General Comments:

This paper calculates photolysis rates using Socrates and evaluates the results through a comparison with two other reference models, UCI-ref and UCI-Jxr. The evaluation was conducted using both Earth’s atmosphere, and an example exoplanet. The absorption cross-sections and quantum yields used were mostly based on expert panel reviews from IUPAC and JPL where available, with other sources used when necessary. The paper is a useful contribution to the field, especially as absorption cross-sections and quantum yields have been updated through this work, however more work needs to be done to extend the analysis of the discrepancies between the models, and improve consistency and readability before it can be published. Additionally, the quality of many of the figures (and their captions) does not seem to be up to GMD standards and should be improved.

Primarily, there are too many technical mistakes. Although many of them are minor, their quantity indicates a more thorough read through and edit is required by the authors (in addition to correcting those I have noticed in the ‘Technical corrections’ section).

A few sections mention the rates of reactions (lines 227,234), but there is no reference of how this was obtained, where are the quantum yields and absorption cross-sections from? Even if this is mentioned in Table A1, it should be referred to in the text.

Additionally, the analysis of the discrepancies between the models is missing depth. Is this work considered better than UCI (which may have quantum yields/absorption cross-sections that are not up-to-date)? Or is UCI already the standard, and this comparison is just to check the reliability of this work? Does UCI use pressure and temperature dependent quantum yield? It is unclear from reading your paper. Differences between the two photolysis models should be explored further.

Finally, figures are not of a high quality and should be improved generally. Here are some specific examples, where detail, readability and consistency are lacking:

• Any mention of molecules should be correctly written i.e. ‘O₂’ not ‘O2’ in Figure 2.
• Ensure that the labelling is consistent: in most figures it should be ‘µm’ instead of ‘micron’, and ‘J rate (s^-1)’ instead of ‘J rate s^-1’
• Many caption descriptions are not detailed enough to stand alone. Some examples: Figure 3 should have clearer references to the legend used in the figure, Figures 4 and 5 bottom row have no mention of where that data comes from (along with all other figures that include a J rate against wavelength), Figure 7 caption is missing detail (legend not explained)
• Some of the figures are misaligned (such as Figures 7 and 16, but there are many examples)
• The text and numbers are very small, should be increased for visibility

Specific Comments:

Line 176: Why was Part 1a used, and not another set-up? Can you provide a sentence or two to explain? Can you discuss how only looking at one very specific scenario ‘clear sky, no aerosols, high sun (Solar Zenith Angle, SZA = 15^◦) over the ocean’ will affect the results?

Lines 226,247: You mention that there are differences due to input data, do you know why absorption cross-sections and quantum yields are different? Is one more up-to-date, do they use a different source?

Lines 294-295: Can you find the cause of the discrepancy, instead of listing potential causes

Line 306: ‘which the reference models may not’ is there no way to know for sure?

Lines 320-324: Can you add a figure (potentially in the appendix) demonstrating the effect of the pressure dependence on a J rate vs wavelength plot, at different pressures? This would be similar to Figure 6 bottom panel, but showing the rate with and without pressure dependence of quantum yield? It would be good to do this for formaldehyde too

Line 330: You mention there is a great deal of uncertainty with the input data, can you expand on this and provide examples? Especially as this is mentioned again in the last conclusion point

Technical Corrections:

Generally:

• Fast-JX: capitalise the ‘X’ throughout the text
• Molecule names should not be capitalised (formaldehyde lines 303 and 304, glyoxal line 326, carbonyl sulfide)

Line 17: units should be separated with a space ‘25 km’

Line 20: ‘At the surface, UV irradiation has implications…’ remove comma

Line 24: This sentence is confusing, what do you mean by ‘elements’?

Line 29-31: This sentence is very hard to read and understand, please rework

Line 90: there is an extra space after ‘remembered’

Line 154: EUV was mentioned for the first time without a definition. Wavelength range covered in extreme UV category would also be useful

Line 202: What is ‘N ³’, is that a mistake?

