Measurements of nitric oxide and ammonia soil fluxes from a wet savanna ecosystem site in West Africa during  the DACCIWA field campaign

Pacifico, Federica; Delon, Claire; Jambert, Corinne; Durand, Pierre; Morris, Eleanor; Evans, Mat J.; Lohou, Fabienne; Derrien, Solène; Donnou, Venance H. E.; Houeto, Arnaud V.; Reinares Martínez, Irene; Brilouet, Pierre-Etienne

doi:https://doi.org/10.5194/acp-19-2299-2019

Articles | Volume 19, issue 4

https://doi.org/10.5194/acp-19-2299-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Results of the project "Dynamics–aerosol–chemistry–cloud...

https://doi.org/10.5194/acp-19-2299-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 4

Research article

|

21 Feb 2019

Research article |

| 21 Feb 2019

Measurements of nitric oxide and ammonia soil fluxes from a wet savanna ecosystem site in West Africa during the DACCIWA field campaign

Federica Pacifico, Claire Delon, Corinne Jambert, Pierre Durand, Eleanor Morris, Mat J. Evans, Fabienne Lohou, Solène Derrien, Venance H. E. Donnou, Arnaud V. Houeto, Irene Reinares Martínez, and Pierre-Etienne Brilouet

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Final revised paper (published on 21 Feb 2019)
Preprint (discussion started on 20 Mar 2018)

Interactive discussion

Status: closed

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

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RC1: 'Pacifico et al.', Anonymous Referee #2, 16 Apr 2018
- AC2: 'Response to anonymous referee #2', Federica Pacifico, 07 Jun 2018
RC2: 'Review comments on "Measurements of nitric oxide and ammonia soil fluxes from a wet savanna ecosystem site in West Africa during the DACCIWA field campaign"', Anonymous Referee #1, 18 Apr 2018
- AC1: 'Response to anonymous referee #1', Federica Pacifico, 07 Jun 2018

Peer-review completion

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

AR by Federica Pacifico on behalf of the Authors (12 Jun 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (02 Jul 2018) by Sally E. Pusede

RR by Sally E. Pusede (13 Aug 2018)

Suggestions for revision or reasons for rejection

Dear authors,

This review is from the Co-Editor.

This paper describes measurements of NH3 and NO fluxes in Benin, West Africa and in June and July, 2016. The article has received one round of reviews. My primary concern is that the new version has not adequately addressed concerns around the NH3 flux measurements. Data of soil fluxes in West Africa are rare and I hope more information can be provided on the measurements so the paper can be published. I have provided other comments as well.

The application of experimental results from Vaittinen et al. (2013) is not yet convincing. The value for PFA of 13.9e12 molecules/cm2 was determined under constant conditions that do not represent the field sites: 10% sea-level atmospheric pressure, a constant temperature of 295K, constant humidity, and ambient NH3 of ~8 ppm (not 8 ppb). Vaittinen et al. (2013) themselves say the reported uptake values are minimums, rather than absolute quantities. Diurnal and day-to-day changes in these conditions could drive significant variability in air-chamber exchange, despite the low standard deviations reported in Vaittinen et al. (2013). If the chambers were frequently washed, then maybe uptake is the dominant direction of the air-chamber exchange; however, a key conclusion is that soils are largely an NH3 sink. For this reason, additional work is need to verify that the reported fluxes are between the air and soil rather than the air and chamber.

Referee 1 makes a valid point in comment 2 (below):

“The authors use the measured NO and NH3 fluxes for a stepwise linear multiple regression analysis, upscaling to country-wide soil fluxes, and comparison with soil emission estimates from the GEOSChem model. These analyses give valuable information on the importance of soil NO and NH3 exchange and our current knowledge about them. However, while the authors state that a process understanding of the NO and NH3 fluxes is not within the scope of the presented study, in my opinion it is important to understand the underlying processes of the measured fluxes. For example, the estimated emissions from soil characteristics only poorly agree with the measured fluxes in some parts, which indicates that a more detailed process understanding is necessary.”

However, the authors have made no substantive changes to the manuscript in response. Or, if they have, they have not indicated this in the formal response. Please clarify.

Rearding Referee 1’s comment:

“The assumption that the concentration in the chamber is equal to the concentration leaving the chamber to the analyzer is questionable. Due to the low flow rate required for the practical use of the closed-dynamic chamber technique, the residence time within the chamber is substantial (17-18 min). As no active mixing (e.g. with fan) is used, the chamber geometry in relation to the positioning of the ambient air inlet and sample outlet is of importance.”

