Review of “Spatiotemporal variations of the d(O₂/N₂), CO₂ and d(APO) in the troposphere over the Western North Pacific” by S. Ishidoya et al, ACPD.

General:

The manuscript presents new combined d(O₂/N₂) and CO₂ values that allows the author to calculate Atmospheric Potential Oxygen (APO). Measurements of these parameters are obtained from flask samplings on board an aircraft between three different stations and an altitude transect at one of the stations. The measurements are analysed for their seasonality and secular trends and are compared to model results. The interpretation adds very valuable information for the understanding of the carbon-oxygen cycle links and helps to improve the budgeting of the global carbon cycle.

The manuscript is very nicely written with detailed information on how the method works and how it is used and applied to data. The figures and their legends are clear and concise.

It was easy to read the manuscript and I would like to congratulate the authors. I have only a few rather minor comments and suggestions. I suggest publishing it once these comments have been taken into consideration.

Minor points:

Abstract: The corrections that are applied to the raw measurements are significant, how robust are these corrections. It is important that the reader gets already an impression of whether the corrections made are robust. I suggest rewording the sentence about the corrections by adding a corresponding statement about the robustness or adding an additional sentence about it.

Abstract: The altitude dependence of d(O₂/N₂), CO₂ are not consistent percent-wise. This is obviously not the case for other locations. This should be discussed and compared to published studies about the altitude dependence in the corresponding section where the altitude dependence is mentioned. See also lines 2018-2019.

Line 111: Eq. 6 describes how you applied the corrections. Why is the correction based on Ar/N₂ and not d¹⁵N, because you have excellent correlations with d¹⁵N and this parameter is stable in the atmosphere over long time periods?

Line 113: The value for a_O2 = (4.57±0.02) is not directly reported in Ishidoya, you may refer here to how you calculated.

Line 116: The overall uncertainty of d_cor(O₂/N₂) was evaluated to be less than 6 per meg, and the effect of the seasonal d(Ar/N₂) cycle on of d_cor(O₂/N₂) was not therefore excluded in this study.  This sentence is unclear to me.  

Line 285: Fig. 11 instead of Fig. 12.

Fig. 1:      One could indicate in this graph that at MNM altitude profiles are taken.

Fig. 10:     It is not clear how the rate change values on the top panel of Fig. 10 are obtained. The values should be positive and negative. What about uncertainties. The spline functions in Figure 4 have uncertainties associated, could you add shading on the derivatives (e.g. Fig. 10) to illustrate these uncertainties for readability reasons only for one curve.

General comments:

Ishidoya and colleagues present ~8 years of airborne observations of O2/N2, CO2, and Ar/N2 from cargo aircraft flights over the North Pacific. The data are significantly impacted by sampling artifacts. The authors correct this data convincingly, and, from the corrected data, calculate the apparent global ocean and terrestrial CO2 sinks. They also show that the seasonal cycle in APO is influenced by interhemispheric transport. An analysis of the interannual variability in the observations shows a signal which can be attributed to ENSO.

I think it could be made clearer in the text that the transported fluxes are derived from climatologies, which have known deficiencies. The Garcia and Keeling fluxes, for instance used the Wanninkhof 92 gas transfer velocity which is now known to be biased. The authors might consider using potential temperature or pressure as a vertical coordinate for binning instead of altitude.

I think the study is well conducted and worthy of inclusion in ACP. I have raised some minor points to be addressed below.

ABSTRACT

L11 and throughout: "amount fraction" is not a term I'm familiar with, I suggest "abundances" for referring to both O2/N2 and CO2, and "mole fraction" for referring to CO2.

L11: "Observations were corrected for significant..."

L18 and throughout: usually "northern hemisphere" and "southern hemisphere" are capitalized, but I would refer to the specific style guidelines of the journal.

L22: "indicated a clear evidence of influence" -- suggest change to "indicated a clear influence"

L24: What is a "C equivalent"? Do you mean simply petagrams of carbon? If so, Pg C a-1 is a widely used unit.

INTRODUCTION

L27: "ratio" should be "ratios"

L27: "marine biospheric activities" is a little unclear to me. I might suggest simply "...early 1990s, for the primary application of constraining the marine and terrestrial exchange of CO2."

