the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Jun 2025
Validation of the Fugitive Emission Distributed Sampling (FEDS) system: A mobile, multi-inlet system for continuous emissions monitoring
Abstract. The National Physical Laboratory (NPL) has developed and trialled a mobile and remotely-operated Fugitive Emission Distributed Sampling (FEDS) system for continuous measurements of emissions at the facility spatial scale. FEDS is capable of both locating and quantifying emission sources over long-term periods and has been deployed at sites around the UK to monitor methane emissions from the natural gas network, landfill, and waste treatment. This work presents validation activities using a controlled release facility (CRF) to test the performance of the measurement system and two reverse dispersion models (Airviro and WindTrax) for emission quantification. Emissions were quantified using prior knowledge of release timings as well as in the absence of this knowledge. High variability in wind direction was shown to negatively impact emission quantification accuracy (especially for Airviro). Emission results were improved by removing periods of high wind variability (low wind persistence) from analysis. Both models performed better when using daily-averaging periods for emissions (Airviro RMSE = 0.37 kg h-1; WindTrax = 0.29 kg h-1) over shorter averaging periods, such as hourly data (Airviro RMSE = 0.77 kg h-1; WindTrax = 2.19 kg h-1). Emission rates were shown to be sensitive to the specified source release height for both models, with discrepancies in model release height relative to the true release height of more than 0.5 m yielding less accurate results. Furthermore, it was shown that emission results were less accurate when using concentration data input from fewer sampling locations, although it would be remiss to recommend a minimum density of sampling locations for a given area based on a single validation study.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-1451', Anonymous Referee #1, 06 Aug 2025
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RC2: 'Comment on egusphere-2025-1451', Anonymous Referee #2, 13 Nov 2025
Validation of the Fugitive Emission Distributed Sampling (FEDS) system: A mobile, multi-inlet system for continuous emissions monitoring
Shaw et al.
egusphere-2025-1451
The paper describes the validation of a monitoring system to be used to quantify methane emissions over long-term periods from large, aerodynamically complex sources (e.g. natural gas network, landfill, and waste treatment sites). The validation was conducted using four controlled emissions (point source; 20-hour duration; fixed emission rate 1 kg CH4 h-1) with the methane mixing ratio measurements being made a short distance (~35 m) downwind of the release over a flat grass fetch. The measurement campaign lasted 18-days in March/April 2022. Results show good agreement between calculated emissions and the controlled emission rate.
The paper is written well but similar results have been published by other studies, research in this field has moved on considerably.
Firstly, there is little novelty in the work. Many systems have been developed that can quantify methane emissions from point sources a relatively small distance away over an aerodynamically simple wind field. New research in this field either investigates using low-cost/low-power technology that can be deployed remotely or novel dispersion modelling approaches. The authors claim the USP of this paper is that it uses a single expensive analyzer connected to multiple sampling locations. However, methane analyzers, such as the one used here (no details were given on the spec and I cannot find it on ABB’s website), are expensive (USD 20,000 - $50,000), require mains power to operate and a controlled climate environment to operate in. Moreover, the methane mole fraction data were used to generate an hourly average – raising the question of whether a trace methane analyzer is actually needed. As a result, those quantifying methane emissions to justify GHG emission estimates (e.g. landfill and O&G operators) are using alternative instruments that are lower cost, lower power and can be adapted to operate in a range of environmental conditions. The data are presented but there is no synthesis or research questions asked other than - does this approach work in the simplest of emissions scenarios.
Secondly, claims made that the FEDS could be used to quantify emissions of gases from other sources are not validated by the data presented in the paper. The controlled release experiments are very basic when compared against emission scenarios generated at FluxLab (https://fluxlab.ca/) or METEC (https://metec.colostate.edu/). The authors do not describe why they have chosen a single, continuous point source emission of 1 kg CH4 h-1, released at ~2 m above the ground for 20 hours or what real-world emission scenario this is meant to represent. This emission not representative of those from either landfills (heterogenous, area source emission) or wastewater treatment sites (both point and area source emissions). This could simulate a source on the natural gas network (usually point source), but in real-life emissions are typically intermittent (e.g. cycling with separator dumps). The aerodynamic complexity of the experiment is a real weakness, and I find it difficult to imagine any real-world emissions scenario that this would represent.
