Methane and nitrous oxide from ground-based FTIR at Addis Ababa: observations, error analysis, and comparison with satellite data

Yirdaw Berhe, Temesgen; Mengistu Tsidu, Gizaw; Blumenstock, Thomas; Hase, Frank; Stiller, Gabriele P.

doi:https://doi.org/10.5194/amt-13-4079-2020

Articles | Volume 13, issue 7

https://doi.org/10.5194/amt-13-4079-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/amt-13-4079-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 13, issue 7

Research article

|

30 Jul 2020

Research article |

| 30 Jul 2020

Methane and nitrous oxide from ground-based FTIR at Addis Ababa: observations, error analysis, and comparison with satellite data

Temesgen Yirdaw Berhe, Gizaw Mengistu Tsidu, Thomas Blumenstock, Frank Hase, and Gabriele P. Stiller

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Final revised paper (published on 30 Jul 2020)
Supplement to the final revised paper
Preprint (discussion started on 28 May 2019)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Methane (CH4) and nitrous oxide (N2O) from ground-based FTIR at Addis Ababa: observations, error analysis and comparison with satellite data', Anonymous Referee #1, 27 Jul 2019
- AC1: 'Methane (CH4) and nitrous oxide (N2O) from ground-based FTIR at Addis Ababa: observations, error analysis and comparison with satellite data', Temesgen Yirdaw, 11 Oct 2019
RC2: 'Review of "Methane (CH4) and nitrous oxide (N2O) from ground-based FTIR at Addis Ababa: observations, error analysis and comparison with satellite data", by Yirdaw berhe, et al.', Anonymous Referee #2, 28 Aug 2019
- AC2: 'Methane (CH4) and nitrous oxide (N2O) from ground-based FTIR at Addis Ababa: observations, error analysis and comparison with satellite data', Temesgen Yirdaw, 11 Oct 2019

Peer-review completion

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

AR by Temesgen Yirdaw on behalf of the Authors (06 Nov 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (07 Nov 2019) by Helen Worden

RR by Anonymous Referee #2 (19 Nov 2019)

RR by Anonymous Referee #1 (12 Dec 2019)

Suggestions for revision or reasons for rejection

This work presents retrievals of CH4 and N2O using a ground-based FTIR instrument at Addis Ababa, Ethiopia with aim to present observations, error analysis, and comparisons with satellite data. The lack of long-term remote sensing observations at Addis Ababa makes this work important. The technique and results of such measurements might be interesting and likely suitable for the journal. However, I still have major comments and do not recommend the current manuscript; revisions are warranted before publication. The quality of the paper needs to be improved before publication.

Major Comments

I have the following major comments:

(1) In this revised versions authors state that the goal is twofold: (1) present a retrieval strategy of CH4 and N2O, and (2) validate the FTIR observations using satellite observations.

Regarding 1, the retrieval strategy applied in this work is different than the NDACC/IRWG recommendations. The authors claim that they use different micro windows because high residuals are obtained if using NDACC recommendations. However, this is not mentioned and shown in the manuscript. It would be valuable for the IRWG/NDACC community, if this in fact is true. By checking the NDACC archive I can see other sites within the latitude range of Addis Ababa and they use the recommended settings. I highly encourage to include a thorough analysis comparing both retrieval strategies (even as supplemental information). The current manuscript just mentions that they use different micro-windows and interfering species but they do not justify. A thorough analysis might consist in showing retrieval fit examples using both methods and at least some months’ worth of data (e.g., linear correlation).

Regarding 2. In the abstract authors state “They reveal the high quality of FTIR measurements at Addis Ababa” and then they show biases in the FTIR with respect to three different satellite measurements.

First, A justification of why these three satellites is missing but highly important. This is crucial if authors aim to validate ground-based FTIR observations using satellite, which normally is the other way around.

Second, the main findings of the paper are summarized in the abstract as follow:

“From comparison of FTIR CH4 and IMK/IAA MIPAS_CH4_224, a statistically significant bias between -4.8 and +4.6 % in the altitude ranges of the upper troposphere and lower stratosphere (15- 27 km) are determined. The largest negative bias in FTIR CH4 is found in the altitude range of 11-19 km with a maximum difference of -0.08 ppmv (-4.8 %) at around 15 km, a positive bias of less than 0.14 ppmv (9 %) is found in the altitude range of 21 to 27 km with a maximum value at around 27 km with respect to AIRS. On the other hand, a comparison of CH4 from ground-based FTIR and MLS-derived CH4 (version 3.3) indicates the existence of a significant positive bias of 2.3 % to 11 % in the altitude range of 20 to 27 km and a negative bias -1.7 % at 17 km. In the case of N2O derived from FTIR and MIPAS_N2O_224 comparison, a significantly positive bias of less than 15 % in the altitude range 22-27 km with a maximum value at around 25 km and a negative bias of -7 % have been found at 17 km. A positive bias of less than 18.6 % in FTIR N2O for the altitude below 27 km is noted when compared to MLS v3.3 N2O. Precision of ground-based FTIR CH4 and N2O in the upper troposphere and lower stratosphere over Addis Ababa are better than 7.2 % and 9 %, respectively which are comparable to the bias obtained from the comparisons.”

