EGU23-14514, updated on 19 Aug 2023
https://doi.org/10.5194/egusphere-egu23-14514
EGU General Assembly 2023
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Measurements of the clumped isotopic composition of atmospheric methane

Malavika Sivan1, Thomas Röckmann1, Carina van der Veen1, Caroline P. Slomp2, and Maria Elena Popa1
Malavika Sivan et al.
  • 1Institute for Marine and Atmospheric research Utrecht, Department of Physics, Utrecht University, The Netherlands
  • 2Department of Microbiology, Radboud Institute for Biological and Environmental Sciences, Radboud University, Nijmegen, The Netherlands

Atmospheric methane (CH4) is the second most important anthropogenic greenhouse gas after CO­2. Global scale measurements of CH4 mole fraction show an increasing trend since pre-industrial times. Various studies have attempted to attribute the temporal change to variations in the balance between different CH4 sources and atmospheric sink reactions. Measurements of bulk isotopic composition (δ13C and δD) are used for this purpose, but due to the overlap of source signatures, it is difficult to distinguish between biogenic, thermogenic, and pyrogenic CH4. With the advancement of high-resolution mass spectrometry, it is now possible to measure the two most abundant clumped isotopologues of CH4: 13CDH3 and CD2H2. The clumping anomalies denoted as Δ13CD and ΔDD can be used as an additional tool to constrain CH4 sources.

Most of the clumped isotope studies so far, have focused on high-concentration samples, which can easily deliver the large quantity of pure CH4 (several mL) needed to measure the clumped isotopologues. But these measurements could be particularly interesting for atmospheric CH4, for which the explanations of the recent variations are still under debate. As shown by a recent modeling study (1), clumping anomalies, especially ΔDD, have the potential to help distinguish between the main drivers of change in the atmospheric CH4 burden.

In our laboratory, we use the 253-Ultra mass spectrometer to measure the clumped isotopologues of CH4. These measurements require 4-5 mL of pure CH4 to achieve a precision of 0.3 ± 0.1 ‰ for Δ13CD and 2.4 ± 0.8 ‰ for ΔDD. For atmospheric air at 2 ppm, this translates to extracting CH4 from at least 2000 L of air.

We have recently developed a method for extracting and purifying CH4 from this large quantity of air, without modifying its isotopic composition. We will present the current capabilities of this extraction system, and the first results of the clumped isotopic composition of the ambient air.

Reference:

1. Chung, E & Arnold, T 2021, 'Potential of Clumped Isotopes in Constraining the Global Atmospheric Methane Budget', Global Biogeochemical Cycles, vol. 35, no. 10, https://doi.org/10.1029/2020GB006883

How to cite: Sivan, M., Röckmann, T., van der Veen, C., Slomp, C. P., and Popa, M. E.: Measurements of the clumped isotopic composition of atmospheric methane, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14514, https://doi.org/10.5194/egusphere-egu23-14514, 2023.