Constraints on global oceanic emissions of N<sub>2</sub>O from observations and models

Buitenhuis, Erik T.; Suntharalingam, Parvadha; Le Quéré, Corinne

doi:https://doi.org/10.5194/bg-15-2161-2018

Articles | Volume 15, issue 7

Biogeosciences, 15, 2161–2175, 2018

https://doi.org/10.5194/bg-15-2161-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Biogeosciences, 15, 2161–2175, 2018

https://doi.org/10.5194/bg-15-2161-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 15, issue 7

Research article 13 Apr 2018

Research article | 13 Apr 2018

Constraints on global oceanic emissions of N₂O from observations and models

Erik T. Buitenhuis et al.

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Final revised paper (published on 13 Apr 2018)
Supplement to the final revised paper
Preprint (discussion started on 19 May 2017)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of Buitenhuis et al.', Anonymous Referee #1, 04 Jun 2017
- AC1: 'reviewer1reply', Erik Buitenhuis, 25 Oct 2017
RC2: 'Review of Buitenhuis et al.', Anonymous Referee #2, 08 Jun 2017
- AC2: 'reviewer2reply', Erik Buitenhuis, 25 Oct 2017
SC1: 'Questions', Gianna Battaglia, 13 Jul 2017
- AC3: 'GBreply', Erik Buitenhuis, 25 Oct 2017
RC3: 'Review of “Constraints on global oceanic emissions of N2O from observations and models” by Buitenhuis et al. for Biogoesciences', Anonymous Referee #3, 14 Jul 2017
- AC4: 'reviewer3reply', Erik Buitenhuis, 25 Oct 2017

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (02 Nov 2017) by S. Wajih A. Naqvi

AR by Erik Buitenhuis on behalf of the Authors (02 Nov 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (21 Nov 2017) by S. Wajih A. Naqvi

RR by Anonymous Referee #2 (05 Dec 2017)

Suggestions for revision or reasons for rejection

This paper includes some valuable calculations and the authors have made some good progress toward clarifying their methodology. However, some of the methodological details remain difficult to follow. It also appears that the authors have used only a small subset of the available pN2O data in MEMENTO, for reasons that aren’t clear. This subset appears to have a summertime bias in the South Atlantic and a wintertime bias in the North Atlantic. Since pN2O is the one of the main constraints used in the model optimization, these seasonal biases may affect the results.

I support publication of this paper in principle, but before it is ready I still think there are a number of details that should be clarified and sections of text that could be written more clearly.
Specific comments

p2L32. The text here mentions 4 methods. Are these 4 enumerated in the lines that follow? For example, is the following method 1,"We extend the global ocean biogeochemistry model PlankTOM10 with additional N cycle processes."? Or are these additional approaches (it seems like there are more than 4 total)? Please start the sentences with transition words like, "First", "Second" instead of just "We" to make this clear. Is the sentence on line35 starting with "Then" approach # 3? If so, please use "Third" This is a key paragraph where the authors lay out what the paper will do, yet I am already lost.

P3L30/P4L1. There are a lot more deep and surface N2O data in MEMENTO than the reported n=8047 and n=6136 mentioned here. Were only a subset of the available data selected and why? Also, were the surface data generally in units of ppb and the deep data in nmol/L?

P4L4 Are deep data converted to delta_pN2O or just the surface data? If deep data are converted too, please mention that this necessitates first converting the deep nM data to ppb using a solubility function. This is a large uncertainty. The statement “we have taken the database at face value” is inadequate for conveying the extent of the uncertainties involved in combining nM and ppb data in equation 3.

P.4 Section 2.4 (Nitrification) and p5 Section 2.5 (NH4+ uptake) appear to be databases 1 and 2 of the 4 mentioned. I was expecting 2.6 then to be MEMENTO deep N2O and 2.7 to be MEMENTO surface data. Instead, we jump to 2.6 N2O production. This is an example of why it remains challenging to follow the methodology of this paper.