I found the above text very confusing and ambiguous. I suggest re-arrange and try to explain better the main findings. Are the findings the same when comparing with the three satellites? it’s not clear to me.

Furthermore, and probably most importantly, the authors compare and report biases at different altitude ranges from about 11 to 27km. I do not believe the ground-based observations are able to retrieve highly resolved vertical profiles. I expect one degree of freedom in the troposphere, where these satellites are not sensitive, and a second degree of freedom in the stratosphere. The abstract and the manuscript sounds like authors claim more degrees of freedom.

Lastly, authors introduce versions of satellite products using awkward names for a paper, e.g., IMK/IAA MIPAS_CH4_224/225, which I found very annoying/distracting.

(2) My understanding is that measurements started in 2009 to present, is this correct? Why you use limited number of years? I highly encourage to include more years and if possible trend analysis. Do the trend make sense?

Specific Comments

Authors use HITRAN 2004, with 2009 and 2012 updates. Why HITRAN 16 is not used?, explain.

In the paper “coincident criteria of ±2◦ of latitude and ± 10◦ of longitude from the ground-based FTIR site in Addis Ababa and within time difference of ±24h”. Please justify these criteria.

I highly suggest to review exhaustively the English along the manuscript.

Review links included in the manuscript, some do not work.

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ED: Reconsider after major revisions (17 Dec 2019) by Helen Worden

AR by Temesgen Yirdaw on behalf of the Authors (07 Feb 2020) Manuscript

ED: Referee Nomination & Report Request started (12 Feb 2020) by Helen Worden

RR by Anonymous Referee #2 (19 Feb 2020)

RR by Anonymous Referee #1 (05 Mar 2020)

Suggestions for revision or reasons for rejection

I have received and checked responses regarding my last comments and suggestions. In general, authors attempt to reply all comments but responses do not fully address the problem or lack in information. I still believe the manuscript could provide useful information, especially since there is a lack of long-term remote sensing observations at Addis Ababa. I highly encourage authors to address my main comments below.

Major comments:

(1) Authors attempt to reply major comment 1. However, in my opinion, it is not complete and/or thorough. The supplement shows a fit example on a single spectrum using micro-windows suggested by the NDACC/IRWG and the micro-windows adapted in this work. It only shows the measured and simulated spectra and residuals; however authors do not mention or compare columns, degrees of freedom, rms, sensitivity, effect of interference species, errors, etc. Showing only the residuals is not a justification that these five windows are suitable or better than the suggested by the IRWG/NDACC. In fact, Figure 1 (supplement) shows that CH4 absorption lines might now be well-isolated (panels 2 and 4) but it is hard to assess since no interference species are plotted. Are the CH4 absorption lines in the micro-windows used in this work well-isolated from other gases? most importantly, from H2O and HDO lines?. HDO might be an important interference but it is not even mention (I asked this before). Furthermore, I recommended to perform the analysis for several months and show an analysis, e.g., correlation of columns/rms, etc between IRWG/NDACC and retrieval setting applied in this work but authors do not mention or show this.

(2) Another major comment I had was about the comparison of profiles authors make between 11 to 27km, even though, for example for CH4, there is one degree of freedom in the troposphere and another in the stratosphere. The abstract and the manuscript sounds like authors claim more degrees of freedom. Authors response: “We took only the intercomparisons where a negative and positive bias are found to show the altitude ranges where volume mixing ratio derived from FTIR is higher (positive bias) and lower (negative bias). In the cases of DOFs, we have already stated in Pg 12, table 1.”. It seems like authors have chosen signs in the bias to report range in altitude, however this might not be the best strategy. It would have been easier to compare upper troposphere lower stratosphere partial columns.

(3) From my last comments I asked why HITRAN 16 is not used and the response is:” This manuscript has been prepared before 3 years ago and even HITRAN 2004 was also not used in this work and rewritten as follows”. It would have been very useful to know how HITRAN 16 compares with previous versions, especially for CH4. It happened to me so many times that I prepare a manuscript and then times fly and by the time I want to submit the abstract I actually have to update so many things, including time series, spectroscopy, etc. Authors mentioned that the manuscript was prepared three years ago, however this does not justify.

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ED: Reconsider after major revisions (06 Mar 2020) by Helen Worden

AR by Temesgen Yirdaw on behalf of the Authors (04 May 2020)

ED: Referee Nomination & Report Request started (07 May 2020) by Helen Worden

RR by Anonymous Referee #1 (10 Jun 2020)

ED: Publish subject to technical corrections (18 Jun 2020) by Helen Worden

AR by Temesgen Yirdaw on behalf of the Authors (26 Jun 2020) Manuscript

Short summary

The retrieved CH₄ and N₂O VMR and column amounts from Addis Ababa, tropical site, are found to exhibit very good agreement with all coincident satellite observations (MIPAS, MLS, and AIRS). Furthermore, the bias obtained from the comparison is comparable to the precision of FTIR measurement, which allows the use of data in further scientific studies as it represents a unique environment of tropical Africa, a region poorly investigated in the past.