Sensor for Determining Single Droplet Acidities in the Venusian Atmosphere

Round 1

Reviewer 1 Report

Overall, the paper describes a technically sound and interesting method for droplet acidity detection, with the context of an upcoming Venus mission and testing a hypothesis regarding partial neutralization of the sulfuric acid cloud aerosols.  The background is well justified and the science question is timely and of high interest.  

General comments:

In several places, pH ranges and H2SO4 concentration ranges are discussed as interchangeable.  However, this is an important distinction to make given that the hypothesis the instrument is designed to test regards neutralization of a high H2SO4 concentration by hydroxide salts.  The authors should explain why it is acceptable to test the sensor's performance with more diluted solutions rather than with actual partially neutralized solutions.  Similarly, the authors should explain how the H2SO4 concentrations used for testing were chosen; it's not clear how (or if) they map onto of expected Venus aerosol concentrations.

Some important specifics of the testing methods are missing and will need to be added: measurement wavelengths, fluorescein concentations, and others noted below in individual comments.

Venus aerosols may contain trace amounts of organics, either through biological presence as hypothesized in this paper or through naturally occurring compounds (e.g., Spacek & Benner 2021).  Organics fluoresce strongly in the UV in sulfuric acid, a potential confound for the instrument/science goal proposed.  The manuscript should address whether this requires future work or other complementary measurements.   

The paper would benefit from a few words regarding compatible aerosol capture or exposure methods, which inevitably create some biases to aerosol size, density, distribution, or relative concentration before sensing.

The "Materials and Methods" section is difficult to follow, and has very little method information -- it's mostly a list of materials, particularly the first part.  Much of the methodology is described in the Results section instead.  Ease of reading would be significantly improved if the basic methods information was at least summarized in the Methods section to provide context for the list of equipment and reagents.

Specific comments:

ln 113 - Please provide the concentration of fluorescein used.

Fig 3: Please provide the UV peak wavelength or type of light source.

ln 115 / Fig 4 - Please provide the emission wavelength for the excitation spectra and the excitation wavelength for the emission spectra.

Fig 4: The axis labels and caption are inconsistent (excitation vs. emission); please correct.

ln 123 - This statement is vague and needs clarification.  What is the "range corresponding to the hypothesis about the partially neutralized droplets"?  Is this a pH range or an H2SO4/H2O fraction range?  Numerically, what is the range?  What is the value for "concentrated sulfuric acid" -- is that the 97% indicated in Fig 3, or the ~80% corresponding to the historical Venus aerosol concentration estimates?

Fig 5: Please mark the ranges referred to in the text on this plot.

ln 127 - "the preliminary experiments" - which experiments?  What is "higher" concentrations?  Please clarify.

Fig 9: Subfigs b, c, and d look like they are taken under special light conditions (UV) - if so, please provide this information.

Fig 9: "While the droplets of concentrated sulfuric acid remain visible on the surface for longer, the fluorescence signal disappears in a couple of seconds" - this is an interesting point and deserves to be in the main text - how stable is the fluorescence signal?

ln 157 - is this referring to LIQUION only, or to all tested matrices?

ln 172 - Please clarify this claim - is it referring to the film being able to fully absorb aerosol liquid? or surface coverage? Both would seem to also depend on aerosol number density, flow rate, and sampling time.

ln 176 - fume hood, or laminar flow hood?

ln 180-182: confusing sentence structure

ln 198: This is an order of magnitude larger than the size range of interest indicated previously in the paper.  What are the implications of this mismatch?

ln 206-207: sentence fragment - was there more to this statement?

Fig 13: These spectra show some pronounced features mostly post-450 nm, not present in the earlier plots.  Are these artifacts from the matrix?

ln 220-222: This seems inconsistent with the previous paragraph.  Are the two subfigures in Fig 13 taken with the same setup?  If so, if it's stable enough to produce a full spectrum, why would it not be stable enough to take two point measurements?  The left subfigure seems to indicate that there is plenty of difference in the relative feature heights.

ln 243: sentence fragment: what was the alternative approach?

ln 265: Please clarify what the "sizes" refers to here is -- the droplet size?  Would this require detection from a micrograph using image recognition?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

In their paper, “Sensor for determining single droplet acidities in the Venusian

atmosphere”, the authors, Laila Kaasik, Ida Rahu, Ellen Marigold Roper, Riika Seeba, Agnes Rohtsalu and Mihkel Pajusalu, present a feasible method for measuring the concentration of sulfuric acid in the clouds of Venus. This method is effective in the lab for a range of sulfuric acid concentrations from 0.01 wt% to 100 wt%, and so is very likely applicable to the entire range of sulfuric acid concentrations for the cloud droplets of Venus. This is an important and useful technique that may well be applicable for constraining Venus cloud droplet chemistry. I recommend that the paper be accepted after some corrections are made, mostly grammar and typos. I have divided my comments into minor corrections and grammar + typos.

 

MINOR CORRECTIONS

 

Abstract: I think it would benefit to be more quantitative in the abstract. Maybe mention the wt% and temperature range for which the technique works.

 

Figure 4: It is strange that the peak shift goes the other way for the 9M case. The authors should note this and, if they have an explanation or hypothesis for why, they should discuss it.

 

Line 127-129: “Besides, it was shown in the preliminary experiments that at higher sulfuric acid concentrations, some reaction (most probably the decarboxylation of fluorescein) takes place, which leads to an irreversible change in fluorescence properties.”

 

More precision is needed here. At what concentrations does the irreversible change take place? Do the authors have an estimate of whether this is a kinetic or thermodynamic effect? In other words, will this happen at lower concentrations, but over longer periods of time, or is there a genuine cutoff?

 

Figure 7: It would be useful to add concentration labels to this figure as with Figure 3.

 

Figure 8: Interesting that, in contrast to Figure 4, there is no wavelength shift to speak of. Do the authors know why there is a wavelength shift in one case and not the other?

 

Figure 13: Seems to be cutoff at ~1e-3 M H2SO4, but that should be fine, since that concentration is most significantly lower than the hypothetical low-end concentrations of sulfuric acid from Rimmer et al.

 

GRAMMAR + TYPOS

 

Line 33-34: “Now there are plans for multiple missions to Venus, including the Venus Life Finder Mission series, which is planned to consists of three missions” ->  “Now there are plans for multiple missions to Venus, including the Venus Life Finder Mission series, which is planned to consist of three missions”

 

Line 127: “Besides” -> “In addition” seems more natural here (“besides” tends to highlight a more stark contrast between what is stated here and what came before)

 

Line 164: “Bar coater…” -> “The bar coater”

 

Line 166: “Later on, 12 μm spiral bar coater was used with 7% PVA.” -> “Later on, a 12 μm spiral bar coater was used with 7% PVA.” 

 

Lines 180-183: “Another matrix was tested Nafion™, because this is used to produce semi-permeable membranes for carrying protons and is highly tolerant to sulfuric acid, but these films tended to crack on glass slides depending on the concentration and would be very difficult to main.”

 

I don’t understand what the sentence means as written, and suspect it is because of the last word of the sentence. Should the last word be “maintain”? I also struggle with the wording at the very beginning of this sentence.

 

Lines 289-290: “This would allow to reduce the size and mass of the sensor significantly” -> “This would allow for a significant reduction of the size and mass of the sensor”

Author Response

Please see the attachment.

Author Response File: Author Response.pdf