Line 209: ‘O(³P)’ not just ‘(³P)’

Figure 8 caption: Needs space between ‘pressure( Pa)’ should be ‘pressure (Pa)’

Line 259: I think it should say ‘400-420 nm’ not ‘300-420 nm’

Line 319: the word ‘significant’ should only be used if it is statistically significant. Is that the case here?

Line 344: FUV not defined

Line 345: space needed ‘121.6nm’

Line 359: NUV not defined, Figure 4 missing capital

Line 382: ‘and so similar to’?

Line 412: 200nm, space needed

Line 459: 50nm, space needed

Table A1: Table caption should be above a table, not below. More description of the table is needed in the caption

This paper reports predicted photolysis rate constants for the modern terrestrial atmosphere using the Socrates radiation scheme and compares them to CCMVal PhotoComp 2011. The Socrates model is then used to predict photolyis rates for the Earth if it were around Proxima Centauri at 0.02 AU, which is taken as an example of an Earth-like exoplanet in the habitable zone around an M dwarf star. The availability of values for exoplanet studies are novel and, if valid, of interest and value to the field. The significance and quality of the work is currently limited, however, as the authors provide little in the way of in-depth explanation for the discrepancies between their solar values and those of PhotoComp. This makes it difficult to be certain that this work is valid, let alone an improvement on previous work. They often ascribe differences between their results and reference models to “model differences” without clarification of whether these differences represent advances in the measurements of photolysis cross-sections, rate constants, quantum yields etc. and therefore provide a significant improvement to the PhotoComp models, or if their model is missing some key features or data.

The paper is generally difficult to read: figures are unclear and need significant improvements (detailed below) before the paper would be suitable for publication. In addition, the language used in the paper is often highly informal and overly verbose, with run-on and grammatically incorrect sentences appearing throughout. I have provided some examples of ways particularly confusing sentences could be reworded to be clearer and more direct, but the paper generally needs significant revisions to improve the clarity and formality of the language used.

Recurring and general issues:

• The figures generally need much improvement. Axis labels are too small to read, chemical species are often not formatted correctly (missing subscripts), arrows are not drawn correctly (using -> instead of → or similar), wavelength units are often given as “micron” rather than “μm”. Many axis labels also fail to differentiate units from titles, e.g. “J rate s^-1”, and need brackets or other means. The readability of plots would be improved by the inclusion of titles or other labels for each subplot, especially in the second half of the paper when rates using solar spectra and Proxima Centauri spectra are shown as separate subplots on the same figure, with no immediate indication of which is which. Line labels in figure legends are unclear and inconsistently formatted, with labels alternating between, for example, “SOCRATES” and “SOC”. Other labels, for example “SOC no T QY” are entirely unintelligible without consulting the main text. Figure captions need additional details for the figures to be understandable without referring back to the main text of the paper. The authors could also consider providing row and column headings for figures with multiple subplots to make it clear that each row applies to the same reaction, and each column shows rate as a function of a different variable/for a different stellar spectrum. The use of subplot numbering or letters should also be considered (Figure Xa, b, etc.) to avoid the current reliance on “top row”, “second row” etc. currently needed to distinguish between subplots. Finally, though this is a subjective point, the authors could consider switching the colours for solar and Proxima Centauri spectra. Using red for the warmer star (the sun) and blue for the cooler star (Proxima Centauri) may make the plots more instinctively readable.
• The model is at times referred to as "Socrates" and as "SOCRATES" at others. This should be consistent throughout the paper. Similarly, “PhotoComp” and “Photocomp” are both used throughout the paper, this should be consistent.
• Figures generally report wavelength in μm (though not always), whereas discussion in the text refers to wavelengths in nm. This conversion leads to mistakes when values in μm are reported in nm (e.g. “wavelength region 0.08-130 nm” (line 382), or “one band centred on 0.2155 nm” (line 427)). Units should be consistent throughout the paper, in both the text and the figures. The use of nm for both seems the most straightforward.
• Acronyms should be defined the first time they are used (e.g. EUV, GCMs)
• When referring to subplots, the italicisation of top/bottom/left/right etc and row/column should be consistent (e.g. Figure 4 caption italicises both words, while the main text generally only italicises the first). Locations should always be in brackets in the main text, e.g. “H₂CO into H and HCO top row, H₂CO→H₂+CO middle row” (line 414) should be “H₂CO into H and HCO (top row), H₂CO→H₂+CO (middle row)”.
• Throughout the paper, NOx, HOx, and Ox should instead be NO_x, HO_x, and O_x.
• “J” is a photolysis rate, so "J rate" is cont technically correct. Labels should be either “J (s^-1)” or “photolysis rate (s^-1)”, not “J rate s^-1”.
• Values and units should be separated by a space (e.g. Figure 12 caption 0.02AU ahould be 0.02 AU)
• The choice of pressure levels for comparison in Figure 6 onwards should be explained.
• When comparing models or datasets in figures, the model that each spectrum or subplot is from must be labelled on the plot.
• “μ m”, when used in the paper, is usually written with a space between the μ and m. It should be all one word and should not be partially italicised.
• From Figure 8 to the end of the purely solar section (4.1), figures do not contain plots of J as a function of wavelength. These appear to instead be grouped with the Proxima Centauri figures. Some explanation should be given for why the pressure the wavelength dependence of J is no longer relevant in this section, but was for earlier species and for the comparison of solar and Proxima Centauri spectra.
• The authors should consider whether the shortening of “Proxima Centauri” to “Prox Cen” is worthwhile, as it makes the manuscript slightly more difficult to read but does not decrease the number of words used and makes only a marginal change to the length of labels etc.
• Figures 4, 5, and 6 are shown with J vs. pressure on the top row and J vs wavelength on the bottom row. From Figure 7 onwards, this changes to J vs pressure on the left and J vs wavelength on the right. A consistent layout of similar figures would make the paper easier to read. The authors could consider changing the layout of Figures 4, 5, and 6 to be consistent with later figures.