L31: suggest "terrestrial biosphere exchanges"

L36: suggest "carbon dissociation effect" read something like "the carbonate buffer system"

L39: I think there is a missing sentence here explaining that airborne observations are useful because they quickly map a large spatial area. Suggest cutting "from this point of view" and moving the sentence to the first sentence of the paragraph beginning with "Aircraft observations"

L40-45: Steinbach (2010) might be worth citing here, since it pre-dates the Ishidoya and van der Laan references. Bent (2014) might be worth citing here as well, since he also reported Ar/N2.

METHOD

This section could use subsections for easier reading.

L76: Suggest changing "Method" to "Methods" in the section heading.

L78: Suggest rewording to "Minamitorishima, Japan, a small coral atoll (MNM; 24.28N, 153.98E)".

L78: "The cruising altitude is about 6km"

L79: "titanium" (not capitalized)

L101: Could the authors specify the scales these species are measured on, at least for O2/N2 and CO2?

L102-103: The authors cite Ishidoya et al 2014, which cites Niwa et al 2014, which cites Tsuboi et al 2013. I will admit I only scanned the papers but it seems Tsuboi is the only refernce that describes the intake and flask sampling apparatus. So I would point directly to this paper on L81 to save the reader time. This paper does not seem to have a diagram of the flask sampler, so unless I missed it in one of these papers I think it would be nice to include either in the text or in a supplement. I think this is important because the fractionation of the samples is quite considerable. I am still not clear on what kind of inlet is actually outside the airfract. From Stephens et al 2021 it was clear that the design and orientation of the inlet is critical for avoiding fractionation. Where is the air conditioning inlet located on the plane, and what does it look like? The thermal fractionation the authors identify is so massive it is probably obscuring other sources of artifacts, like at the inlet or somewhere in the air conditioning system. Since sample air passes through this, could the authors include it in the plumbing diagram? I am also surprised the authors don't have serious problems with surface effects, given that the teflon tubing is used, flasks are only partially dried, they are pressurized fairly high to 0.4 MPa, and then analyzed (I think) without a push gas. I am sure the authors have worked all this out, and probably have already published details on it, but without details here or specific citations it's hard to understand the sampling conditions.

L113: Perhaps cite here that the ratio of the scaling factors 4.57/16.2 is close to the Keeling et al 2004 diffusion factor for (Ar/N2)/(O2/N2) and results in the same tracer d(O2/N2)*

L115: Since all of the samples are from the same region, why not use the monthly mean at Tsukuba? I expect there will be some lag to consider, but otherwise it seems like this is introducing an unnecessary approximation.

L116: How was the uncertainty evaluated, and what terms are contributing to the total uncertainty? Are you accounting for natural variations in Ar/N2? Is this what is meant in L117? How much does the annual mean of Ar/N2 vary? Have the authors considered forcing to a constant value of Ar/N2? This might be preferable since the paper deals with interannual trends.

L119: This seems like the beginning of the "Results" section to me, since data is presented.

L119-125 and Fig3: Could the authors include a panel showing the vertical profiles of detrended (O2/N2)cor, perhaps separated by season? It is hard to evaluate the quality of the data when only shown as a time series. Also, I don't see what the bottom plot of panel b in Fig 3 is adding, since the data are also shown in the bottom plot of panel a. The authors could replace this with profiles or I change the y axis of Ar/N2, since there is not much that can be seen at that scale. Is it correct that the red line is the annual mean Ar/N2 for the Tsukuba time series?

L124: It would be good to indicate which samples were taken with which type of aircraft with a vertical line or some other indicator. Is it correct that only two aircraft were used? Could this be given in the Methods? It appears that the change caused the thermal fractionation effect to be reduced by more than 400 per meg.

L150: is the "seasonal anomaly" of O2 the Garcia and Keeling 2001 climatology?

L153: Where does the 1.35 value for the global OR for fossil fuel combustion come from? It is lower than the values given in Keeling and Manning 2014 and I think lower than what the CDIAC data would suggest.

L156: I don't understand this sentence: "driven by an annual mean air-sea O2 and N2 fluxes...that was considered by Tohjima...was ignored". Also, in the Tohjima et al 2012 reference there is an unnecessary hyphen in "annual". In the Tohjima paper this seems refer to the Gruber et al ocean inversion O2 fluxes? Was there a separate run of the Gruber fluxes? Or just two products: 1) Garcia and Keeling + Takahashi + CDIAC and then 2) simulated - observed APO?

