Review of the manuscript : « Molecular level Insights on the Photosensitized Chemistry of Nonanoic Acid in the Presence of 4-Benzoylbenzoic Acid at the Sea Surface Microlayer” by Ahmed Abdelmonem et al.

General comments:

This work presents a surface specific approach using Sum-Frequency Generation spectroscopy method to a photochemical reaction described in previous work from Tinel et al., 2016. The latter work presented photoproducts and their reaction mechanisms based on bulk measurements in the gas and liquid phase. Here, the study focusses on surface changes of chemical functionalities, which could help understand the surface specific mechanisms that are difficult to study by other methods. The manuscript further uses MS techniques to identify the main reaction products, in gas as well as liquid phase. The abstract announces a study of 4-BBA, a photosensitizer and NA, a fatty acid, in function of pH and salinity.

However, the reader remains frustrated as the study focusses on results at pH 5.4-5.6 since their solution at pH 8 did not sufficiently buffer to work with this pH under stable conditions. Further, the announced study in function of the salinity, is contradicted and resumed by a simple sentence ‘The SFG study also showed that the salt concentration of the bulk accelerates the photo reaction. It is not the focus of this paper to discuss the details of the salinity effect on the photochemistry at this surface, we only register the observed phenomenon’ (P11,L28 - P12,L2). This is contrary to the statements in the conclusions and the abstract of the paper, and really limits the scope of the paper. It would have been very interesting and novel to explore more in detail the influence of salinity on this reaction, especially since the preliminary results in the SI seem encouraging. Since the experiments discussed finally neither fully explore the effect of pH nor salinity, the paper can no longer claim that it is providing insights for interfaces of diverse natural water systems.

There are further no citations in the discussion section, although quite a few works work using this photosensitizer exist which could help clarify the observations made. This is particularly the case when the authors propose reaction mechanisms that seem quite original and rather unlikely. Citing previous works that have observed or proposed mechanisms would make their propositions more convincing.  In several places, the discussion section is repetitive and the reasoning seems to turn in circles, in particular when discussing the SFG spectrum features of the aromatic band and the OH group. The discussion section needs thorough reworking, in light of all remarks in this review (here and following). Once this is done, the abstract and the conclusion section should be reworked as well to better reflect the main points developed in the work.   

Specific comments:

L10: ‘volatile organic compounds (VOCs) can make up a significant fraction of atmospheric aerosols, particularly in urban and industrial areas.’ As is, this statement is incorrect to my knowledge. I think the authors mean that the secondary reaction products of VOCs can contribute significantly to aerosols. Please adjust.

L19-20:  the CDOM references are very old and it has since been shown that CDOM is not always enriched at the surface, see for Van Pinxteren et al.¹, Stolle et al.² or Tilstone et al.³ for more recent work on this. Please modify the sentence to reflect this.

1. van Pinxteren, M. et al. Marine organic matter in the remote environment of the Cape Verde islands – an introduction and overview to the MarParCloud campaign. Atmospheric Chemistry and Physics 20, 6921–6951 (2020).
2. Stolle, C. et al. The MILAN Campaign: Studying Diel Light Effects on the Air–Sea Interface. Bulletin of the American Meteorological Society 101, E146–E166 (2020).
3. Tilstone, G. H., Airs, ruth L., Vicente, V. M., Widdicombe, C. & Llewellyn, C. High concentrations of mycosporine-like amino acids and colored dissolved organic matter in the sea surface microlayer off the Iberian Peninsula. Limnology and Oceanography 55, 1835–1850 (2010).

P4, L3-4: please add some more recent and environmentally relevant references for the existence of SML and its contribution to seaspray (e.g. review by Tinel, L. et al. Impacts of ocean biogeochemistry on atmospheric chemistry. Elementa: Science of the Anthropocene 11, 00032 (2023) and  Sellegri, K. et al. Influence of open ocean biogeochemistry on aerosol and clouds: Recent findings and perspectives. Elementa: Science of the Anthropocene 12, 00058 (2024))

P4, L18-20: the list of publications is not well reflecting the work ‘in recent years’ – please update the references, in particular the paper by Freeman-Gallant, et al. (Photooxidation of Nonanoic Acid by Molecular and Complex Environmental Photosensitizers. J. Phys. Chem. A 128, 9792–9803 (2024)) is of interest for the current manuscript and should be included in the introduction.

P5, L13-15: this sentence relates results obtained and does not seem at its right place here; I suggest to remove.

P5, L18: what do the authors consider a ‘normal pH’ ? Please specify

P5, L23: Did the authors take any precaution as to the purification of nonanoic acid? This is important to precise, since Saito, S. al. (Impurity contribution to ultraviolet absorption of saturated fatty acids. Science Advances 9, eadj6438 (2023)) reported that impurities can play a role in the photochemistry observed.

P6; 2.2: how was the temperature kept stable upon irradiation?

P7, section 2.4: the transfer of the sample with a syringe of a surface-active solution can induce biases due to more or less sampling of the surface layer and adsorption to the syringe (or filter). Can the authors detail what part of the solution was sampled with the syringe and if they assessed the reproducibility of this sampling + measurement method?

P10, L1-9: Have the authors tried to increase the strength of the buffer solution to try and stabilize the pH at 8?

P11, L26-P12, L2: this does not seem an appropriate conclusion to this section. Please state clearly what the main observations are from the study under different wavelengths. The influence of the salinity should be discussed separately or be omitted altogether.

P13, section 3.2.1: the authors discuss aqueous products under irradiation of NA solution and NA+BBA solutions, however in the previous section the changes in spectra with only NA at pH 5.8 are not discussed. Did they see changes there and expected products to be formed from pure NA solutions? Do they see evidence of impurities as well, as discussed by Saito, S. et al. (Impurity contribution to ultraviolet absorption of saturated fatty acids. Science Advances 9, eadj6438 (2023))?