Specific issues and suggestions:

• Line 20: “there are many other trace gases in the Earth’s atmosphere that can undergo photolysis.” Consider listing some examples here.
• Line 32: “Cross sections and quantum yield data are measured in laboratory experiments, with their subsequent recommended
• values collated in various literature sources.” These can also be predicted from quantum calculations to fill in the gaps in laboratory data, is there a reason the authors have ignored such sources?
• Line 37: The clause “...includes both a calculation of the radiative heating rates, and photolysis rates within a simulated atmosphere.” Is grammatically incorrect and should be reworded to something like “includes calculation of both radiative heating rates and photolysis rates within a simulated atmosphere.”
• Line 41: Define “LFRic”
• Line 42: Define “GCMs”
• Line 52: “wavelengths down to far and extreme UV” – give wavelength ranges for these regions
• Line 53: “O₂, N₂, and O absorption” implies these species are being absorbed, not that they are absorbing.
• Line 60: “high resolution” – specify the resolution or refer to a table or figure where it is shown
• Line 66: “Next” is not necessary here, and feels overly informal.
• Line 72: “Extra species relevant to exoplanets are included in an extra category” - specify the section here.
• Figure 2: In addition to the general comments on figures above, “cm²molecule⁻¹” in caption should have a space in: cm²molecule⁻¹
• Line 84: “Essentially, ...” this sounds highly informal and should be removed or reworded
• Line 97: no comma after “where”
• Line 98: The sentence starting “The flux in terms of ...” is unclear and should be reordered to something like “The flux is converted to units of photons m² s^-1 (A) by dividing by the energy of a photon with wavenumber of the midpoint of the sub-band.”
• Line 110: all non-unitless variables have their units defined here except wavelength, so units should be added for consistency
• Line 126: Some explanation should be provided for the switch from wavelength to wavenumber
• Line 136: Some additional explanation should be provided to explain why species “such as HNO₄” are worthy of note.
• Line 141: Sentence beginning “ There are hydrocarbon species...” is informal and should be rephrased, e.g. “H₂O and hydrocarbon species (e.g. CH₄) also have relevance for early-Earth-like exoplanets”
• Line 145: “not suitable” and “can be neglected” seem to be opposites: if Rayleigh scattering by air is a poor model (i.e. not suitable), it should not be used. If the effects of it are so small as to be inconsequential, it can be neglected. The two are not the same. If both somehow apply, this should be further explained.
• Line 147: Four sources are listed, not three (Burkholder et al., Atkinson et al., Venot et al., and Hébrard). “Prominent” seems an odd choice of adjective here given one of the sources referred to is private communication – this seems the opposite of prominent.
• Line 150: “...data was needed and sourced elsewhere.” Where the data is from, or reference to where that information is available (Table A1?) should be included in this sentence.
• Line 153: “...must adjust accordingly via these factors.” This sentence does not make sense, should “via” be “due to”?
• Line 153: “This process was only included for oxygen as the data was available and predominantly comes into effect for EUV wavelengths”. Rephrase to clarify the meaning of “only included for O₂ as the data was available” here: the data was only available for O₂ (and not other species), so corrections were only made to O₂, but would ideally be performed for other species; or the correction was applied to O₂ because the data was available, not because it is required as its effect is negligible?
• Line 154: EUV should be defined here, if not before
• Figure 3: In addition to general points about figures, TOA should be defined, and labels should clarify which model is being used in each case. Does the green line show the Photocomp TOA flux, or is it the Socrates flux at 1 Pa, a value chosen to match the Photocomp TOA?
• Line 177: Sentence beginning “There they assumed a clear sky...” this sentence is very difficult to read and grammatically incorrect, reword.
