Measurements of the clumped isotopic composition of atmospheric methane 

<p>Atmospheric methane (CH₄) is the second most important anthropogenic greenhouse gas after CO&#173;₂. 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 (&#948;¹³C and &#948;D) 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 &#916;¹³CD and &#916;DD can be used as an additional tool to constrain CH₄ sources.</p> <p>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 &#916;DD, have the potential to help distinguish between the main drivers of change in the atmospheric CH₄ burden.</p> <p>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 &#177; 0.1 &#8240; for &#916;¹³CD and 2.4 &#177; 0.8 &#8240; for &#916;DD. For atmospheric air at 2 ppm, this translates to extracting CH₄ from at least 2000 L of air.</p> <p>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.</p> <p>Reference:</p> <p>1. Chung, E & Arnold, T 2021, 'Potential of Clumped Isotopes in Constraining the Global Atmospheric Methane Budget', Global Biogeochemical Cycles<span aria-owns="pdfjs_internal_id_66R">, vol. 35, no. 10,</span> https://doi.org/10.1029/2020GB006883</p>