Please modify the text so the reader does not need to read another paper to understand the fundamentals of the experiment. Consider this comment broadly and add a brief overview of all relevant information from Delon et al. (2017). You may refer readers to Delon et al. (2017) for additional detail, but provide the basics here.

For the new text: “This is different from Delon et al., 2017, but fluxes were anyway superior to this value.” Change the word superior to “greater than.”

Regarding Referee 1’s comment (a concern also raised by Referee 2):

“This assumption seems brave if it was not tested with a set of test experiments. Although the microbial activity is reduced due to the dry conditions, there is a chance that NH3 volatilizes with the drying of the soil sample material.”

I see you have added a reference to this phenomenon, but discussion for how this relates qualitatively and quanitaviely to your results is still needed.

New text: “Daily means of NO concentration vary from 1.28 to 5.40 ppb for all sites.” In Fig. 2 (and 4), NO concentrations appear be equal 7-9 ppb on multiple days.

As far as I can see, Figs. 2 and 4 are identical. Is this an error?

New text: “Average NH3 concentration is 6.28 ± 3.90 ppb for bare soils.” Over bare soil, it appears the NH3 concentration never exceeds ~3 ppb.

Regarding Referee 2’s comment:

“While I am sure location and resources had much to do with this, air-drying may result in large changes to ammonium concentrations. Additionally, significant changes in the amounts of ammonium can take place over prolonged storage at room temperature, even if soils are dried. It seems that the authors are aware of this issue and attempted to justify their method by citing a meta-analysis of warming experiments on N-cycle activity. (Bai et al. 2013). However, this meta-analysis found that warming and moisture reduction had no significant effect on mineralization (Bai et al, 2013: Table 1), indicating even in dried samples, pools of inorganic-N may change over time. To remedy this, the authors could have compared their ammonium concentrations to similar studies from this region; however, this was not included in the results/discussion.”

Please expand the new discussion, which at just two sentences (line 255 and line 360), is insufficient.

Other comments:

All plots are of poor visual quality, can the authors remake the plots with a program capable of higher resolution graphics and that allow the authors to improve the readability of the plots?

Abstract: I don’t follow the logic of the first sentence.

Line 20: change aerosols to aerosol.

Abstract should be a single paragraph.

Best,
Sally Pusede

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ED: Reconsider after major revisions (13 Aug 2018) by Sally E. Pusede

AR by Federica Pacifico on behalf of the Authors (23 Sep 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (24 Sep 2018) by Sally E. Pusede

RR by Anonymous Referee #4 (11 Oct 2018)

Suggestions for revision or reasons for rejection

The manuscript ‘Measurements of nitric oxide and ammonia soil fluxes from a wet savanna ecosystem site in West Africa during the DACCIWA field campaign’ reports measurements of nitric oxide (NO) and ammonia (NH3) soil fluxes from four different types of land cover in rural Benin, West Africa in June and July of 2016. The paper is appropriate for ACP and of interest to the journal’s readers. It is well written and readable though too many important details of the measurements are missing or in the Delon et al. 2017. I found myself having to refer to Delon et al. 2017 too frequently to try to understand the experiment presented here and making assumptions that the information given in Delon et al. 2017 is applicable to this study.

I am, however, concerned with the lack of characterization of the chamber with respect to the NH3 measurements. NH3 is sticky gas-phase species that both adsorbs and desorbs from surfaces depending on environmental conditions making it difficult to quantitatively sample. As such, NH3 measurements need detailed characterization to assess the quality and uncertainty of the data. As discussed below this manuscript is lacking in these details with respect to the NH3 measurements. In the case of NH3, additional evidence or laboratory work is necessary to ensure that the observations reflect interaction between the air and the soil and not the air and the chamber walls.

Lines 140-143 – Here it is stated that a Thermoscientific 17i (ThermoFischer Scientific, MA, USA) to measure the concentration of NO and NH3 through chemiluminescence. However, this instrument does not directly measure NH3 via chemiluminescence. Rather it measures NO, NOx by converting NO2 to NO via a molybdenum converter, and N(total) by converting NO2 and NH3 to NO on a stainless steel converter. NH3 is derived via the following equation NH3=N(total)-NOx. A better instrument introduction would improve the manuscript and help the reader understand later details, such as why the instrument was calibrated with NO2.

Lines 151 – 164 ¬- In this paragraph the calibration of the analyzer is discussed. It is good to see the authors calibrate a commercial instrument. However, the details of the calibration need further explanation. As I understand the instrument was calibrated with a NO standard only twice, pre and post-mission. The manuscript should state how these two calibrations compare. The NO2 converter efficiency and the NH3 convert efficiency were each only evaluated once post-mission. This is a weak point in this study. First, the conversion efficiencies of each should be stated. Second, the authors need to show some evidence that the conversion efficiencies do not drift significantly over time because a drift in either could significantly affect the NH3 derived from the instrument.

