Review of van Pinxteren et al. 2023:

Amino acids, carbohydrates and lipids in the tropical oligotrophic Atlantic Ocean: Sea-to-air transfer and atmospheric in situ formation

General comments:

This manuscript describes the measurement of relatively labile biogenic organic compounds of three major classes (amines, carbohydrates, and lipids) in three natural media relevant to the air-sea exchange of organic compounds: bulk surface seawater, the sea surface microlayer, and submicron aerosol. The measurements were made at or near the Cape Verde Atmospheric Observatory, a coastal site surrounded by oligotrophic waters. Roughly half of the organic carbon in the aerosols was quantified, and a major result is that the bulk of this material was lipids.

These measurements are certainly of value to the community of air-sea-exchange and marine aerosol composition researchers, and I hope this work can ultimately be published in ACP. Measuring the relevant compounds at a location from bulk seawater through SML to aerosol is a powerful approach that is worth pursuing.

I have one main concern about the results, and that is the potential role of gas phase adsorption onto (and potentially revolatilization from) the aerosol filters in the high-volume sampler. This is of particular concern given the apparent importance of lipids in the sampled aerosol. The major contributors include hydrocarbons and free fatty acids. Both n-alkanes and fatty acids appear to be subject to gas-surface partitioning effects in ambient aerosol filter samplers (Kavouras et al., 1999; Lawler et al., 2020). The authors need to raise this issue in the manuscript, explain any steps they took to mitigate these effects, and characterize as best as possible what errors (if any) they expect may be associated with this issue. In general there needs to be more description of the sampling methods (see below).

The grammar and phrasing are by and large OK, but sometimes awkward enough that the meaning is unclear. The work would benefit from a going-over by a native English speaker.

To summarize, these results are certainly relevant to the problem of understanding sea spray aerosol composition. Some further description and likely analysis are needed to either show that the aerosol lipid concentrations and enrichment factors should be taken at face value, or what the approximate error may be. If the authors are able to address this satisfactorily, I would recommend this for final publication in ACP.

Specific comments:

L48: “The same compounds studied in the seawater…” Sentence unclear. Reword, make 2 sentences?

L53: relative composition of the single organic compounds. Maybe relative “abundances” of the organic compounds?

L79 awkward sentence.

L175: Please describe the aerosol sampling setup in more detail. Was there any attempt to remove gas phase species from the sampler, e.g. a denuder? Is there any way to assess breakthrough of compounds revolatilized from the filter? What is the timescale/frequency of the sampling? What are the handling procedures for the aerosol filters? Are there blank filters of any kind to correct for backgrounds? I realize some of this info is in Triesch et al. 2021b, but please include more info here and cite that paper for the rest.

L178: for a boat “to motor” is probably better than “to drive”

L306 Please explain the difference between DAA and FAA in this and the Triesch study. It’s not clear to what extent those are to be considered the same or different.

L727 Seems to state too much. Maybe it’s “reasonably representative” of most of the ocean surface? I agree it’s at least better than coastal sites with upwelling e.g. It may be worth mentioning that this extrapolation to the globe also neglects any potential seasonal changes.

L759 “with respect to sea salt” maybe is meant?

Table S5: Please make it clear in the caption that these relative mol fractions are relative to the total of each type (DAA, DCHO, and DL) analyzed.

References:

Kavouras, I. G., Lawrence, J., Koutrakis, P., Stephanou, E. G. and Oyola, P.: Measurement of particulate aliphatic and polynuclear aromatic hydrocarbons in Santiago de Chile: Source reconciliation and evaluation of sampling artifacts, Atmos. Environ., 33(30), 4977–4986, doi:10.1016/S1352-2310(99)00281-2, 1999.

Lawler, M. J., Lewis, S., Russell, L. M., Quinn, P. K., Bates, T. S., Coffman, D. J., Upchurch, L. and Saltzman, E. S.: North Atlantic marine organic aerosol characterized by novel offline thermal desorption mass spectrometry: Polysaccharides, recalcitrant material, and secondary organics, Atmos. Chem. Phys., 20(24), 16007–16022, doi:10.5194/acp-2020-562, 2020.

