a thorough revision.

There is a missing Table A3 in the manuscript. Please double check.

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Citation: https://doi.org/10.5194/egusphere-2026-1873-RC3

Theresa Barthelmeß, Karine Sellegri, Benjamin Pontiller, Lindsay Scheidemann, Karl Safi, and Anja Engel

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https://doi.org/10.5194/egusphere-2026-1873-supplement

Theresa Barthelmeß, Karine Sellegri, Benjamin Pontiller, Lindsay Scheidemann, Karl Safi, and Anja Engel

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Short summary

Sea spray transports marine organic particles into the atmosphere where they contribute to climate-relevant processes such as cloud formation. We investigated small biogenic gels which were enriched in sea spray aerosols in dependence on the surface sea water conditions as e.g. defined by phytoplankton composition and different organic matter pools. Moreover, the composition of these gels was size-dependent and did not directly mirror the composition of their marine precursors.


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