RESULTS

I suggest to cut some of the L161-177 text, the decrease in O2/N2, the rise in CO2, and their seasonal cycles are well known. I would start the paragraph at "The average rates of change..." with figure citation.

L171/Fig4: there are multiple fit lines to the data but this is not explained in the caption. Maybe a legend? I think it

L186/Fig 6: I think it would be better to plot each latitude bin as a separate plot with observations and model together, it is a little difficult to compare as is. It would also be nice to see the seasonal cycle in the detrended observations along with the fits.

L194: Suggest changing "anti-phase nature in the seasonal APO cycles" to the "opposing phase of the seasonal APO cycles"

L197: I don't understand "superimposing the anti-phase seasonal cycles through the inter-hemispheric mixing of air". From this I would think you are running only the SH flux, but the "w/o SH flux run" would imply it was northern hemisphere fluxes only. Earlier in L192 it says "northern hemisphere flux only".

L198/199: should read "the seasonal cycle in CO2 mole fraction" or just "seasonal cycle in CO2".

L205: From figure 6 it looks to me that most of the seasonal cycle is due to NH fluxes, as one would expect...it does not look that much smaller to me. Can you give the amplitudes of the two runs?

L213-215: I don't fully follow this--it's the gradients in the fluxes, with contributions from atmospheric transport, that causes a gradient in the amplitude in the atmosphere. I think it would be clearer to say simply that the SH makes a significant contribution to the amplitude and phase of the lower latitude observations. I would also caution against over interpreting the model results--the transport model could be over or underestimating the interhemispheric transport.

L224: Suggest changing "highly" to "more"

Fig8/9: Typically one plots altitude on the y axis, but I leave it up to the authors.

Fig10: This looks like a fit to the data, can you include the points as well? The data look a little odd, I would expect observations of the APO growth rate to look noisier.

L265: This is a rough approximation of the thermal component of APO, which is a combination of air-sea fluxes of O2, N2, and CO2 caused by solubility changes. The "netbio" APO will also have a contribution from fossil fuel burning/CO2.

L273-287: I am not fully convinced that this exercise accomplishes more than very roughly constraining the global APO and O2 flux. The number of simplifying assumptions is extensive, and using aircraft observations from a comparatively small region, sparse data coverage, and enormous sampling artifacts is not an ideal approach. To me what this shows is that the corrected data have a coherent signal the authors can explain, and helps to prove that the data are of good quality. But I would caution against over interpreting and overselling the data.

L301: I thought 1.35 was used?

L312: Where does the 4-5 year period come from? Just visual inspection? Missing here is a citation for the Nevison et al 2008 study, which (also) showed that the errors from assuming a constant zeff over short time scales (e.g. 5 years) are quite large.

CONCLUSIONS

I would change this section heading to "Summary"

L327: Better something like "Regular air samples were taken on cargo aircraft flights from..."

L341 and throughout: "Superposition" is a slightly strange choice of words, I might suggest something simple like "combination".

 general comments

This manuscript presents 8 years of d(O₂/N₂) and CO₂ observations and calculated APO values from aircraft flights over the North Pacific. The data were corrected for significant fractionation effects on O₂ and N₂. The data were then well-analysed to find latitudinal and altitudinal, seasonal and secular trends in APO and the authors have demonstrated the influence of inter-hemispheric mixing on the seasonal APO cycle through comparison to model data. This manuscript is well-designed and well-written, and the discussion and interpretation of results contributes to the understanding of global atmospheric carbon and oxygen processes. I can recommend this manuscript for publication in ACP, with some minor comments below.

specific comments

Line 24: Units are usually written as Pg C a^-1, if this is what is meant by C equivalents

Line 24: I suggest that here, and elsewhere, a space should be added between the number ± and the uncertainty value for ease of reading. E.g. “1.9±0.9” changed to “1.9 ± 0.9”. I would also suggest removing the brackets around all of your quoted values, especially as the units are stated outside of the brackets.