Overall, I would recommend: 1. that the authors reanalyze their data to present something more novel (e.g. down-sampling the mole fraction data to assess the highest sensor detection limits that still yields the same results – how does this affect the cost of the system and are there available sensors that could be used instead of the LGR?); and 2. be very clear in the conclusions what the limitations of these validation experiments are and place the controlled emission scenario you have used in a realistic, real-world context.
Citation: https://doi.org/10.5194/egusphere-2025-1451-RC2
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Review of: Validation of the Fugitive Emission Distributed Sampling (FEDS) system: A mobile, multi-inlet system for continuous emissions monitoring.
General: The work presented here feels a little “work in progress” rather than a true validation of the system. Whilst it is a useful step in the validation process of FEDS, I am a little concerned that it doesn’t demonstrate the capabilities in a truly rigorous way in comparison to the intended use scenarios and that a follow-up piece of work is needed. I would request that the term “validation” is removed from the title as I think more is needed – maybe “first assessment” is more appropriate?
I believe this work is worth publishing after revisions, as it lays the groundwork for the FEDS system. A part II controlled release study paper would be welcomed – possibly alongside the first site level measurements?
There are a number of significant improvements and increase in scope that I would like the authors to consider. Some should be feasible within the boundaries of the current dataset, but others I would like them to be mindful of as they design their next controlled release experiments to assess and improve the system.
Detailed line by line:
Abstract:
L15: Add some details of the CR - number of sources, release rate ranges, what is being simulated.
L19: Without knowledge of the true emission rate these values are a little meaningless. If the emission rate is 100kg hr, then these are wonderful. If it is 2kg hr, less so... Maybe replace throughout with % of true emission.
L21:23: Feels more like discussion that abstract.
Main Text:
L31: efficient, accurate and transparent monitoring
L41: Discuss snapshot vs continuous
L46: Put in context of reporting requirements such as the new EU regualtions or voluntary programmes such as OGMP2.0? These are already in place and have specific requirements around needs.
L74: The CR needs to be better described here. Is it fully blind, single point or multiple point etc...?
L79: I wouldn’t consider custom Gaussian plume out of reach of commercial teams.
L83: Specify what is considered high-performance
Fig 1: Instrument labelled as uMEA
L114: The FMEA may sample at 1Hz, but what is the turnover time of the cell and therefore what is the true sampling resolution of the system?
L125: Expand this section so that the reader can understand the lags and data invalidation periods. 60s seems like a very long time to clear a portion of line if the lines to each node are being continually pumped. I’d like to see some of this analysis in the main paper.
L129: Is this really sufficient for traceability? I think this quality of calibration is quite poor and I would have expected better QC on this. Can this be replicated in the lab now with more cal gases to demonstrate that instrument performance is as expected over the measurement range seen.
L169: I do have questions around wind persistence and whether it would become less of a controlling problem if you weren’t having to average everything to hourly timesteps. I suspect that there isn’t sufficient data to dig into this further, but I would consider looking at minute by minute enhanced methane readings and correlating with wind direction to see if that gives better data for locating an emission source.
L200: As this is stated to be a validation study, I would like to see more information and justification around the design of the experiment – especially given that it is stated that this system will be used for landfill quantification, among other things.
Fig 3. Can the wind direction be plotted as dots so that there is no 360 jumps in the data.
L260 section: I’d like to see a probability of detection metric determined for the set up if at all possible.
Fig 4. This is not at all intuitive and I would give serious consideration to redefining how this is presented and instead look at methane excess over background as the primary metric rather than just atmospheric mole fraction as I think it would provide much more immediate understanding
L434: Reference issue
L458-462: Whilst these models may perform to the stated stats, I don’t get a sense of understanding as to why they are performing well or poorly (with the exception of the discussion of the wind persistence). Is there more to the modelling than this / more nuance?
L497: Surely measuring more methane downwind from the source can’t be considered a conclusion?
L513: Give % errors so that we can see how performance was without having to know what the release rates were.
Conclusions: General – I’d suggest shortening and tightening up once the major corrections are sorted. It is a little long and unconcise as it stands.