The GEMINI-UK network will measure column average dry air mole fractions of CO₂ and CH₄ with EM27/SUN spectrometers at ten sites across the UK, with the goal to reduce the uncertainty in regional net fluxes estimates. In this study, Kurganskiy et al. present its network design, co-located measurements of one of its instruments with TCCON and the networks theoretical benefits for reducing net flux uncertainties of CO₂ and CH₄. The authors determine the networks potential benefit by quantifying the uncertainty reduction in inversions of simulated measurements.

The network constitutes a substantial contribution to the overall atmospheric GHG observing system in the UK. Presenting the network design and instrument performance is a valuable step for later usage of the network data. Estimating the networks benefit does not only provide insights of the specific network but is also informative for initiatives in other regions. The methods used here are an appropriate tool to investigate this question and the criteria applied to simulate the data yield are realistically chosen. However, the results depend strongly on the uncertainty assumptions, which are only little discussed. I recommend publication of the article after a major revision, addressing the following comments.

Major Comments

The theoretical network performance is evaluated in a closed-loop experiment based on measurements which are simulated with the forward model used for the inversion. While this works well to evaluate the spatial structure of the uncertainty reduction, the magnitude of the uncertainty reduction depends mainly on the values chosen for a priori uncertainty and measurement/transport error. In the manuscript, both are not sufficiently justified and discussed given their importance. The observation error is not specified, only described as “scene-dependent”. The sensitivity of the results to the assumptions should be presented.

Throughout the analysis, the theoretical network performance is evaluated based on the relative change of the uncertainty. However, the ultimate goal is to improve the knowledge on the net fluxes. For this, evaluating the total change in uncertainty is much more informative. Especially for evaluating the results shown in Fig. 7 and 8, which show significant spatial differences. This will also make the argument in L414 clearer. An estimate on the uncertainty reduction of the national estimate could be instructive.

No other sources of error to the flux estimates are discussed. This underestimates the benefit of the total column measurements. For example, a misrepresentation of the boundary layer would induce errors in the flux estimates to which the total column measurements would be much less sensitive compared to in situ measurements. This and other types of systematic errors can not be represented in this closed-loop experiment. However, they should still be discussed.

The result of section 3.1 is that “CAMS in situ” provides the best fit to DECC measurements. On what quantitative result is this conclusion based? The analysis before reads inconclusive.

The bias and standard deviation of the XCO₂ and XCH₄ measurements by the GEMINI-UK EM27/SUN with respect to TCCON are in line with the findings of Frey et al. 2019. However, the standard deviation is larger than what is expected for the random error of EM27/SUN and TCCON measurements (Frey et al. 2019, Wunch et al. 2011). It should be made clear, that the standard deviation presented here is not caused by random but by systematic errors. Additionally, it would be helpful to set bias and scatter in relation to the expected XCO₂ and XCH₄ variability and the expected gradients though out the network. The results should be used to inform the measurement error assumed in the network performance analysis.

No results are shown for the network design process. Especially the results of the sensitivity analysis for the final selections would be interesting for the reader. It would be help to visualize the influence region for individual sites and the overall network sensitivity.

Minor comments

For the forward operator H, the modeled columns are convolved with the averaging kernels. For this, two averaging kernels are used (one for January, one for July). This implies that the SZA dependence of the averaging kernel is neglected, even though L269 names “scene-specific averaging kernels”. Generally, the SZA dependence of the EM27/SUN averaging kernels for CO₂ and CH₄ are not negligible (Hedelius et al. 2016). If it is neglected here, what is the justification for this?

The weatherproof enclosure is mentioned in the manuscript and described in Appendix A. It is mentioned that the optical dome has no adverse effect on the tracking performance. Were tests performed (for example through simultaneous measurements with a standard EM27/SUN) that demonstrate the absence of adverse spectral effects, i.e. on the retrieved columns? This would be especially interesting, since integration into COCCON is planned.

It should be addressed that transport/representativeness error is typically different (lower) for total column measurements than for in situ measurements, since they are less sensitive to very local effects.

In section 2.2, the term “theoretical calculations” is unclear. For which part of the study is real or simulated in situ data used?

The term “baseline calculation/model” is unclear. What is this baseline contrasted by?

L196 and L330 imply that the lateral boundary condition is given by the global GEOMS model. In L379 it is specified that “CAMS in situ” is used for subsequent calculation. Does this refer to future studies or to the subsequent sections of this study?

Please use clear names for the lateral boundary conditions. Does CAMS in situ refer to vCAMS-73? Additionally, it is not clear what vCAMS-73 refers to. The references (Chevallier, 2023; Segers 2023) do not contain a DOI or URL, and the Atmosphere Data Store does not list anything under that reference.

In L214 it is stated that the threshold for well-mixed boundary layer is “analyzing the observed time series”. For what is the time series analyzed?

In L229, why is the a priori termed “background”?

In L290-L297, the notation is quite confusing. Are the cursive and bold “a” in the exponent the same? If so, what is the difference between (6) and (7). If not please change the notation. Additionally, what is N?

In L335, what does “this contribution” refer to?

In L344, how is the optimal subset defined? Is this the subset with maximum S, or do you have other criteria (i.e. related to the spatial distribution)?

In Figure 2 and 3, the different panels are hard to distinguish on one glance. Please simplify the title and indicate the lateral boundary condition used in the respective panel or row.

A checkerboard pattern is visible in Fig. 6. What causes this?

Technical corrections

• L81 EM27 Sun
• L144 instrumentsorldwide
• L195 frmo
• L213 methane. These
• L231 condtions
• L388 substantally
• L428 within within
• Words missing in the sentence L370-L372

References

Frey et al. 2019, https://doi.org/10.5194/amt-12-1513-2019

Hedelius et al. 2016, https://doi.org/10.5194/amt-9-3527-2016

Wunch et al. 2011, https://doi.org/10.1098/rsta.2010.0240