|The authors have made a number of improvements to the original manuscript, and they have included additional details and raw data. In my opinion, the manuscript is now publishable without further review on my part. However, I do offer some additional comments for the authors to consider if they wish to clarify and revise any of the following remaining issues.|
Abstract: P1, L32-33: I suggest adding the very important clarification in caps: “These results FROM THE DEEP TROPICS suggest the following.”
Abstract: P1, L33-35 The authors state: “The time scale of horizontal mixing in the tropical middle stratosphere is sufficiently large for in-situ photolysis of N2O, mainly because of strong upwelling and transport barrier between the tropics and extratropics.” What does this sentence mean? Taken literally, it doesn’t make any sense to me and is oddly phrased. I am guessing that authors are trying to say that the epsilon values in the Rayleigh analysis for the N2O isotopocules are as large (or almost as large) as those for photolysis in a closed system because air in the middle tropical stratosphere is relatively isolated from the mixing in of older, photochemically-aged air from the extratropics (which serves to decrease epsilon values from the Rayleigh limit). If I were to just try to rewrite their sentence (which I think would not be as clear), then it would be more accurate to say something like: “The time scale for quasi-horizontal mixing between tropical and midlatitude air in the tropical middle stratosphere is sufficiently slow relative to the tropical upwelling rate that isotope fractionation approaches the Rayleigh limit for N2O photolysis.”
There is a similar problematic wording in the Conclusions (see below).
P3, L32: The authors state that (Kaiser et al., 2002a) “estimated that a much larger fraction than 10% is removed by photooxidation at least in the lower stratosphere” Can the authors please give a number for the larger fraction that Kaiser et al suggested? How much larger than 10%?
P3 L33: The authors state that “similar but a little simplified analyses by Park et al. (2004) and Toyoda et al. (2004)” were performed. Can the authors please be more descriptive in what they mean by ‘but a little simplified relative to Kaiser et al"?
P6, L4: The authors say that they used CO2 mixing ratios to estimate the mean age of air of an air sample and cite Engel 2009 for the measurements, but is this true for all flights (i.e., that CO2 mixing ratios were measured for all samples by Engel et al. (2009))? Or perhaps they just mean to state how a mean age can be obtained from CO2 mixing ratios? This is unclear and should be clarified in either case.
P9, L11-12: The authors state: “The slope, which corresponds to the ratio of epsilon values, is very close to the one expected for photolysis. This confirms that photochemical decomposition of N2O is mainly caused by photolysis (Minshwaner et al., 1993)…” I think most stratospheric chemists will think it is a stretch that this isotope data needs to be used to confirm what has been known for some time. In other words, I don’t believe the fact that the majority of N2O is destroyed by photolysis needs to be confirmed in 2017. This should be reworded so that it doesn’t seem like this is an important, new, or controversial finding.
P9, L27-28 The authors state here: “Because the vertical ascent rate in the tropics is much faster than quasi-horizontal transport, there is an apparent transport barrier between the tropics and extratropics (Plumb, 2007)…” Sort of… It would be better to take out the ‘much’ from “much faster” and then qualify by saying “Because the vertical ascent rate in the tropics is faster than quasi-horizontal transport out to the extratropics and much faster than the quasi-horizontal transport of extratopical air into the tropics, there is an apparent transport barrier between the tropics and extratropics…” On the other hand, what one really wants to get across for this isotope study is that the tropics are relatively isolated than the rest of the stratosphere – especially the middle tropical stratosphere since little extratropical air is mixed back in (i.e., the vertical ascent rates and the entrainment of tropical air out to midlatitudes are both much faster than the transport of older air back into the tropics…).
P11, L11: The authors state: “Low values are obtained near the TTL over the Equator.” Note that the TTL is usually tropospheric air, not stratospheric. So are these samples in the troposphere (below the tropopause) or above the tropopause in the stratosphere ?
P11, L11: The authors state: “This result confirms the indication by Kaiser et al. (2006) that the photooxidation sink has a much larger fraction than 10% in the lower stratosphere.” As I noted earlier, please give a number for ‘much larger’? And ‘confirms’? I would not agree yet that the analysis presented here confirms that at all. So I agree with the authors, who state below, that this will require more study. But, in light of this, perhaps they could use a word other than ‘confirms’ which carries a strong meaning scientifically, which is a bar I do not think they have met yet. It’s fine to publish this idea, and the case they present, but I think “confirm” is too strong a word to use just yet.