• Figure 4: In addition to general points about figures, the model being shown in the bottom row should be indicated on the plot.
• Figure 5: In addition to general points about figures, models should be specified on plots and labels should be consistent within a figure. For example, the bottom left panel refers to the green line as “~1 Pa”, whereas the bottom right panel refers to the green line as “Photocomp TOA ~1 Pa”
• Line 187: For readability, “Sections 4.1.3, 4.1.4, 4.1.5 and 4.1.6 ” could be “Sections 4.1.3 – 4.1.6”
• Line 207: “Finally, we only present results for the main species in the main part of this paper.” This implies the existence of results for minor species elsewhere. I have not been able to find any supplementary information, should this be available somewhere?
• Line 209: “(³P)” should be “O(³P)”
• Page 10 footnote 1 and 2: These links contain spaces and therefore do not work. The data is also not available from the corrected links without permission. If the aim of this paper is to provide an informative comparison of models, it would be useful for this to be generally available if possible, or for the paper to contain instructions on how to request access if not.
• Line 213: “Hartley bands 200-310 nm”: Wavelength range should be in brackets
• Line 214: More precise wavelength ranges for the Huggins and Chappuis bands should be provided
• Line 220: “...between 220-290 nm, for the lower/mid-atmosphere” there should not be a comma here, and “for” should be “in” instead.
• Line 226: “...caused by differences in the input data (cross sections, quantum yields and stellar spectrum) between the models.” This needs much more detail. Which are different and how do they differ? Are the changes an improvement or not?
• Line 228: informal, remove “slightly”
• Line 234: Sentence beginning “The top left panel of Figure 5...”: the greatest disagreement between the rates in Figure 5 occurs at high pressures (> 10⁴ Pa), which is not addressed. What causes this difference?
• Line 247: This line refers to small discrepancies due to differences in input data, which are not expanded on in any significant detail either here or in the section referenced.
• Figure 7: In addition to the general points about figures, the line labels in this figure are particularly unclear. The model names are inconsistent across the subplots (SOC vs SOCATES), labels such as “SOC no T QY” and “SOC O₃, O₂ mmr only” are impossible to understand. The pressure levels chosen in the right-hand column change between each subplot and the reason for the choices of levels are not made clear.
• Line 250: N(⁴S) is not explained
• Line 250: “..as a function of pressure ( Pa, left panels) with UCI-Jxr added (purple line) and as a function of wavelength (μ m) in the right panels.” Reorder this sentence to make it clear that UCI-Jxr is added to the left hand panels to avoid confusion with the magenta line in the right hand panels
• Line 252: there is clear disagreement in the shape of the profiles below 10³ The cause of this should be explained.
• Line 255: “The absorption spectrum for NO₂ is quite complex”. This is vague and imprecise. What is meant by “quite complex”? Presumably that it has fine structure and therefore needs a high resolution spectrum to reproduce the cross section accurately?
• Line 257: “This is beyond the threshold limit for this photolysis reaction to occur which is 398 nm, but can be accounted for...” This sentence is very difficult to read. Reword the first part of this, e.g. “Beyond the 398 nm threshold for the photolysis reaction to occur...”
• Line 258: “This reflects the behaviour of the quantum yield which decreases down to zero between 300-420 nm.” Rephrase and/or clarify: is the quantum yield zero from 300 to 420 nm, or does it decrease near 300 to a value of 0 between the two, then increase towards 420 nm? Or does it decrease from a non-zero value at one extreme of the wavelength range to 0 at the other?
• Line 270: “shows” should be “show”.
• Line 282: “discrepancies lie with different input cross sections and model differences.”: This needs to be expanded on. What model differences? Why are different input cross sections used? Are they improvements?
