The manuscript presents results from the retrieved Aerosol Optical Depth (AOD) by a EM27/SUN spectrometer and its spectral dependence in the 873-2314 nm spectral range from COCCON measurements at the Izaña Observatory as well as its capability to simultaneously infer trace gases and relate their common emission sources. In addition, the authors performed a cross-validation with AERONET measurements, during the same period in the common spectral range, which revealed a good agreement.

The manuscript is well structured and well written and fits into the scope of AMT. The authors give proper credit to related work and support the current study. The manuscript is worth to be published, however, there are a few comments that are listed below.

General comments:

• In general, I would recommend to try to have an uncertainty analysis of the retrieved AOD.
• AOD at 1640 nm needs a GHG correction. Is this already done and do you use actual FTIR GHG concentrations to do it?
• Finally, is the purpose of the paper the investigation of using FTIRs for AOD retrieval? Would that help increasing AOD networks or would it have an added value for COCCON?

Specific Comments:

1. Lines 24-25: “...the most recent assessment report by the Intergovernmental Panel on Climate Change (IPCC)” – If possible, please provide a reference in order to support this statement.
2. Line 31: “SI-traceable measurement technique” (the acronym SI is not defined).
3. Lines 50-51: If possible, please add a brief comment introducing the near-infrared (NIR) and short-wave infrared (SWIR) spectral regions at this point.
4. Line 84: The comment in the parenthesis “an IFS 125HR”, could be replaced by “an IFS 125HR spectrometer”.
5. Lines 131-132: Is there any further information about the uncertainty of the instrument or the performance according to the last calibrations available?
6. Figure 1 (page 6): The exact spectral range that each detector is sensitive in (InGaAs-1 and InGaAs-2), could be introduced in the instrument’s technical description in Section 1.
7. Figure 1 (page 6): “Note that both detectors have different gains …the measured radiation.” This part could be included in the main passage instead.
8. Lines 166-167: The explanation of the selection of these specific wavelengths was essential and well placed here by the authors. However, the sentence “In this study, an additional channel (B1) ...” might firstly give the impression that another channel (apart from the 8 already mentioned) was added. To avoid confusion, this sentence could be rephrased as: “Seven of the presented spectral bands (B2-B8) were selected with respect to those presented in Barreto et al. (2020), while an additional channel (B1) has been incorporated for the purposes of this study due to the wider coverage range of the EM27/SUN InGaAs detector.”
9. The names of the two detectors are introduced in lines 168-169: “hereafter referred to as InGaAs-1 and InGaAs-2, respectively”, however, they have already been mentioned as “InGaAs-1” and “InGaAs-2” in Figure 1. A suggestion would be to move Figure 1 below this paragraph.
10. Lines 170-171: “in this study the EM27/SUN solar spectra were neither calibrated nor referenced to any traceable lamp”. In section 3.1 it is mentioned that proper calibration of all COCCON spectrometers is performed.
11. Table 2 (page 14): could be moved after the end of the paragraph (line 303).
12. Equation 2 (page 14): It was mentioned previously that “m” is the air mass, please define if “m_a” stands for a different parameter.
13. Figure 5 (page 15): It is a bit puzzling why during the first period (2020 – 2021) the uncertainty is higher than the second one. Is it a matter of calibration?
14. Figure 6 (page 16): I would recommend to be presented in Section 5.4, probably after line 346.
15. Figure 8 (page 20): the numbering of each cell is in the same position (bottom right) except for 8f & 8g which are on the bottom left side.
16. Figure 8 caption is not inside the page boarders.
17. Author contributions: It is suggested to change O.G. to O.E.G. (Omaira E. Garcia) and S.L. to S.F.L.L. (Sergio F. Leon-Luis).

The manuscript presents a portable and compact low-resolution Fourier Transform Spectrometer (EM27/SUN), the method and the results from the retrieval of AOD (Aerosol Optical Depth) from COCCON measurements at the Izaña Observatory (IZO) with EM27/SUN. The validation of the AOD retrieval method is done with an intercomparison to AERONET sunphotometer measurements at IZO. Case studies with simultaneous trace gases (CO2, CO) and AOD retrievals are presented for a local volcanic erruption (La Palma, September 2021) and a local forest fire (Los Realejos, Tenerife, July 2022)

The manuscript is of high scientific interest regarding the scope of AMT. It is very clear, from a pedagogic and linguistic point of view, well structured, well written, english is good, almost no syntax/grammatic/orthographic/typos  mistakes. Current related works are well cited, the litterature study is well lead. I recommand this manuscript for publication after very few minor corrections and after answering some questions. These comments (for minor correction)and questions are listed below.

* General comments:

1) I would recommend to add an accronym tables, in order that the readers can find quickly what the different paragraphs are talking about, since there are a lot of accronyms used in the manuscript.

