Dear Nønne and Samuli

Thank you for your reply to the referee comments and for submitting an updated manuscript. Generally, I found that the referee comments were well answered and addressed and therefore I decided not to send the manuscript out to the referees again, but to accept the manuscript for publication after a minor revision (based on my comments below).

Before going into details, I’d like to thank you for submitting to ACP. I’m was very delighted to find a submission of such a detailed laboratory study using and showing the power of XPS to atmospheric scientists. I fully support the goal to make these results better visible to the environmental science community and stress the relevance beyond the pure physical chemistry aspect. I also acknowledge that this is a difficult task, as the delicate balance between giving details and increasing readability by simplifications needs to be found. Infact, I’m facing a similar situation with a submission to ACP’s sister journal. There, we also probed NaCl, looking at phase changes and supercooling at the interface (https://tc.copernicus.org/preprints/tc-2020-253/).

Referee 1 had highlighted the excellence in scientific significance. The comments addressed mainly the processing and analysis of the XPS data raising questions on the confidence of the findings and conclusions.
Referee 2 mentioned an outstanding scientific quality and asked for minor revisions to make the conclusions easier to gasp, to give more details on the peak fitting, and to broaden the introduction.


Thank you for your detailed answer to the referee comments. I was particularly impressed by the addition of the MC simulation to discuss uncertainty in the fitting procedure in more detail. My main remaining comment also addresses the uncertainty. May I ask you to elaborate in slightly more detail on the BE shifts that you observe and on the peak-ratio with water vapr and rule out the following hypothesis. Please don’t get me wrong, I think this is a very clever and novel analysis with a clear finding. However, the I believe the manuscript would be stronger if the following arguments are tackled explicitly:

• What is the origin of the shift you observe? I must confess, that I find the presentation of the results slightly confusing. I tend to understand that “Shift” is defined as change in observed or apparent binding energy relative to that at 0 mbar in the manuscript While this is correct; I’m used to a definition of “shift” relative to un-charged samples (or literature values). NaCl appears to have a N1s BE of 1071-1072 eV; I would therefore argue that the 0mbar samples in Figure 1 are shifted, but the 2 and 5 mbar are not (or less). So, there is less (or no) shift with increasing water vapor pressure – as opposed to my understanding of lines 199-201. As your write, this shift is caused by charging of the sample in UHV and is reduced in presence of gas-phase water. The importance of this, I think, becomes evident, when looking at Figure 5 which describes the C1s of Sucrose. The C-H is precisely where one would expect it at 285eV. Apparently this sample is not charged even at 0 mbar. Then, is my point, this would explain why the BE does not change in presence of water as the sample has already been dis-charged at 0 mbar. This brings me to some questions that I ask you to discuss in your manuscript.
o Can you quantify how much dis-charging you would have expected by gas-phase water alone as compared to adsorbed water? That is an interesting finding, does this explain why and result in water gas being so much more efficient in discharging than nitrogen gas or even oxygen gas.
o How well are the samples electrically conductive? I assume that gold is very conductive, but have no experience with silicon wafers. Could it be that those are less conducting and therefore you observe a charge with the NaCl samples at 0 mbar and not with the organic samples on gold?
o How did your sample look like? Could it be that you sample adventitious carbon mostly at the gold surface -without any charging even at 0 mbar - and sucrose as deposits with are charged at 0 mbar and with reduced charging at 1 mbar? I think this point might be stressed more in the manuscript to explain the different behavior of the individual features in the C1s spectra with respect to changes of the BE at varying RH.
o How certain are you about the BE of the C1s features. What is the impact on fitting (width, peak ratio, position contains as well as background treatment) on the BE for each feature and its change with RH?
o Adventitious carbon often also has some oxidized functionalities (Barr, 1998). Would one not need to differentiate more clearly between the CO peaks of your sample and those from the adventitious carbon (line 350). I agree that this gives too much freedom to the fits though.
• Was there no adventitious carbon on the NaCl samples? By the way, why is O1s not shown for the NaCl samples to quantify the water uptake? Might masking by carbon also explain the trend seen in Fig. 4 and line 286-290 – assuming that carbon built up with time or that the data are from different sample spots?
In summary, I think it would be beneficial to address the charging as well as the adventitious carbon and potentially the different types of sample preparation and sample holder in more detail. From my point, it is important to mention them – I do not see a reason at them moment to re-write or add substantial data to the manuscript.

