the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Aug 2024
A new portable sampler of atmospheric methane for radiocarbon measurements
Abstract. Radiocarbon (14C) is an optimal tracer of methane emissions, as 14C measurements enable distinguishing fossil from biogenic methane (CH4). However, 14C measurements in atmospheric methane are still rare, mainly because of the technical challenge of collecting enough carbon for 14C analysis from ambient air samples. In this study we address this challenge by advancing the system in Zazzeri et al. (2021) into a much more compact and portable sampler, and by coupling the sampler with the MICADAS AMS system at ETH, Zurich, using a gas interface.
Here we present the new sampler setup, the assessment of the system contamination and a first inter-laboratory comparison with the LARA AMS laboratory at the University of Bern.
With our sampling line we achieved a very low blank, 0.7 µgC compared to 5.5 µgC in Zazzeri et al. (2021), and a sample precision of 0.9 %, comparable with other measurements techniques for 14CH4, while reducing the sample size to 60 liters of air. We show that this technique, with further improvements, will enable routine 14CH4 measurements in the field for an improved understanding of CH4 sources.
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Status: open (until 24 Sep 2024)
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RC1: 'Comment on amt-2024-123', Anonymous Referee #1, 26 Aug 2024
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The presented manuscript highlights an improved setup for the radiocarbon analysis of methane from atmospheric air samples. Due to the low atmospheric concentration of methane and the fact that other C-bearing gases are contained in the atmosphere it is difficult to obtain a sufficient amount of sample for 14C analysis, that is also free of contamination.
The manuscript is well-written and structured, the presented methods are state of the art. Similar and historic approaches to the tasks fulfilled by the setup are referenced properly. The results are discussed properly and data are corrected for appropriately by the typical assessment of extraneous C via a model of constant contamination.
However, some questions arised while reading the preprint that should be adressed for clarification:
1. The introduction refers to the setup presented by Palonen et al. (2017), who built a similar portable CH4-oxidation setup to sample CH4 from respiration chambers. The authors herein conclude that the Palonen et al. setup is only usable for concentrations >100 ppm. How was this conclusion reached? While it is true that respiration chambers used in temperate climate wetlands will contain much higher CH4 concentrations, I think the setup could in theory be used for different approaches, although sampling will take a long time.
2. In the method section it is stated that the setup is battery powered, which is obviously useful when working in remote areas. What type of battery was used and how long can the system run until the battery is empty?
3. The setup uses a Nafion membrane to remove H2O from the gas stream. Was the amount of H2O in the sample quantified? I think it would be relevant to mention because 13X zeolith, that is used to trap the CH4-derived-CO2, is also able to adsorb H2O. In addition to that, the oxidation of CH4 to CO2 will also produce H2O that will adsorb onto the sample trap.
4. What was the reason for the catalyst that was used? Palonen et al. (2017) for example used Pd-based catalysts that yielded sufficient results at lower furnace temperatures, potentially saving more battery power.
5. The assessment of contamination is valid, however, wouldn't it have been more realistic to produce a mixture of gases to test contamination as well as trapping efficiency? For example, create a gas mixture that contains fossil CH4 and Ox-II derived CO2 as a sensitive measure for sufficient separation of gases.
6. How was the evaluation using the best-fit for contamination in Fig. 3 and 4 done? Did you use a script or automation to assess the best fit or was the fit eyeballed by manually manipulating input parameters such as mass of contamination and 14C-content?
If the authors clarify the mentioned questions above I endsorse the script for publication, it is valuable to the community for further development of methods for atmospheric CH4 dating or rather CH4 dating in general.
Citation: https://doi.org/10.5194/amt-2024-123-RC1
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