General comments:

This paper has estimated methane emissions from offshore gas facilities in the Southern North Sea based on airborne observations of CH₄, CO₂, C₂H₆. A mass balance approach was applied to the observed CH₄ enhancements downwind of the targeted facilities to derive the emissions. Furthermore, the estimated emissions were compared to inventory studies, operator-based facility-level reporting, and other relevant and similar studies. Top-down quantification of CH₄ emissions from offshore oil and gas facilities is much needed to independently evaluate various CH₄ emissions, either expected or unexpected, and these efforts are very useful to help mitigate CH₄ emissions. The manuscript is well structured and well written, and will be suitable for publication after addressing my comments below.

• Assuming a vertically well-mixed plume within the PBL and observing the enhancements that vary significantly across the transects at different heights are contradictory. It may be justifiable to assume the entrainment flux is small, and the top most transect can be extrapolated to the top of the PBL. Can the author justify the extrapolation to the sea surface? Note that the surface layer usually has distinct mixing scheme compared to the PBL. Also, LES simulations indicate that plumes downwind of point sources are not fully mixed kilometers from the source, and only time-averaged plumes show the Gaussian shape, e.g., Raznjevic et al., 2022.
• The comparison of the mass balance estimates with operator-based reporting is highly appreciated. From the main text, I understand that operator-based reporting is provided daily. As maintenance activities and planed venting may vary over hours, I wonder the detailed info was provided by the operator as well. This may potentially explain the still quite large discrepancies between the mass balance estimates and the operator-based reporting.
• The derivation of flux uncertainties is well described. However, some important info is lacking for a reader to understand why the uncertainties of the estimated fluxes are rather small compared to other studies, e.g., what are the wind speed and the wind speed and direction variabilities? Has the upwind transect been considered as backgrounds? How much differences are found in the calculated fluxes if a Kriging method is used to interpolate/extrapolate the plumes? What’s the flux uncertainty caused by the uncertainty in the PBL height? How is the uncertainty of the sum emissions of all facilities derived from the individual uncertainties?

Some details:

• Both Line 115 and Line 302 refer to the Aerodyne instrument using the same technique, please be consistent with the name of the instrument; also, please be consistent in using American and British spelling, e.g., analyzer, analyser, tuneable or tunable
• L130: should be mole fractions? As analyzers report concentrations in mole fractions, not molar ratios. Also for other occurrences throughout the text.
• L131-132: not clear how backgrounds are calculated, please rephrase the sentence. It seems there is a typo here to interpolate between plume edge and side of the plume.
• L197-198: Can the authors clarify how the level of detection was derived? Not sure how the maximum uncertainty of all flux calculation parameters is defined?
• L291: As P9 is reported offline, no CO₂ plume was detected, can the authors exclude flaring emissions from P9?
• L329-330, Figure 3, it would be nice to see the scatter plots of CH₄ CO₂ and CH₄ vs. C₂H₆.
• L455: eq.(A3) is found nowehere.

I am posting this reviewer comment as the editor of this manuscript. During the open discussion it has been difficult to find a second anonymous referee to provide a review of the manusript. But based on the one available anonymous review and the additional open comment by Glen Thistlethwaite, I am satisfied that the paper could meet the standards for publication in ACP, being reconsidered after major revisions. I am posting this comment so that the open discussion phase can be closed, allowing the process to continue and the authors of the manuscript to post their responses to all reviews and to provide a revised manuscript for reconsideration.