This paper describes a relatively new TCCON station located near Nicosia, Cyprus. This location fills an important gap in carbon cycle monitoring, and it is therefore a welcome addition. The paper describes the location, measurements to date, and some comparisons with coincident AirCore profiles. A reference for these measurements will be very helpful, so I recommend publication after addressing the following comments.

Specific comments:

TCCON typically chooses 45 cm OPD for its spectral resolution to allow the spectra to distinguish the absorption lines of interest from the interfering absorption lines across a broad wavelength range while maintaining a high signal-to-noise ratio. You’ve chosen 64 cm OPD, presumably as a compromise between the TCCON- and NDACC-style measurements you wish to collect. Do you have a sense of how that choice impacts the TCCON measurements? Have you truncated the interferograms to show whether the additional 20 cm OPD improves or degrades the CO₂ retrieved? Please justify your choice of 65 cm OPD.

Figure 2: What is the cause of the substantial increase in Xluft spread (stdev) in late 2019, and early-to-mid 2023? Did the density of data decrease in mid-2021 when you began NDACC-like measurements?

Figure 3: What caused the sparsity in measurements in early 2021?

Section 3.2.2: There are several reasons proposed for the seasonal cycle in CH₄, but earlier in the paper, it was stated that Nicosia measures outflow from Europe, Africa, and Asia. Could you perform an analysis that distinguishes airmasses from each of these continents to confirm your earlier assertion? A back-trajectory analysis or a climatological analysis would be helpful to interpret your results. Can you make use of your HCHO and HCN measurements to strengthen your argument that CH₄ enhancements are caused by fire activity? Is CH₄ expected from agricultural waste burning?

Figure 5: I find this to be a difficult way to visualize the comparison between the AirCore and TCCON measurements. It would be helpful to show the TCCON data time series spread out across each panel as a function of the hour of the day, with horizontal lines in black and red representing the medians of the public TCCON and custom TCCON retrievals, respectively. Then, the AirCore diamond (in blue) should be positioned at the time of the lowest altitude AirCore measurement, so we can compare any trends in the TCCON measurements over the +/- 1 hour with the AirCore columns. With the current visualization, the reader cannot see if there are trends in the TCCON measurements throughout the comparison period. Also, what is the cause of the disagreement in XCO₂ on June 29? According to Figure 4, there’s a substantial near-surface CO₂ enhancement on June 29, and the AirCore profile does not appear to provide data below ~2 km on that day. Do you have surface data to fill in the bottom of the profile on that day?

Figure S8: How are the profiles extended down to the surface? The text seems to imply that the lowest AirCore measurement is dropped straight to the surface (“flat-extrapolation”), but replaced in the bottom two grid levels by the surface in situ measurement. However, the right panel of Figure S8 does not appear to show that. Please clarify whether these are two different profiles, or why the near-surface assembled profile is >8 ppm different between the left and right panels.

Technical comments:

L30: “mid-infrared (MIR) spectrum” should be “mid-infrared (MIR) spectral region”

L74: “north hemisphere” should be “northern hemisphere”

L93: remove “shall”

L96: TCCON Network seems redundant -> use just TCCON or just Network

Equation (1): missing ‘gas’ subscript on the central equation

L182: It is not just spectroscopic errors that cancel in the ratio. Alignment errors, pointing errors, some spectroscopic uncertainties can partially cancel.

L187: The O₂ is retrieved from a 250 cm^-1 wide window that is centred at 7885 cm^-1, and not from a single line. The retrievals are based on multiple O₂ absorption lines. See Mendonca et al. (2019) for reference for this comment and the previous one (L182).

References:

Mendonca, J., Strong, K., Wunch, D., Toon, G. C., Long, D. A., Hodges, J. T., Sironneau, V. T., and Franklin, J. E.: Using a speed-dependent Voigt line shape to retrieve O₂ from Total Carbon Column Observing Network solar spectra to improve measurements of XCO₂, Atmos. Meas. Tech., 12, 35–50, https://doi.org/10.5194/amt-12-35-2019, 2019.

This article reports on the installation of a new TCCON site in Nicosia and describes quality control procedures applied on the measured TCCON data starting in 2019.

In the introduction and conclusions, the site is advertised as strategic due to its unique location: it can measure different airmass from Europe, Asia and Africa and bring new insights in regional sources and sinks for the EMME region. With three AirCore profiles and their back-trajectory analysis, the material presented in the article demonstrates the stated  benefits of this site to a limited extent. This is the main concern with the paper which lead to the overall manuscript rating:  the authors should include material that supports their claims made in the conclusion and introduction.  

More detailed comments:

• L139: please add a motivation why this max opd deviates from the TCCON standard of 45cm
• L182: I believe this cancels not only spectroscopic, but any systematic error common to both gas and O2 columns
• L198: I was confused with this sentence: at the beginning of the § the problematic period is in 2022 (L194), but here the start of the period is April 2021. Please clarify.
• L214: “This evaluation exercise is only visual”: why?  If the CFs are derived from a larger ensemble of insitu profiles, it would remain meaningful to interpret the observed differences between the TCCON and AirCore measurements (cf the comments on Eq(2)).
• Eq(2): how can gamma be determined from the public TCCON data? 
• Eq(2): the main purpose of calculating AC.Xgas and its comparison to TCCON is a reduction in the comparison error budget (cf Rodgers 2003). This paragraph should be extended with an uncertainty budget estimate on the difference between the AirCore and TCCON data (not the full detail of the error contributions (eg spectroscopy, noise ,… ), but the text must link it to the uncertainties reported along with the measurement data so that a reader may reproduce the results).
  
  Also which pairs of Xgas values should be considered so that their difference allows an error budget reduction: public.Xgas minus AC.Xgas, or public.Xgas minus the unchanged AirCore, or  … ?
  
  Which uncertainty should be used to evaluate the differences for each such pair? Eg is the smoothing uncertainty part of the uncertainty budget on the difference? This information is not available here nor in the supplement.
• L231 “Differences amongst these two quantities will be due to the difference in the measurement principle”: this is unclear and must be clarified (which quantities?, which uncertainty term is canceled? cf the previous remark on Eq(2)). In Figure 5 the different Xgas values are plotted and the author presents it as a “visual-only comparison”: this is not sufficient for a scientific publication: an uncertainty budget must be specified to properly interpret a comparison.
• L227: typo: The results presented here are obtained using the “pressure weights” method 
• Discussion in §3.2 would benefit from a more clear link with the plots in Fig 3: eg where in the plot is the location of the minor peak in xCH4 around mid-spring (L287)
• Suggestion to revise the document to follow the GUM terminology and replace “error” with “uncertainty” where necessary (see §2.2 from GUM 2008)