2) I recommand to develop more information about the reference Fourier Transform Spectrometer: High resolution IFS 125 HR presented and validated in Barreto et al. 2020, and not only to cite Barreto et al. 2020 each time IFS 125 HR is mentionned. For instance, explain the resolution of IFS 125 HR when the resolution of EM27/SUN is discussed. Also during the validation of the AOD retrieval with EM27/SUN, since IFS 125 HR is presented as reference and Barreto et al. 2020 continuously mentioned, the authors should better give some values of the statistics of Barreto et al. 2020 regarding intercomparison AERONET vs. IFS 125 HR, and discuss and interprete these results to the intercomparison results of AERONET vs. EM27/SUN presented in this manuscript.

* Specific comments/questions

Line 4 or Line 13: You menton "low resolution" -> Maybe specify "0.5 cm-1" in brackets

Line 42 and Line 51: Specify the resolution of IFS 125 HR (line 42) and of "low resolution" EM27/SUN (line 51).

Line 114: You give the instrumental resolution in cm-1 (0.5 cm-1). Maybe specify how it is in nm (for SWIR and NIR bands), since the rest of the study and the comparison with AERONET is given with wavelength and spectral band width in nm. -> This inconsistency is very visible in the legend of Figure 1: "EM27/SUN solar spectrum for the 870-2500 m ... resolution of 0.5 cm-1"

Line 140: "Solar/lunar and sky measurements are normally taken every ~15 minutes" -> Can you verify this information, in my opinion it is more often (every 5 minutes)

Line 181: Typo: "3-year period" -> "3 years period"

Line 201: Formula V_lambda = V_0,lamdda * d-2 * exp(-m*tau_lambda) -> Since V_0,lamdda is later (Line 203) defined as the "instrument's signal at TOA", and not at the sun, the term "d-2" has to be cancel from the formula of Line 201 and from the description of Line 203.

d-2 is allready integrated in V_0,lamdda, since V_0,lamdda = V_sun,lambda * d-2 (signal measured at the sun)

Line 251-254: Why should an event increasing atmospheric turbidity lead to a lower V_0,lamdda TOA signal? The aim of the langley-plot method is to get rid of the atmospheric turbidity. It can be, that because of these events, the turbidity is to high and unstable, and then we cannot do Langley-Plot, but if we can do it (not too much turbidity and stable during sun rise / sun set) = langley plot (ln(I) va airmass) is a straight line, then the result should not be lower because of it. -> Can you please consider this question and give explanation. If not I do not agree with "... could also cause a loss of signal" (Line 253), at least if "signal" = TOA signal (V_0,lamdda)

Figure 4:

1) please make two figures, one with 2019-2022 (whole period) and the other one with Dec 2019 - Dec 2020, with the open markers and the plain one, it is too confusing to interprete the graphic.

2) In the legend "open circles" -> "open markers" (there are other open symbols than circles)

Lines 301-302: You recommand ideally one calibration / month -> Then you cannot use the system as an operational system on a site without opportunity of Langley-Plot calibration (urban areas, turbid areas, not high mountains, ...) -> Do you have to suggest other methods of calibration for these non langley-plot compatible sites?

Lines 322-325: The authors seem to be satisfied with the agreement EM27/SUN to AERONET, even if the WMO criterium (U95) that has been mentionned is not satisfied. Maybe here it is worth to give some explanation about what values (percents of occurence in U95 or which softer criterium than U95) is expected from the authors to be satisfied. here maybe it would be interesting to compare the performances of EM27/SUN to the one of IFS 125 HR, mentionning the values of the performances explained in Barreto et al. 2020

Lines 328-330: I disagree with the assumption, that since U95 is defined for UV, it should be harder in SWIR+NIR. No, in the contrary: U95 is a criterium set in the absolute AOD difference that is larger in UV than in SWIR+NIR, since the AOD itself is larger. U95 should be in my opinion, from a statistically point of view easier to reach in SWIR+NIR since the AOD is lower. Of course, from an instrumental point of view it is different, but this has to be justified with other argument (signal noise ratios of the photometers, etc...)

Lines 341-342 vs Line 352-353: Line 341-342 mention that older studies (Toledano et al. 2019 and Barreto et al. 2020) say that for high dusty events, there is no Angstroem law, than at lines 352-353, you mention that this study has same results as older studies (the same: Toledanoet al. 2019 + Barreto et al. 2020) and you have more interspectral correlation for high AOD and dusty. But: Angstroem law should not be a source of increasement of interspectral correlation? Can you develop / explain (not in manuscript but in comment) where should the higher interspectral correlation come from, if not from Angstroem law?

 

Lines 455-457: You give quantificated values of the evolution of calibration values for EM27/SUN and mention IFS 125 HR as reference... But without mentioning values of the stability/evolution of calibration values of IFS 125 HR.

Lines 462-463 vs Lines 483-486: At lines 462-463 you mention the need of monthly calibration of the system (that only works on some few calibration sites) and lines 483-486 you mention that the system should be applied in a measurements' network -> Most of the station of measurements' network are not compatible with Langley-plot calibration -> Which method do you suggest for these stations to keep the instrument well calibrated without sending it to IZO or another calibration site every month?