P4L27. Does Yool provide an actual data base, or simply assume a constant rate of 0.2 /day everywhere (as written, the sentence implies the latter)?

Section 2.4. Given that a cost function of 2 means that, on average, the model deviates from the observations by a factor 2 (Section 2.3), does this paragraph suggest that the model deviates from observations by a factor of > 4 on average? In other words, it provides no real constraint. Shouldn’t that be stated explicitly somewhere in this section? Is there any meaningful difference between a cost function of 4.22 and 4.16?

P5L34. So, the diagnostic model dN2O/AOU ratios are not optimized against the MEMENTO database using the cost function? The intro and the mention of the 4 datasets had led me to expect they would be.

P6L11-12 “We indenpendently optimised the ratios of N2O production and consumption from denitrification” These are minor terms in the budget compared to N2O production from nitrification. Why wasn’t that ratio/coefficient optimized?

P6L14 “The ratios of both submodels were optimized using the databases of observed N2O concentration and pN2O” Is this referring to the deep N2O or the surface N2O data or both?

Figure 3 caption. Please specify where these values are from (model, MEMENTO, etc).

Figure 3 annotation shows 0.0815 + 2.7551, but this is reported on p3L15 as 81.5 ± 1.4 μmol/mol. First, the error has completely changed. Second, there is a switch from nmol N2O and umol AOU in the figure to units of umol/mol in the text, with a factor of 1000 thrown in to add to the confusion. Better to be consistent across figure and text.

Figure 6/Section 2.5. The model is credited with reproducing “the large scale pattern of surface NH+4 concentration (which) shows an increase with latitude.” However, the performance seems pretty poor and the pattern could equally well be described as high in the Southern Ocean (where nutrient utilization is known to be low) and around continental boundaries where there is nutrient input from land. Similar to Section 2.4, the cost function of 3 seems quite large and suggests there’s no real constraint here.

Figure 9 and 11 caption and Equation 5 on p. 6. What does MSE stand for? Please spell it out in all these places. (The captions should be understandable without referring back to the text.)

Section 2.8 Should Equation 6 be presented before 5? It seems like 5 builds upon the definition of MSE introduced in Eq 6.

P7L26. I’m confused by the use of “even though” here. Given that the prognostic model represents N2O consumption in the OMZ, why would that be expected to improve (i.e., increase) the concentration of N2O between 200 and 1500m?

P8L5-7. These sentences belong in the methodology. Also, as mentioned above, why isn’t the nitrification N2O/NO3 ratio optimized too? That seems like the most important term in the model.
P8L14 I would suggest writing as 0.183 mmol N2O, to avoid switching units, which is confusing for the reader.

P8L16-17. “pN2O provided a better constraint than the N2O concentration distribution” Back in Section 2.2, deep N2O is mentioned as dataset number 3 used to optimize the fluxes. The sentence just cited suggests that deep N2O is not actually used. Please clarify.

Figure 10a) This figure represents only 6136 data points (I think there are a factor of 10 more surface N2O data in MEMENTO than that), yet the figure suggests extensive coverage of the global ocean, and near complete coverage in the Atlantic. Have the data been binned and gridded and if so how? It seems like a single data point has been expanded as a ~5x5 pixel, which implies much better coverage than may really exist. Also, what is the seasonal distribution of the data? I suspect the South Atlantic data are all from austral summer, while the North Atlantic data are mainly from fall/winter. I don’t think the Atlantic is undersaturated to the extent implied by this figure on an annual basis. Can the data be binned by season and plotted in a 4 panel plot?

Figure 10b) Following on the above comment, it is confusing to plot the “same months where there are observations, and annual averages everywhere else,” especially if there are strong seasonal biases that differ by region. I would again suggest 4 panel seasonal plot for the model results.

Figure 10b) Also, the red contours appear to smear over the black continents.