Review of the paper:

“Amino acids, carbohydrates and lipids in the tropical oligotrophic Atlantic Ocean: Sea-to-air transfer and atmospheric in situ formation” from M. van Pinxteren and co-authors.

This paper presents a very interesting work on the sources of organic compounds in the atmosphere from marine surfaces that are still badly characterized. The authors investigated the sea-to-air transfer by measuring a selection of organic compounds both in the surface sea water and in submicron aerosol particles collected at a coastal site (Cape Verde Atmospheric Observatory). Those classes of biogenic compounds are amino acids, carbohydrates, and lipids; they are strongly related to the marine biological activity. The enrichment of those compounds between the 3 sampled natural media (bulk sea water, sea surface microlayer, submicron aerosol) are evaluated in this study.

These measurements are original and provide to the community very important information on the potential mechanisms leading to the generation of organic aerosols in the atmosphere: enrichment of the surface microlayer by biotic and abiotic processes, selection of compounds according to their physical properties (hydrophobicity), etc.

My major concern is relative to the fact that the various samples that have been analysed have not been collected at the same period. The conclusions of the article are based on average values analysed over the period of the field campaign. Moreover, the “link” between the two sites allowing the collection of sea water and atmospheric particles is not analyzed in this study. The authors need to mention the limits of their approach more explicitly concerning these two points.

However, the main conclusions of this work are relevant for the scientific community working on the chemical composition of sea spray aerosol that clearly play a role on the climate system (radiative aerosol properties and formation of cloud droplets and ice crystals). If the authors take into consideration these main points and consider the more minor comments listed below, I recommend accepting this article to ACP.

1 Introduction

- Lines 79-81: please rephrase this sentence.

- Lines 82-84: could you add more information on the role of marine chemical composition on aerosol-cloud interaction (i.e., climatic effect).

- Line 149: You write “atmospheric in situ formation”. I think this is more “atmospheric in situ transformation”. Could you confirm?

- At the end of this section, you should better describe your methodology. You followed the concentrations and speciation of OC from the sea to the atmospheric particles; this helps to evaluate the evaluate the enrichment factors of the various targeted compounds between the compartments. By this way, you can better assess, as you mentioned, “the coupling of marine aerosol particles to the sea surface biological and chemical properties”.

2 M&M

2.1 Sampling strategy

- You describe in this section your sampling strategy.

It could be helpful for the reader to have a table where you indicate for all your samples:

• The type of samples: SML, ULW, AERO…
• The sampling period
• Meteorological information when available
• The sampling procedure (short description)
• Which kind of analysis has been performed for these samples: AA, CHO, Lipids, DOC/WSOC, IC, EC/OC, …
• The analytical procedure (filtration, chemical treatment, analytical instrument…) (short description)

This table could be implemented in the manuscript or in the SI, as you prefer.

- Line 188: You mention that your glass plate and bottles were rinsed several times. How do you evaluate possible contaminations? Was it possible to do some blanks (or not!)?

- Does several SML and bulk water samples have been performed at the same time? And analyzed in the lab?

- What are the storage conditions before the analysis?

- It could be interesting for the readers to give information on the backward trajectories of the air masses that have been sampled during the PM1 collection (see my comment below). If you add this in the SI, please give the relative information for these calculations (backward traj.) in section 2.1.

2.2 Chemical analysis

- Line 199: “DOC is the fraction of OC that passes through a filter of 0.2–0.7 μm pore size”. I don’t understand here… What does it mean? In my opinion, DOC is measured by filtration via a 0.7 μm pore-sized filter only?

- Does the analysis been performed several times (triplicates)?

- Could you add some information regarding the LOD (limit of detection) and LOQ (limit of quantification) from your analytical procedures?