The authors state in line 163 that ‘Multipoint (at least 4 points) calibrations between 50 to 250 ppb were done to ensure the linearity of the response, obtaining regression coefficients over 0.9993 for both NO and NO2.’ Does this mean that the multipoint calibrations were done only with the NO and NO2 standards and not the NH3 standard? The ppm level standards were diluted to the 50 to 250 ppb for the multipoint calibrations. What is the uncertainty in the dilution procedure? Were these calibrations performed with the 4m Teflon tube that connected the chamber to the instrument in place? Understanding the effect of this line on NH3 could be important to the NH3 flux determination.

Looking through figures 2-5 the measured ambient levels of both NO and NH3 are much lower than 50 ppb. Especially in the case of NH3, which is difficult to sample quantitatively, the authors need to justify their assumption that the instrument response for both is linear below 50 ppb to the ambient levels measured.

Line 164 states the global precision of the analyzer is 0.4 ppb. Is this experimentally determined or just the manufacturer’s specification?

Lines 166 – 192 – The description of the chamber is lacking in important details. Here the manuscript would benefit with a good schematic showing the locations of the air inlet and the outlet to the chamber, i.e. the geometry and layout of the chamber. The residence time of air in the chamber is given as 20 minutes. Yet the chamber is maintained in place for only 10 minutes, i.e., less than 1 turnover of air in the chamber. Without a fan and depending on the geometry of the inlet and outlet, it seems possible that air in the chamber could be layered such that the mixing ratio of NO or NH3 going to the instrument is equal to that in the chamber. How does this affect the flux determination? Why do the authors feel this is not an issue?

The manuscript cites Vaittinen et al. 2013 to support their claim that the effect of the Teflon walls on NH3 is negligible. However, this is not thoroughly convincing because the experiments performed in this study were with NH3 mixing ratios in the hundreds of ppb to low ppm range, not applicable to the atmospheric levels seen here. Furthermore this study was performed in a much more controlled environment and thus not directly comparable. Similarly, the experiment in Appendix A is promising though the authors should better relate the laboratory conditions to those experience in the field. One aspect that does not seem to be covered by the Appendix A experiments is the temperature of the Teflon itself. When sitting in direct sunlight, the temperature of the Teflon surface could differ from the air temperature and affect the adsorption or desorption of NH3. Additionally, the 4m Teflon tube from the chamber to the instrument could exhibit surface affects that differ from the chamber walls. The effect of cleaning the chamber daily should also be investigated. Though it appears that no chemicals were used in the clean, does the daily wiping remove NH3 or add NH3 from the chamber surface? If NH3 is removed, then, are observations of deposition really reflecting NH3 deposition on the chamber wall not the soil? Perspiration is a significant source of NH3. In the act of clean could the walls adsorb NH3 and then come off during a sampling period. These are hypothetical but could affect the flux determination, nevertheless.

Line 225 – More in-depth reasoning should be given as to why interaction with particulate matter is ignored, especially in regards to NH3.

Lines 227 – 235 – Here the manuscript could be improved by actually showing chamber data, i.e., how the measured concentration varies with time. No explanation is given why the R^2 threshold for NO is 0.8 while that for NH3 is 0.4. Is this indicating that there are other influences on the NH3 mixing ratios in the chamber such as wall adsorption or particulate matter interaction? Is there a difference in derived fluxes for NH3 when the R^2 is 0.4-0.6 compared when the R^2 was above 0.8?

Tables – Tables 3, 4, and 5 summarize the soil measurements. There are no comparable tables for the atmospheric observations, e.g., the average NH3 mixing ratio over the various land types (line 468).

Figures – It is difficult differentiating the different symbols in Figures 2-5. Consider using color as in Figure 6 to help the reader more easily follow the data.

Minor comments

Line 36 – Oxide does not need to be capitalized.

Line 166 – It is simpler to say internal volume instead of useful volume as useful is not defined in this context.

Line 191 – There is no Davidson et al (1990) in the reference list. Please change to appropriate reference. Also, I suggest showing the conversion of the flux to ngN m^-2 s^-1 either here or in an appendix.

Line 220 – I fail to see how low calculate values of O3 near the soil indicate that O3 deposition is of secondary importance when these calculated values have not been verified.

Line 410 – Here daily means of NO are reported. However, measurements were not taken over the course of a full day. Rather, line 135 states that each location was sampled during the daytime. I’d suggest changing here and elsewhere to ‘daytime means’.