Figure 10d) Please make green and red lines thicker. They are illegible.

Figure 11. Are the different symbols (0.06, 0.11, 0.17, 0.34 for A) and (0.07, 0.12, 0.17, 0.34 for B) essentially the same thing or is the similarity in numbers pure coincidence? If the former, why the slight differences (e.g., 0.06 v. 0.07) and inconsistent shapes? Also, why does A say “different symbols indicate different low O2 ratios” while B says “points with the same symbols have different N2O ratios for nitrification.” I thought the nitrification ratio was not optimized.

P8L23-24. This implies that the oxic nitrification ratio is optimized, at least in the prognostic model. Yet as far as I can tell, the Methods only describe optimizing the denitrification and suboxic N2O parameters.

P8L23 and L32. Are deep offshore and near-shore non-coastal synonymous? Please use consistent terminology to avoid confusion.

P9L8 Again, it is important to know whether the surface pN2O data used were originally in ppb units or in nM. If the latter, these measurements likely have an uncertainty of +/- 0.5 nM or more, such that the 3% cited here (which may be accurate for the solubility function per se) greatly understates the actually uncertainty in ppb due to the solubility conversion.

P9L19-20 “In light of this, we decided to recalculate the N-cycle-based N2O production based on currently available data. We find that we can estimate all the relevant steps in the N cycle with observational data” These seem like non sequiturs. The logic here is difficult to follow.

P9L26 NO3- is the electron ACCEPTOR in denitrification. Organic C is the electron donor.

P9L28. “this estimate” Better to restate the numbers, e.g., 4.6 +/- 3.1 vs. 2.5 +/- 0.8. More importantly, these two errors are estimated in very different ways, such that it is not clear they are directly comparable. The authors are comparing a global back-of-the-envelope calculation, with large global errors on any given parameter, to a more grid-specific calculation. Of course the latter will have smaller uncertainty.

P9L30. Please use “further” only once in this sentence and perhaps break into 2 sentences.

P9L30 What does “their” refer to?

Discussion section in general. The writing and organization of thoughts could use improvement. Please avoid starting sentences with “This” unless the antecedent is clear.

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RR by Anonymous Referee #1 (21 Dec 2017)

ED: Reconsider after major revisions (23 Dec 2017) by S. Wajih A. Naqvi

AR by Anna Wenzel on behalf of the Authors (26 Jan 2018) Author's response

ED: Referee Nomination & Report Request started (28 Jan 2018) by S. Wajih A. Naqvi

RR by Anonymous Referee #2 (23 Feb 2018)

Suggestions for revision or reasons for rejection

The paper is much improved, although I still find that the description of the optimization of N2O production via nitrification in well-oxygenated waters is somewhat unclear. However, I'm ready to support publication with a few minor revisions.

Specific comments
p2L27-. This paragraph, in which the authors lay out what the paper will do, is greatly improved and now provides a clear blueprint of what to expect.

P3L13 uses => used

P4L2-3 It was very useful to clarify that the 227463 raw data points reduce to only 6136 on a 1x1x12 grid. By my calculations, assuming that 60-70% of grid cells are ice free ocean, this means that only 6136/(360x180x12*~0.65) = ~1% of possible monthly 1x1 grid cells have an N2O measurement. I think this lack of coverage should be mentioned somewhere, perhaps near the presentation of Figure 10b, which conveys the impression of extensive coverage, especially in the Atlantic Ocean. (It is good that the Fig 10 caption mentions the 5x5 pixels, but I think the lack of coverage needs to be acknowledged more explicitly.)

P6L16 indenpendently misspelled

Figure 9. Please explain in the caption what the different symbols and values are. Are these consumption rates? Please give units.

