Overview:

The paper by Taquet el al reports on measurements from Mexico City, including three sites, two within the city itself and one at the high-altitude site at Altzomoni. This group is very experienced in FTIR columns measurements, using both high resolution FTIR spectrometers at fixed locations as part of the NDACC/TCCON networks, but also the use of portable EM27/SUN FTIR spectrometers in the COCCON network. This study uses quite an extensive set of data and a methodology based on previous work reported by Stremme et al in 2013. The data set and instrumentation are described in good detail, and any reader who wishes to understand the exact procedure will need to refer to the Stremme paper for details. It is my impression that what the authors produce here is a very promising report of Mexico emissions over a 5-year period and are able to compare this with in situ data as well as satellite based TROPOMI measurement. Within the constraints of their method, that is a simplified approach that avoids the use of complicated modelling, these data sets seem to compare well, given the spatial and inherent uncertainty limitations of the ground-based column measurements.

In terms of principal criteria, the manuscript is rated as good (3) for scientific significance (the methods are not new but are followed with a highly experienced team), rated excellent (4) for scientific quality, an in general rated as good(3) to excellent(4) in presentation quality (see technical corrections below)

In general, the manuscript is well written, gives a very good scientific motivation for the work, has clear description of the measurements and references the paper by Stremme where a very detailed account of the method can be read (not required to be repeated here).

Comments:

Given that understanding of emissions in mega cities is a global question, is the study method only limited to Mexico City? Could this method be applied to others cities or does Mexico city offer very unique geography that means this cannot really be applied elsewhere?

Why not use a chem/trans model in this study? Clearly there is the yet to be published study by Che et al presumably on a subset of this data. Are there specific reasons why a more complicated modelling exercise is not undertaken for the entire measurement record by these authors? Is the suggestion that this simpler approach here should be adopted elsewhere?

A few other comments;

1. Page 19, line 561: are these low costs sensors different to the CO2 sensor mentioned on page 7?
2. Page 21, lines 619/620: what is the significance of these slight decreases?
3. Page 21, line 622: possible reasons for the low ratios?
4. Page 22, line 654: are there other independent traffic patterns that might shed light on the lack of lock-down signal?

Technical Corrections:

1. Page 6, line 179: Nation -> Nafion
2. Page 7, line 215: what is the CO2 sensor (ie, type, model etc)?
3. Page 9, line 279: remove “the”
4. Page 9, line 295: remove “the”
5. Page 10, line 324: “in order of” probably sounds better with “of order of”
6. Page 18, line 553: “the total columns XCO2 and XCO” –> “the total column mole fractions XCO2 and XCO…”
7. Page 24, line 684: “ upwind the city…” -> “upwind of the city…”
8. Page 24, line 689: “ ..in the Stemme…” -> “ ..in Stremme …”
9. Page 24, line 698: “… mountain around …” -> “ … mountains around...”
10. Page 24, line 709: “… would be ..” -> “ …is…”
11. Page 24, line 711-714: does it matter though if these uncertainties have both systematic and random components?
12. Page 24, line 717: what are these instrumental and retrieval effects, just briefly? What size are these factors?
13. Page 25, figure 11 caption, line 731: is that t/year or kt/year?
14. Page 27, lines 807/808: The mention of other components is presumably industrial and domestic burning as described in the next sentence? Need to link these two sentences more clearly.
15. Page 28, line 837: “.. effects to the advection, …” -> “ ..effects of advection, ..”
16. Page 29, line 861: “redaction” is not the correct term which means to remove text for publication, so “writing” is better here.

Taquet et al. investigated the variability of CO₂ and CO in the Mexico City Metropolitan Area (MCMA) on different (annual, seasonal, and diurnal) time scales, based on ground-based in situ and remote sensing measurements. Enhancement ratios (CO/CO₂) were derived from both the in situ and remote sensing measurements and used to estimate CO₂ emissions in the MCMA by combining them with TROPOMI CO data. The estimated annual CO₂ emissions showed the reduction in 2020, likely due to the COVID-19 lockdown, which in not yet reflected in the emission inventories.

The topic of this manuscript is important and relevant to the scope of Atmospheric Chemistry and Physics. In addition, the analysis method is appropriate, and the writing structure is well organized. I recommend that this article be published after addressing the following concerns and questions.

Specific comments

Abstract: The abstract only describes what was done in this study, so please write what was revealed.

L97: What does the “ground-based satellite produce” mean?

L129-131: Please add latitude, longitude, and elevations of the VAL, UNA, and ALTZ stations.

Figure 1: What do the triangle and cross symbols represent?

L179 and L210: Nation air dryer → Nafion air dryer

L276: VRM-scaling → VMR-scaling

L281: The degree of freedom for the CO retrievals in the MIR region is not expected to be as large. Do you evaluated the impact of using a single prior in the profile retrieval of CO?

L419: The description of Figure 4 in the text precedes Figure 3. Please swap the order of Figures 3 and 4.

Figure 3: Figures 3C and 3D are not explained in the text. Please add their explanations or omit these figures.

L479-480: To understand what is described in this sentence, which figure should readers refer to?

Figure 5: What factors contribute to the difference in the diurnal patterns in ΔXCO₂ between the UNA and VAL sites? Can this difference be explained by differences in the spatiotemporal patterns of wind direction within the MCMA?

L529 and L798: Please define the “MGRA”.

L530: What does the “ELD” represent? Please add the explanation.

Figures 7A and 7B: Are the “Surface” and “From FTIR Tot.col.” legends reversed?

L648: Fig. 9 instead of Fig. 7?

L683: Over what domain is (CO_MCMA – CO_bgrd) integrated? Area?

L694: What does the “mixed layer column” mean and how is it defined?

L717: The factors related to instrumental and retrieval effects would also affect the CO columns. How do these factors affect the CO emission estimates?

L758: Please define the “GRA”.

Supplementary file

Caption of Figure S2: after aplying the calibration factors → after applying the calibration factors

Table S1: Are the digits of the calibration factor of “VERTEX-XCO MIR” insufficient?

Caption of Table S3: *corresponds → The asterisks (*) correspond