Further comments (that you may or may not consider). I add a few references from our work because I think they might interest you. As editor it is not my intention to make you cite them.

Line 35: I could not agree more on the importance of the surface to volume ratio (Artiglia, Nat. Commun. 2017)

Line 50: Again, very important point. The presence of organics indeed may have profound impacts on heterogenous chemistry, in particular the solubility of ozone in the liquid aerosol phase and thus the reaction rate of halogen oxidation by gas-phase ozone (Edebeli, Env. Sci. Process Impacts 2019).

Line 65: Here, I wonder how much aerosol deposits as you elegantly used differ from experiments with slurry considering that one exposed the samples to UHV and then humidifies again. Probably a topic to discuss during a conference – I hope we have the chance soon. (Lampimäki, J. Phys. Chem. 205; Orlando J Phys Chem Lett, 2019; Orlando Top. Catalysis 2016)

Line 95: sampling nano-scale particles I have a practical question: How did you find the sample for a good XPS signal? Was it covering the whole sample holder at the measurement spot?

Line 139: please define ESP

Table 1: Please define mü, sigma, etc in the caption.

Line 200 ff: Water uptake before deliquescence has been observed before. Please cite Wise 2008, Aerosol Sci Technol 42, 281-294

Figure 1: mention and explain blue peak in caption.

Line 293 “This behavior” are you referring to the ratio at 0 mbar or the shift with RH?

Line 310: Maybe it is worth to compare the surface coverage also to the findings by Ewing 2005 (H2O on NaCl: From single molecule, to cluster, to monolayer, to thin film, to deliquesence. In chapter 12, springer-Verlag, 2005

Line 338 that -> than (?)

Line 358. Do you observe a difference in peak with for gas-phase and condensed phase peaks? If so, I would add this information as strong argument.











Thank you again for your submission,
This has become a long reply, please take it as interest in your work. Again, I do not expect major changes to the manuscript or the data analysis; but please, may I ask you to mention or discuss your view on the raised topics more explicit in the paper.

I wish you a g merry Christmas and a good start in 2021 with hopefully some more conferences to meet and exchange ideas.

Thorsten

Dear Jack

Thank for for the detailed response to my comments. I'm sorry to insist, I do still see the need to address 2 points explicitly in the manuscript:

* Please mention the possibility of adv.C in the NaCl samples and mention the impact on the attenuation (possibly on page 12-13). I feel that this is important as the audience of this journal might otherwise get the impression that adv. C. is unique to the organic samples.

* I still find the reporting of the binding energy shifts hard to understand for a broader audience. It appears that the relative shifts to 0 mbar are compared in absolute shifts for the different samples. My argument here is, that because the 0mbar spectra are less shifted for the organics as compared to the NaCl - for reasons related to the sample holder and/or the sample - the shift when introducing H2O vapour can not be as large as that observed for NaCl. In this understanding the H2O (adsorbed and in the gas-phase) "only" reduces the shift caused by charging - because it establishes a better charge transfer (conductivity) via the gas phase and via the interface. The BE can not change further than typical values for well conducting surfaces. I think we agree on this, don't we? Looking at Figure 3, I get the impression that the shifts you observe (at low RH) for the organics are already 100% of these expected changes in BE. Increasing the RH further, I would argue that you can not detect further changes in BE even if more water would adsorb. I don't see this explicitly mentioned or discussed in the paper.
I kindly ask you to mention the reason for the BE shift at 0 mbar more explicitly so that a non-expert audience can follow and then shortly elaborate on the aspect I tried to mention above. I think the reader needs to clearly see how much the 0 mbar BE shifted (due to charging), how much the BE can thus change with increasing RH (going back to the uncharged value) and how much change you observed.
Again, I hope you agree that his will not change the interpretation, discussion, outcome of your manuscript. What I have written is already in your manuscript, but I think not in a way that a broader audience can understand it and follow the results easily.

Best, Thorsten

Dear Jack et al.

Thanks for discussing and tackling my thought on your manuscript. I appreciate this a lot and have accepted the submitted revised version.
Coming back to our email exchange, please verify the binding energies.