|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. |
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?
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.