Line 31: Here, and throughout, you have referred to CO₂ amount fraction – this is usually referred to as CO₂ mole fraction

Line 81: Although the sampling methods are described in full in the stated references, I think a brief summary of a few lines would be helpful to the reader here and then direction to the references for a full detailed description

Line 83: What scale is the CO₂ measured on?

Lines 88-90: at some point here, or in the figure 1 caption you could include how many air samples were collected in total. You have said that 17-20 are collected per flight, but not how many flights total.

Lines 93-97: While these equations are correct, I would suggest writing them in full i.e. (sample-standard)/standard. The form shown here is a mathematical simplification and results in some loss in understanding of the principal behind the equation

Line 99: Which scale have each species been calibrated to?

Line 116: how was this overall uncertainty calculated? Is this from the measurement uncertainty and the stated uncertainties in the coefficients from equation 6?

Line 117: I found the phrase “was not therefore excluded in this study” difficult to comprehend. I would suggest rewording to “was therefore not excluded in this study”, or “was therefore included in this study”

Figure 3: The legend on 3(a) shows this studies data as an open red circle, whereas in the figure I am assuming that they are coloured by altitude (as they are in 3b), I would suggest adding the altitude colour bar to 3(a) also. The bottom panel of 3(b) is not referred to in text and is showing the same data as the bottom panel on 3(a) so could be removed if the red reference point line were added to the bottom panel of 3(a). Is the red line reference point of d(Ar/N₂) the annual mean value from  Tsukba in 2013? If so this information could be added to the figure caption for further clarification, if not, what is it?

Line 123: I don’t think “but” is the correct word here, as that implies that the reduction in fractionation since 2018 is linked to the larger fractionations at higher altitudes before 2018 – unless this is the case, and if so this should be reworded to make this clearer

Line 124-125: The word “however” implies that the lack of systematic data gaps across 2018 mean that the change in aircraft may not be the cause of the reduction in fractionation, I don’t understand this. If this is the case, could you suggest another cause of this reduction in fractionation - is the change in aircraft the only change that occurred in 2018? The reduction in fractionation is substantial so further discussion of this would be useful.

Line 153: A value of 1.35 for fossil fuel OR is not given in Keeling and Manning (2014) or in Keeling (1988) which is referenced therein, where is this value from?  Typically, the value used for the weighted global average for fossil fuel consumption is higher than this

Figures 4 and 5: I think the scale differences between panels (a) and (b) in each of these figures needs to be explained explicitly in the methods section when discussing NICAM-TM. I would also suggest adding to the figure caption to note that the x-axis scales differ

Figure 5: Add reference to different altitudes in the figure caption e.g. observed in the troposphere over MNM at various altitudes

Line 155-158: I would suggest further explaining what is meant by ignoring the dAM(APO), particularly as this is frequently referred back to in the results/discussion, and I don't think this sentence fully explains this

Line 175: To avoid confusion I would suggest referring to “the figure” by figure number, it is not immediately clear which figure you are referring to as in the previous sentence you referred to both figures 4 and 5.

Figure 8: figure caption states “relative to the corresponding values at 6 km” but in text it says “relative to surface values”?

Line 235: Why are these values from figure 9(b) relative to the corresponding values at 6 km, but the values in figure 8(b) are relative to the surface? If there is no reason for this, I would suggest being consistent between the figures

Figure 10: the scale size for the bottom panel (12 per meg a^-1) is smaller than that for the top and middle (-14 per meg a^-1. I would suggest changing this so they are visually comparable

Line 301: why has 1.37 been used as the OR here, but 1.35 above?

 technical corrections

Line 52: change “artificial fractionation on O₂/N₂” to “artificial fractionation of O₂/N₂”

Line 69: I don’t think western should be capitalised here, should read ”western North Pacific”

Line 71:”heigh-altitude” to “height-altitude”, or “altitude-latitude” as you have referred to altitude throughout the text

Line 126: change detail to detailed

Line 130 – 132: this sentence is hard to comprehend due to the number of and’s, I suggest rewording

Line 132: Change have to has

Line 262: change to “is a global average”

Line 285: change Fig. 12 to Fig. 11

Line 298: Pg C, here and elsewhere

Line 431 and 440 : Formatting of references is not consistent, for all other references publication tear is at the end of the reference. These two references also say “and co-authors”, rather than having a full author list which should be present