P11, L13-15: Figure S6: It is an important new figure component that the authors now provide in this revised manuscript the total loss rate for N2O in relevant regions in the stratosphere. I still believe they are not interpreting it nor their data in as rigorous a way as they could, but, as they note, they leave further work up to future studies. However, in the authors’ response to review they note “[we] deleted a part of the plots of tropics and all the plots of February Arctic region so that the plots show the parameters in the region where photochemical N2O loss really occurs.” I do see tropical parameters in their Fig S6, so I am wondering what 'part' was deleted. I think it is very important to include a complete Fig S6 so that readers can evaluate and follow up on the arguments that the authors are making here.
P 11, L28-29 “Unlike other region of the stratosphere, enrichment factors for isotopocules in the middle equatorial stratosphere (25–30 km, or [N2O] < 260 nmol mol-1) agreed with those obtained with laboratory photolysis experiments, suggesting that the isotopocule ratios are determined mainly by photolysis because of weak vertical or horizontal mixing in the tropical upwelling…” As I noted in a comment to the authors above, the wording seems to mix up cause and effect. The middle tropical stratosphere is relatively isolated; the lack of transport of air of many different transport time histories makes the isotope fractionation appear almost Rayleigh (as in an -- almost -- closed system). The specific arguments about weak vertical and horizontal mixing are not especially accurate, nor are they really needed.
P12, L3-5: The authors state: “Further observations of temporal variations and comparison with ACTM simulation will be needed to examine the change in the meridional circulation…” What change in the meridional circulation? This is the first time the authors have brought this up as a motivation or next step. There is a vast literature on this hot topic but no context is presented here at all, and no references are given. If the authors wish to motivate that this is something that N2O isotopocules could provide insight into, they should provide at least a sentence or two of context as well as some references, and a more complete argument of how N2O isotopocules could help.
Even more minor comments:
Polishing of the English usage of definite (the’s) and indefinite (a’s) articles could be helpful for readers.
Pg2,L24; “transportation” should be replaced with “transport”
Pg 3, L8-9: “Although these observations are limited to ca. 20 km altitude, vertical profiles can be obtained for horizontally wide areas such as Arctic polar vortexes (pg 3, L8-9).” It is unclear what is meant by this nor why this is an important distinction.
P7, L25-28: “The height of the tropical tropopause layer (TTL) was 14-18.5 km (Fueglistaler et al., 2009). I presume since the measurements were made in 2015, then the authors are using Fueglistaler to mean “typically,” and what the TTL is rather than a reference that has the actual tropopause measurements for these flights in it. So saying ‘typically’ and putting an “e.g.,” in the reference could clarify this.
P8, L19: The authors state: “The equatorial values of epsilon almost coincide with those of photolysis…” If the authors mean “almost as large as” it would be more clear to state it that way (since it gives the direction, and what is expected); if it is noisy, then I suppose “almost coincide” would be ok.
P8, L21-22: The authors state “latitudinal and year-to-year or seasonal variation are slight compared to those of the middle stratosphere in the lower stratosphere”: I had to read this several times, as the order is confusing. I think the authors mean mean: “latitudinal and year-to-year or seasonal variation are slight IN THE LOWER STRATOSPHERE compared to VARIATIONS IN the middle stratosphere”
P8, L22-25 The authors state: “Although the similar latitudinal and altitudinal dependence of epsilon has been reported previously for the latitudes ranging from 18°N to 89°N (Park et al., 2004; Kaiser et al., 2006), our equatorial data showed that the change in epsilon at altitude with higher N2O mixing ratio and the epsilon value is exactly what would be expected during the N2O photolysis.” I have several suggestions to make this sentence more clear. First, putting “show” in present tense (instead of past tense “showed”) clarifies that the authors are referring to the new deep tropical data presented here. In addition, the very last part of the sentence I think is a preview of what will be discussed in the next section, not a statement that needs to be supported here.
P8, L29: We NOW discuss causes (not ‘then’).