• Line 283: “The gas NO’s photoabsorption cross section” Don’t use possessive apostrophes for gases, reword to something like “The photoabsorption cross section of NO gas”
• Line 288: “Therefore a pressure and temperature dependence, of the absorption coefficients, were included in our calculations.” This is not a separate clause and the commas should be removed.
• Line 294: “This could indicate a missing component either in the wavelength range included or the quantum yield, or the use of different input data.” It seems quite important to know which of these is the case, as one highlights unknowns that need to be addressed in this field of study going forward, whereas the “different input data” could reflect an improvement compared to previous work.
• Line 297: Sentence beginning “For HNO₃, we adopt the quantum yield...”. This paper would benefit from the quantum yields used being more readily apparent, In cases such as this, where such large changes arise from the choice of quantum yield alone, it would be useful to have the Socrates quantum yields readily available (without having to find the relevant quantum yield in Table A1, find the reference, and find the quantum yield from the reference).
• Line 299: “...significant discrepancy between the Socrates and reference rates, unless we adopt a quantum yield of one and the near–infrared cross sections and quantum yields are omitted.” This should be clarified: In all of these cases, are the reference models wrong and they were missing factors (variable quantum yields, IR cross sections etc.) and this work is a clear improvement due to the inclusion of these things, or are the expected quantum yields adopted in this work in some way uncertain or untrustworthy?
• Line 302: S(³P) is not explained (ground state vs excited)
• Line 307: “mid atmosphere” should be “mid-atmosphere”
• Line 309: Sentence beginning “The main difference in our rates and those of...”: Is this difference in choice of input data an improvement (for example, using higher resolution data, capturing the fine structure of the cross-section) or a limitation (for example, lack of pressure and temperature dependence)?
• Line 313: carbonyl sulphide should not be capitalised
• Line 314: “... which implies that the reference also included this.” This sentence suggests that part of the aim of this paper is to critique the reference models, back-out their assumptions and choice of values. If this is an aim of the paper, it should be mentioned earlier and receive more consideration throughout the paper. If not, what is the cause of the agreement worthy of note? Presumably the most important this is that the correct values are used going forward?
• Line 326; “Glyoxal” should not be capitalised
• Line 328: “The rates calculated by Socrates CH₃ONO₂ are also significantly...” Something is missing to make this sentence make sense, possibly the word “for”?
• Line 342: “This is an appropriate total incoming flux for a planet in the habitable zone around Prox Cen.” Consider adding further information here about corrections to the flux for planets at different distances from their stars. Are the values used here still valid if the total incoming flux is rescaled? What corrections would be necessary in that case?
• Line 344: “FUV” must be defined here, if not before
• Line 345: “This has implications for the photolysis rates of certain species...” List the species or refer to the sections where this will be discussed further
• Line 349: “Figure 13 shows the actinic flux at three different atmospheric pressure levels, namely the TOA (blue)...” Presumably this section uses an identical atmosphere to the Earth’s, but placed around Proxima Centauri? This should be explicitly stated somewhere in the introduction to Section 4.2 to make this clear.
• Line 356: “...on a log scale as a function of pressure (Pa, top panels) and wavelength (μ m, bottom panels) for just the Prox Cen spectrum.” The figure contains rates as a function of pressure for both Proxima Centauri and the Sun in the top panels. The wording of this sentence currently implies the figure shows only data for Proxima Centauri, so should be corrected to make it clear when both stars are plotted.