Line 466 – I do not understand what is meant by ‘NH3 concentration vary from 0 to 12.46 ppb for all sites’. How can this instrument report 0?

Line 468-468 – ‘and 3.68 ± 2.13 440 ppb for forest.’ Appears to be a typo

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ED: Reconsider after major revisions (29 Oct 2018) by Sally E. Pusede

AR by Federica Pacifico on behalf of the Authors (07 Dec 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (20 Dec 2018) by Sally E. Pusede

RR by Anonymous Referee #5 (22 Jan 2019)

Suggestions for revision or reasons for rejection

This review is focused specifically on the experimental design and interpretation of the NH3 flux measurements during this study. It is well-known that NH3 is susceptible to adsorption and desorption processes that are temperature and humidity-dependent and the authors acknowledge this issue throughout the manuscript. Several lines of evidence are provided to evaluate the potential for artefacts in the flux measurements. Given how rare direct measurements of NH3 flux are, and how challenging they can be to obtain, I support the publication of the data in this manuscript, however I have some suggestions for the authors to slightly reframe their discussion of the results.
On lines 175 – 180, the authors state that calibration linearity at lower concentrations is assured by a 4-point linear calibration between 30 and 200 ppb, and the response to a ‘zero air calibration’. However, several studies (e.g. Whitehead et al., 2008, Ellis et al., 2010) have reported that biases related to adsorption and desorption become relatively more important at lower concentrations, so one cannot simply interpret linearity and high concentrations and a clean background as evidence that there are no issues with quantification at lower concentrations. Given that the ambient mixing ratios are mostly below 10 ppb, the authors should be more cautious in their interpretation.
In Appendix B, the authors present a set of laboratory experiments to evaluate the potential impact of adsorption or desorption from the chamber walls under a narrow range of temperature and RH. I believe that the concentration differences between the through chamber (TC) and direct (D) measurements seen here would be a better way to estimate of the detection limit for fluxes than the 0.4 ppb instrument precision limit that the authors are currently using. For example, one could say that an absolute difference of 1.6 ppb or a relative difference of 12% can arise simply from the effects of adsorption or desorption. In this case, ambient flux measurements that generate such small differences in NH3 should be viewed as not statistically different than zero.
A consequence of this more conservative approach will be that the number of NH3 flux measurements above the threshold for quantification will decrease. The authors expand on this idea in the discussion in Appendix C, indicating that 23% of the fluxes measured were below a value of 0.5 ng N m-2 s-1. The authors go on to speculate about the consequence of excluding these low flux values from the database – the resulting average flux value is larger. I think that the low values should not be excluded – they contain important information about periods where the fluxes are small. Rather the authors should use the results from the experiments in Appendices B and C to derive a more realistic flux detection limit for NH3 and then clearly report that for positive or negative fluxes of smaller magnitude, the flux is statistically indistinguishable from zero. These near-zero values are still meaningful – for example they can be compared to the predictions from the compensation point framework described in Section 2.7, or compared to the much larger fluxes measured over fertilized maize fields in other parts of the world.

Additional specific comment: The statement on lines 53-55 implies that application of synthetic fertilizer is the largest source of ammonia from agriculture whereas it is actually animal husbandry. The sentence should be modified accordingly.

Whitehead, J.D., M. Twigg, D. Famulari, E. Nemitz, M.A. Sutton, M.W. Gallagher, D. Fowler (2008) Evaluation of laser absorption spectroscopic techniques for eddy covariance flux measurements of ammonia, Environmental Science and Technology, 42, 2041-2046.

Ellis, R. A., Murphy, J. G., Pattey, E., van Haarlem, R., O’Brien, J.M., and Herndon, S. C. (2010) Characterizing a Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS) for measurements of atmospheric ammonia, Atmos. Meas. Tech., 3, 397–406.

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ED: Publish subject to minor revisions (review by editor) (24 Jan 2019) by Sally E. Pusede

AR by Federica Pacifico on behalf of the Authors (01 Feb 2019) Author's response Manuscript

ED: Publish as is (06 Feb 2019) by Sally E. Pusede

AR by Federica Pacifico on behalf of the Authors (09 Feb 2019)

Short summary

Biogenic fluxes from soil at a local and regional scale are crucial to study air pollution and climate. Here we present field measurements of soil fluxes of nitric oxide (NO) and ammonia (NH₃) observed over four different land cover types, i.e. bare soil, grassland, maize field, and forest, at an inland rural site in Benin, West Africa, during the DACCIWA field campaign in June and July 2016.