Controlled release testing of the static chamber methodology for direct measurements of methane emissions
Abstract. Direct measurements of methane emissions at the component level provide the level of detail necessary for developing actionable mitigation strategies. As such, there is a need to understand the magnitude of component level methane emission sources and test methane quantification methods that can capture methane emissions from component level sources. The static chamber method is a direct measurement technique that is being applied to measure large and complex methane sources such as oil and gas infrastructure. In this work we compile component level methane emission factors from the IPCC emission factor database to understand the magnitude of component level methane flowrates, review the tested flowrates and measurement techniques from 38 controlled release experiments, and perform 64 controlled release testing of static chambers methodology with mass flowrates of 1.02, 10.2, 102, and 512 grams of methane per hour (g/hour). We vary the leak properties, chamber shape, chamber size, and usage of fans to evaluate how these factors affect the accuracy of the static chamber method. We find that 99 % of component level methane emission rates from the IPCC emission factor database are below 100 g/hour, and that 76 % of previously-available controlled release experiments did not test for methane mass flowrates below 100 g/hour. We find that the static chamber method quantified methane flowrates with an overall accuracy of ±14 %, and that optimal chamber configurations (i.e., chamber shape, volume, usage of fans) can improve accuracy to below ±5 %. We find that smaller chambers (<20 L) performed better than larger volume chambers (>20 L), regardless of the shape of chamber or usage of fans. However, we found that the usage of fans can substantially increase the accuracy of larger chambers, especially at higher mass flowrates of methane (>100 g/hour). Overall, our findings can be used to engineer static chamber systems for future direct measurement campaigns targeting a wide range of sources, including landfills, manholes, and oil and natural gas infrastructure.
James Philip Williams et al.
Status: open (until 17 Apr 2023)
- RC1: 'Comment on amt-2023-27', Jesper Christiansen, 13 Mar 2023 reply
James Philip Williams et al.
James Philip Williams et al.
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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?
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.
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.