• Line 358: “...are much lower than Solar because the visible part contributes a larger proportion of the total rate and there is much less NUV when compared with the Solar spectrum results...” Some additional explanation should be included here to explain this point better, especially as the photolysis cross sections of species at all wavelengths (i.e., without the effect of the stellar spectra included) are not available in this paper. Are the photolysis cross sections lower in the visible range compared to the near-UV, and Proxima Centauri has more power in the visible range and less in the near-UV compared to the Sun, meaning photolysis is less efficient for Proxima Centauri?
• Line 359: “figure 4” should be capitalised
• Line 368: “the major contribution is from wavelengths > 175nm”. Some explanation here of why the Lyman-α line is not the dominant contribution despite having a photolysis rate three orders of magnitude higher in Figure 15 (bottom right) than values above 175 nm.
• Line 382: This should presumable by “~80-130 nm”, not “~0.08-130 nm”
• Line 394: “For the Prox Cen case oxygen absorption...” requires a comma after “case”
• Line 400: Comparisons between the Solar and Proxima Centauri spectra would be easier if they were included within the same figure, as is the case for many other reactions.
• Line 402: Sentence beginning “As there is very little absorption...” “different pressures is almost entirely” should be “different pressures are almost entirely”
• Line 407: Sentence beginning “As the Prox Cen spectrum...” The large number of clauses in this sentence makes it difficult to read. Consider re-ordering to something like “As it is a lower temperature star, Proxima Centauri has a higher fraction of its flux at longer wavelengths, and rates are therefore sensitive to the temperature of the atmosphere.”
• Line 414: Sentence beginning “Figure 20 shows rates...” Some reactions are written out in words (H₂CO into H and HCO), others are written as reactions (H₂CO→H₂+CO). This should be broadly consistent throughout the paper, but must be consistent within a sentence.
• Line 414: Sentence beginning “Figure 20 shows rates...”: “top row”, “middle row”, “bottom row”, “left column”, “middle column”, and “right column” must be in brackets
• Line 425: “top row” should be in brackets
• Line 425: “4” would be better written as “four” here
• Line 426: “the methyl radical-CH₃” does not make sense and should be either “CH₃”, “the methyl radical”, or ideally “the methyl radical, CH₃,”, or “the methyl radical (CH₃)”
• Line 427: “0.2155 nm” should be either 0.2155 μm or 215.5 nm.
• Line 456: Sentence beginning “Whereas the photolysis rates...” This is not a full sentence and should be expanded upon, reworded, or combined with the previous sentence.
• Figure 12: Including the name of the star in the legend would make this plot much easier to read (e.g. “Proxima Centauri (MUSCLES-Ribas)” and “Solar (CMIP6)”)
• Figure 12: CMIP6/cmip6 should be consistent across the figure and description
• Figure 12: provide a value for the total incoming flux, e.g. “The value of 0.02 AU was selected to provide a total incoming flux of [value], consistent with the Solar spectrum.”
• Figure 13: Some sort of labelling of which is the solar spectrum and which is Proxima Centauri on the plot itself would improve readability immensely.
• Figure 14 (and onwards): Presumably the solar case results here are for Socrates? This should be explicitly stated as numerous models for the solar case have been used in this paper
• Figure 16 (and onwards): subplot titles or captions on the figures must be included here to identify what reactions, stars, and models each subplot is showing.
• Figure 18: For clarity, could these be shown as insets in the respective subplots of Figure 17? Could similar zoomed-in sections be shown for the solar case (on a secondary axis/scaled to comparable values)?
• Table A1: Caption should be before the table
• Table A1: Second words of two-word species should not be capitalised (Hydrogen Peroxide, Nitrogen Dioxide, Nitrous Oxide, Nitric Oxide, Nitrous Acid, Nitric Acid, Carbon Dioxide)
• Table A1: OH(X₂π) is written as OH(X²Π) in the paper. This should be consistent.