P8 Section 3. An ongoing point of confusion for me is why the Results only mention the N2O production at low O2. What about the N2O production at higher O2 and total oceanic N2O production? I guess this is explained on p8 29-33, but perhaps make the point more clearly by saying something like, "We used the surface ∆pN2O distribution to constrain N2O production via nitrification in well-oxygenated waters and thus (by summing with the N2O production at low O2 described in Section 3.1), the total global N2O flux, …"

P9 paragraph 2. It's not clear what the point of this paragraph is. Is it to argue that coastal areas are not strong sources of N2O? The MEMENTO surface pN2O dataset includes few data in the coastal region, so the calculations in Table 2 may not be well constrained.

P9L27 it's => its

Fig. 10 I still find it very confusing to plot the model results as annual averages in general but as the same month as observations where there are data (what happens if there are obs in 2 or more months in a given grid? Is an average of those months plotted?) For this reason, I recommend including the sets of 4 panel plots provided in the review response in the Supplementary Information. Also, based on those 4 panel plots, there appear to be strong summertime maxima in model dpN2O in both hemispheres. Please comment on whether this is due to enhanced production in summer or simply to thermal effects.

P9 last paragraph. Please provide more information about Cohen and Gordon's calculation. How was it based on N assimilation? What was their total estimate?

P10L4 "nitrification, which uses O2 as the electron acceptor."

P10L5 "needed to nitrify NH4+ to NO3-, the electron acceptor"

P10L21 Is N really in the 0 and +2 states in N2O? I thought both Ns were in the +1 state.

P10L35/P11L1. This claim is not obviously supported by Figures 10-12. The fact that the depth profiles are significantly off seems like a red flag that low O2 production is underestimated, given how sparse the surface dpN2O data are and their sensitivity to air-sea transfer assumptions.

P12 Paragraph 2. This is mainly a repeat of the previous paragraph

The reviewers gave the same response to 2 of my previous comments.
My review #2 comment was: P6L11-12 "We indenpendently optimised the ratios of N2O production and consumption from denitrification" These are minor terms in the budget compared to N2O production from nitrification. Why wasn't that ratio/coefficient optimized?
The authors responded: Both databases were used to optimise separate parameters. To clarify this, P6L14 was rewritten: "The low O2 ratios of both submodels (supplementary material Section 8.7) were optimised using the database of observed N2O concentration (Sect. 3.1) and the oxic ratios of both submodels were optimised using the database of observed DeltapN2O (Sect. 3.2)".
However, this does not really address my comment. Furthermore, they gave a nearly identical response to my next comment: P6L14 "The ratios of both submodels were optimized using the databases of observed N2O concentration and pN2O" Is this referring to the deep N2O or the surface N2O data or both?
The authors responded: Both databases were used to optimise separate parameters. To clarify this, P6L14 was rewritten: "The low O2 ratios of both submodels (supplementary material Section 8.7) were optimised using the database of observed N2O concentration (Sect. 3.1) and the oxic ratios of both submodels were optimised using the database of observed DeltapN2O (Sect. 3.2)".
I think there was a typo in which the authors pasted the same response to 2 of my comments, without actually addressing the first comment

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ED: Publish subject to minor revisions (review by editor) (24 Feb 2018) by S. Wajih A. Naqvi

AR by Anna Wenzel on behalf of the Authors (08 Mar 2018) Author's response

ED: Publish as is (11 Mar 2018) by S. Wajih A. Naqvi

Short summary

Thanks to decreases in CFC concentrations, N₂O is now the third-most important greenhouse gas, and the dominant contributor to stratospheric ozone depletion. Here we estimate the ocean–atmosphere N₂O flux. We find that an estimate based on observations alone has a large uncertainty. By combining observations and a range of model simulations we find that the uncertainty is much reduced to 2.45 ± 0.8 Tg N yr⁻¹, and better constrained and at the lower end of the estimate in the latest IPCC report.

Constraints on global oceanic emissions of N2O from observations and models

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Interactive discussion

Peer-review completion

Constraints on global oceanic emissions of N₂O from observations and models