Review report Williams et al.

I enjoyed reading this manuscript. Generally, the manuscript is well written, identifies the knowledge gap clearly on missing information on chamber-based measurements and performs well-documented research to add new knowledge that is highly applicable.

Language is also good, but it becomes rather monotonous in the Results section to read over and over again the same phrase “We observed…” etc. Not that this choice of words disqualifies the results, but the authors could think of varying the language in this respect. It is a minor thing and totally up to the authors. Just a point of view from the reader.

Figure quality can be improved and below I suggest adding more figures on the actual chamber designs and chamber measurements to make it more tangible for the reader.

I have marked "reconsidered after major revisions" although the major work is not revising the text for flaws per se, but more adding new figures and text to increase the value of the manuscript. I hope you can see my point here as outlined below.

Materials and Methods

Your description of the experiment is clear, but I was missing pictures of your chambers in the field. This can provide valuable information for the reader wanting to do the same as you, but also help the reader/reviewer assess critical design aspects of your chambers that are not apparent, directly from your description in text. Also, since the chamber testing is the central element in your paper it deserves to be highlighted. Therefore, I suggest to make a new Figure 1 with four panels each showing the chambers A – D. Here it will also be obvious to show the methane delivery system and other important details.

Another thing I was missing was the description of how the chambers are sealed to the ground? Sealing of the chamber to the ground is an essential part of the chamber based measurement and is critical when measuring under windy conditions. Even a lower windspeeds between 5 – 15 kph, as you report, you can have a relatively large disturbance of the chamber headspace concentration that will negatively impact your flux calculation, by leading to underestimation if chamber headspace is diluted which could be a likely scenario in your case with very high CH4 concentrations. Please add a description of this in text and this will also be aided by having the pictures of the chambers as suggested.

For others that want to measure CH4 fluxes at the component level it becomes quite relevant to know your reflections on the sealing. Also, as I could imagine that surfaces are not always smooth and flat, but can constitute more complex 3D structures?

Results

Short and to the point and I do like the use of the error, so the results become comparable across chamber types, flow rates, with/without fans.

The violin plots are good to show the overall performance of the tests, but I was missing some more detailed plots on how actual chamber measurements (CH4 concentration vs time) looked in case with good agreement (~1% error) and poor (>30% error) for both the small and large chambers under calm and windy conditions. In the main manuscript I would just highlight some examples and then in supplementary materials provide all 64 chamber enclosure measurements. In such a figure you could add the theoretical CH4 concentration (e.g. what is dictated according to the release rate) and the actual observed concentration.

Discussion

Again, well written and to the point and you summarize the main findings well. However, I think some more in depth technical discussion on chamber performance and simulation of methane release is needed to put the value of your study in perspective.

I was surprised to see the extreme variation in error for your experiments which seem to be present no matter the combination of mass flow rates, volumetric flowrate and chamber design. You do not discuss this issue, but only use the median error. I think this is a mistake and you should discuss in more depth the reasons why you can have such large errors in some case. Here showing the individual enclosures to the reader would demystify this and bring our all your results in the open.

I would assume that the large error is something to do with the design of your release experiment and/or in combination with chamber design. There is no real reflection on the representativeness of how you release the CH4 so it simulates emissions at the component level. This was done quite detailed in the soil release studies you refer to where the entire validation of the chamber design hinged on simulating the physical nature of the flux, e.g. diffusion through a porous media. A more detailed discussion of this is needed as it will only increase to the value of your study in terms of application.

The large chamber types you use may also be the cause of the large errors, as the collapsible chambers are more susceptible to pressure pumping in the headspace by the wind acting on the walls of the chamber. You do not show results of this nor discuss it, which is really at the core of the applicability and pros/cons of certain chamber types. Here again showing the chamber enclosures and identify the disturbances will help you to discuss this.

If the above mentioned results and discussion are added to the manuscript I believe the value will be increased a lot.

This work builds upon previous research on the assessment of the performance of static chamber methods for methane emission quantification. Such controlled release testing is important in interpreting methane emissions measurement data needed to assess the magnitude of emissions and progress toward emission reductions. The focus on component-level measurements is also important, given the methane policy implications. In addition, the detailed assessment of the various factors that could influence measurement accuracy contributes to the novelty of the work presented here. A few comments and suggestions for revisions are included below:

• Some additional statistical assessment/visualization could help improve the data interpretation here. A regression analysis/parity plot of the performance of the static chamber method against metered flow rates is standard in these kinds of controlled release experiments, and it is not clear why the authors excluded this in their presentation. These parity plots (and accompanying goodness of fit tests) are easily accessible to the lay person than, say Figure 5, showing the percentage error correlation with metered flow rates. The authors should consider including such statistical analysis and visualization in the revised manuscript.
• In the discussion section (e.g., Page 13, lines 289-296), some more direct comparison with previous studies could be useful. Is a median percentage error of 14% consistent with similar previous studies? Similarly, how does the quantification performance of the static chamber method as assessed here compares with other indirect quantification methods?
• On Page 13, lines 297 to 307, it is not clear, and was not tested here, whether static chamber methods can accurately quantify emission rates beyond e.g., greater than 1,000 g/h. Is there a threshold beyond which this method does not work? If so, there needs to be a discussion of that limitation here, otherwise the reader is left with the impression that this method can be used to quantify all kinds of emission rates > 100 g/h.
• The visualization of the percent error (Equation 2, Figures 3-5) is potentially misleading. Because these are presented on an absolute basis using equation 2, Figures 3-5 could be interpreted that all quantified emission rates are greater than metered rates. One could assume that there were quantified emission rates that were less than metered rates, which would lead to a negative percent error, in some cases. The authors should consider revising Figures 3-5 to include those values that were quantified both above (positive percent errors) and below (negative percent errors) the actual metered emission rates.
• Line 19-20, please include a reference for the Global Methane Pledge.
• In the introduction, suggest including specific examples of “components” that can be quantified using the static chamber method (e.g., wellheads at oil and gas well sites). Also, can static chamber methods quantify total facility level emissions, which could be thought of as an aggregation of emissions from individual methane emitting “components” at the facility?

This was a very interesting read and the author shows the value of improving methane emission quantification at the component level. The paper compares previously used emission factors and evaluates previous controlled direct and indirect controlled release tests.  The study also examines direct measurements techniques from static chambers for estimating methane emission rates. These types of techniques have solely been tested and examined in detail which provides useful information for scientists studying methane emissions looking for the right techniques.

In general, the paper is a nice contribution to this research area.  The writing and structure is clear and well organized, but “we found” can be used less.  The paper could use more visuals and photographs of the tested experiments and measurements.  Overall, I approve of this paper with the minor revisions:

Line 26:  add a more detailed description of what component level means or provide distinct examples.

Line 41:  list the recent paper on satellite estimations by de Foy et al., 2023 (DOI 10.1088/1748-9326/acc118).

Lines 100-135:  A visual would be useful for the reader here. I would suggest a general diagram or image of the static chambers with the main components and also a schematic or field photo of how the controlled release was set-up.  The author could also include a material list and description of the construction in the SI with what each component/material was used for and why.

Discussion: It would be beneficial to include more comparison to the other static chamber studies mentioned in the introduction and describe how the results from this study could explain outcomes in the other studies.