- Line 234: Leucine and Isoleucine are isomers. How do you distinguish these two compounds?

- Line 225: could you explain more why the addition of ascorbic acid can avoid “the oxidation of the obtained AAs”?

- Line 239: change “Seawater” to “seawater”.

2.3 Evaluation of the enrichments

- Here, one important information is the thickness of the sampled SML; it can be surely calculated as the quotient between the sampled volume of the SML and the sampling area of the glass plate. You indicate page 10, line 351 that the thickness is around 100 µm. Maybe you can add in this section how it is evaluated?

- Those factors are calculated based on the concentrations of the analytes in the different compartments that present uncertainties. You can mention here how these uncertainties are taking into account in the final calculations of EF_SML and EF_Aero ?

3 Results

- Line 294: “bulk water” instead of “Bulk water”.

3.1.1 SML and bulk Water

- It is essential here to refer to the number of samples that have been analyzed and to mention that the pool of samples for the various analysis (AA vs CHO vs Lipids) are different. Please indicate on Figure 1 the number of samples that have been analyzed allowing to create each boxplot. If the DOC in SML and BW have been measured (I’m not sur because nothing is indicated in section 2.2.2), please add this information on Figure 1 and comment this regarding the different classes (AA vs CHO vs Lipids).

- Only two samples (03/10/2017 and 07/10/2017) present a full chemical characterization (AA, CHO and Lipids) if I’m right. During these two days, you also observed a higher enrichment factor of DAA in comparison with other classes of compounds? You should mention this in the text.

- If it is possible, some statistical analysis must be performed; this will help to give more robust conclusion on the difference between groups (i.e., between the class of compounds). For example, Mann-Whitney nonparametric tests allow to validate significant differences between two groups.

- Figure 2. You have drawn the relative compositions of the various classes of compounds in %mol. The concentrations in the aqueous phase are in µg L^-1 (of water) and concentration in PM1 are in µg m^-3 (of air). Please explain your calculation in the SI, this will help the reader.

- Table 1. EF are presented in this table. For comparison, you mentioned the work of Rastelli et al. (2017) for EF_aero. Could you add other references in this table for EF_SML (but maybe there is no previous works looking at your targeted classes of compounds)?

- Line 306: You compare DAA with FAA. Do not forget to explain in the manuscript the difference.

- Lines 333-335: Your conclusions should be made with more caution. The number of samples is rather limited, and a statistical analysis could help to conclude.

3.1.2 Discussion on the SML

- 362-363: You mean that you cannot specifically sampled the first nano-layer on the very top of the surface? Therefore, you dilute the lipids that could be highly concentrated in this nano-layer? Could you her rephrase please to be more precise?

- 373-388: You explain that “non-surface active and soluble compounds” can be more concentrated due to co-adsorption some surfactants (lipids) leading to their enrichment. In the title, you mention also “complexation”. You mean what? Complexation with some specific compounds? Could you add information about this point?

- Line 399: you mention that Glu can be produced by biotic and abiotic mechanisms, and you refer to Jaber et al. (2021). However, this paper deals with the incubation of AAs with bacteria isolated from cloud water and representative of this kind of environment. This is different from the SML.

- Line 402: Which stressors can modify the microflora metabolic activity?

- Lines 424-426: You indicate that cyanobacteria can generate amino acids. Do you have biological data regarding those bacterial strains during the MarParCloud campaign that could link biological activity with the AA enrichment?

3.2 Aerosol particles

- Please add on the boxplots (Figure 3) the number of samples that have been analyzed for each class of compounds.

- Lines 470-473: please re-phrase this sentence that is unclear.

- Lines 461-469: This seems strange to me why you focused your comparison specifically on polar regions. Carbohydrates, amino acids have been investigated in many other studies at sites characterized by different environmental conditions (marine, rural, urban, polar)… You should restrict your comparison to marine sites or to sites under marine influence(?)