P13, Fig 6: the caption currently reads ‘concentrations of each of the photoproducts’: since quantifying with CIMS is particularly challenging, especially for compounds for which no standards are available, I suggest to replace this by ‘normalized intensities of each…’. Further, it is unclear what the error bar for the sum of products indicates here. 

P13, L24-26: the much larger increase of some products under O2 conditions, is probably also indicative of primary and secondary products formed; hence it looks like C8H12O4 could be one of the primary products formed and the other products secondary products, that are more limited due to the lack of oxygen available. Did the authors also see new products formed under oxygen low conditions? This would be interesting to help elucidate formation mechanisms.

P17, L13-14: the authors state that the study was done in conditions ‘…similar to the real environment at sea water and droplets in the atmosphere.’ I’m not convinced this is indeed the case, as most experiments were done without no salinity and at pH 5.8 and irradiation at AM0, conditions that are neither representative for aerosol, droplets nor sea surface. I suggest to nuance this statement.  

P18, L22-25 and P25, L13-15: do the authors suggest that differences due to different wavelengths (through geographical location, time etc) are currently not taken into account in models? What kind of models are they thinking of here? The reference is not correctly cited as well.  

P18, L31: the authors suggest that the reason for the rapid decrease of the pH is the decarboxylation of 4-BBA. This is a little surprising to me, as this is not the main pathway for 4-BBA reactions (which is expected to be a reduction of the carbonyl group, e.g. Calvert & Pitts, 1966), and should thus only be a minor CO2 source. It should be noted that NA can also undergo decarboxylation, which is a likely a major pathway, as expected for an aliphatic organic acid (and seen some major products observed are C8). Could the authors make a quantitative estimate of how much CO2 they need to change the pH as observed and what sources they think contribute (solubilisation of gas vs decarboxylation) ? This could help better understand how much 4-BBA or NA is expected to disappear.  

P19, L12: ‘Normally NA is only partially dissociated’ Can the authors please precise here what ‘normal' they referred to? It would be good to relate this behaviour to the pKa of NA at 5.23, pH at which NA will thus be present at a proportion of 50/50 in ionic and deprotonated form.

P20, L2-5: The formation of the C9H10O5, suggested to be an aromatic compound, from irradiation of 4-BBA alone is quite surprising. Are the authors sure this is not contamination from a previous experiment (e.g. rinsing several times, repeated results)? If precautions were taken to avoid contamination, further investigation is needed to better understand this unusual reaction mechanism. The reaction mechanism proposed P22, L11 (and also L16) indicates an addition of bicarbonate. The probability of this reaction taking please again depends on the concentration of CO2 expected to be dissolved (see remark above). Bicarbonate would also be majorly under its fully protonated form at pH 5.4, and should thus better be written as H2CO3.

P22, L8-11, L13-17: it would be very helpful to have a developed reaction mechanism schematic instead of the brut formulas here. There are several steps that seem unrealistic, notably the additions of bicarbonate as mentioned before, and in particular L16 where the addition of bicarbonate on an RO2 radical leads to further unsaturations. This could also help clarify the quite unclear discussion of the mechanism P23, L10-14.

L23, L1-6: this seems like an unnecessary repetition and can be removed

P23, L20-P24, L25: The explanation for the appearance of the aromatic band given here is not very convincing; it seems a little far-fetched to use an observed gas-phase product (benzaldehyde) to explain changes in the SFG spectrum at the surface and link these to the lower OH band. It seems like the aromatic band should form with and without NA. Since it is only observed in the presence of NA, this could be due to having less NA at the surface, as it will be more oxidised and hence more solubilised, which would lead to an increase in the aromatic signal from 4-BBA. The appearance of the aromatic band could also be due to other liquid phase products formed from a self-reaction of 4-BBA alone; however, these products are not discussed in the paper explicitly and would not explain why the aromatic band is not observed with BBA alone. Furthermore, benzaldehyde may interact with OH groups from water in a similar way as 4-BBA, since they both have a carbonyl function. This discussion should be revised thoroughly; quantitative (or estimations) of photolysis yields for primary photoproducts from 4-BBA or estimated loss rates of 4-BBA could be helpful in this discussion.

P24, L26-31: it’s unclear what the authors are trying to convey in this paragraph; is this meant to further explain the dangling OH band feature? Or is this supposed to explain photoreaction products (which should be mainly radical reactions however)? It’s also unclear why the presence of catalysts is invoked here.

Language and minor edits:

P4, L24: nonanoic acid should not take a capital here, to be consistent with other chemicals.

SI: S6, Table S3: pH =MilliQ, please replace by the measured pH value of the ultrapure water.

P8, L5 & L23: neglectable: should be negligible

P9, L25: seens the following discussion, I suggest to remove ‘simply’

P10, Fig 3a: the legend indicates AM0 but the caption indicates these are dark experiments. Please correct.

P11, L1: one and a half hours: please write as 1.5 h

P16, L12: ‘…(see Table S5). Only…’ this is the same sentence, please remove the capital.

P16, Fig. 11: please indicate for which solutions these results are shown.

P17, L18: the year of the reference is missing

P18, L28: for solutions with a higher pH value of 8; the whole sentence is not grammatically correct

P22, L5: improved: please replace by increased

P22, L1: please add ‘…the solution with 4-BB and NA’

P24, L16: afterword: should be afterwards

P24, L29: I think I should be a conjugate base, not acid?

SI, S5, Table S1: last column: please correct the units to mM.