See the review from Matos et al. (2016) and the supplementary information given in Renard et al. (2022) for AAs; see also for example the paper from Samaké et al. (2021) and Zhu et al. (2022) for sugars.

Matos, J. T. V., Duarte, R. M. B. O., and Duarte, A. C.: Challenges in the identification and characterization of free amino acids and proteinaceous compounds in atmospheric aerosols: A critical review, TrAC-Trend. Anal. Chem., 75, 97–107, 2016.

Renard, P., Brissy, M., Rossi, F., Leremboure, M., Jaber, S., Baray, J.-L., Bianco, A., Delort, A.-M., and Deguillaume, L.: Free amino acid quantification in cloud water at the Puy de Dôme station (France), Atmos. Chem. Phys., 22, 2467–2486, https://doi.org/10.5194/acp-22-2467-2022, 2022.

Samaké, A., Jaffrezo, J.-L., Favez, O., Weber, S., Jacob, V., Albinet, A., Riffault, V., Perdrix, E., Waked, A., Golly, B., Salameh, D., Chevrier, F., Oliveira, D. M., Bonnaire, N., Besombes, J.-L., Martins, J. M. F., Conil, S., Guillaud, G., Mesbah, B., Rocq, B., Robic, P.-Y., Hulin, A., Le Meur, S., Descheemaecker, M., Chretien, E., Marchand, N., and Uzu, G.: Polyols and glucose particulate species as tracers of primary biogenic organic aerosols at 28 French sites, Atmos. Chem. Phys., 19, 3357–3374, https://doi.org/10.5194/acp-19-3357-2019, 2019.

Zhu, R.-G., Xiao, H.-Y., Cheng, L., Zhu, H., Xiao, H., and Gong, Y.: Measurement report: Characterization of sugars and amino acids in atmospheric fine particulates and their relationship to local primary sources, Atmos. Chem. Phys., 22, 14019–14036, https://doi.org/10.5194/acp-22-14019-2022, 2022.

- Lines 470-526: I was wondering if FT-ICR-MS analysis were also performed on marine aerosol. Using van Krevelen diagrams, you can have access to the proportion of some classes of compounds (lipids, proteins, carbohydrates, amino-sugars…). This could be included in your discussion here.

The recent paper from Renard et al. (2022) analyzed with this technique the organic matter in cloud waters under marine influence. They indicate that “Lipids are correlated with the sea surface for air masses transported within the free troposphere, confirming the long-range transport of marine biogenic sources.”. Their conclusions confirm your results concerning the predominance of lipids.

- Lines 583-598: this paragraph highlights the limitations of the comparison between seawater and aerosol samplings. You mention the sampling periods that are different but also the link between CVAO and the sea water site. Some previous works are indicated in this section to validate the possible link between the upper ocean and the aerosol particles.

You analyzed in this study specific days of sampling during the field campaign. Is it possible to add in the SI information about for example backward trajectories, meteorological information during this specific period?

3.4 Contribution to OC

- I think that organic carboxylic acids other than oxalic acid have been also quantified (?); what are their contributions to the OC?

4 Conclusions

- Line 778-779: Dominutti et al. (2022) have also shown that AAs detected in cloud waters sampled at a tropical site under strong marine influence contribute to a small % of the DOC. Sugars were more significantly present than in the present study.

Dominutti, P. A., Renard, P., Vaïtilingom, M., Bianco, A., Baray, J.-L., Borbon, A., Bourianne, T., Burnet, F., Colomb, A., Delort, A.-M., Duflot, V., Houdier, S., Jaffrezo, J.-L., Joly, M., Leremboure, M., Metzger, J.-M., Pichon, J.-M., Ribeiro, M., Rocco, M., Tulet, P., Vella, A., Leriche, M., and Deguillaume, L.: Insights into tropical cloud chemistry in Réunion (Indian Ocean): results from the BIO-MAÏDO campaign, Atmos. Chem. Phys., 22, 505–533, https://doi.org/10.5194/acp-22-505-2022, 2022.