Good BUDS (Boulder Utrecht Deuterium System): Hydrogen Isotope Measurements of Atmospheric Methane Through International Collaboration

J. Ortega¹, T. Leon¹, R. Clark¹, S. Michel¹, C.V.D. Veen² and T. Roeckmann²

¹Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309; 303-492-5495, E-mail: john.ortega@colorado.edu
²Institute for Marine and Atmospheric research Utrecht (IMAU), University of Utrecht, Netherlands

For over fifteen years, methane abundance has increased and the carbon isotope ratio of atmospheric methane has decreased, and these changes are accelerating at record pace: from 2020 to 2022 methane abundance has increased by an average of 15.4 ppb yr^-1, and d¹³C_CH4 has decreased at a rate of 0.09 ‰ yr^-1. The hydrogen isotope of methane, expressed as d²H_CH4, provides additional information on different source and sink processes. However, measurement of d²H_CH4 is more challenging than d¹³C_CH4, and data are very limited as a result. Here we show 2022-2024 data that have been obtained by transferring NOAA Global Greenhouse Gas Reference Network (GGGRN) flask samples to FlexFoil bags and sending them to collaborators at the university of Utrecht. The transfer process involves 3 cycles of filling the bags with pure nitrogen and pumping them out before transferring the air from the flasks into the bags. This process combined with the previous handling of the flasks for other measurements (GHG concentrations, d¹³C_CO2, d¹³C_CH4) introduces potential for biases. Measurement comparisons of CH₄ mole fraction and d¹³C_CH4 between Boulder and Utrecht allow us to flag samples that show significant differences.  Over 90% of the samples pass these quality tests. The data show clear latitudinal gradients and seasonal cycles in d²H_CH4, highlighting the power of these isotopes to constrain changes to the methane budget. Meanwhile, INSTAAR is making progress on the revival of its d²H_CH4 system. Flask measurements of many more samples from the GGGRN should soon be possible.

Figure 1. A time series of d²H_CH4 measurements from Barrow, AK (BRW), Tutuila, American Samoa (SMO), Cape Grim, Tasmania (CGO), and Mauna Loa, Hawaii (MLO). These samples were transferred from GGGRN flasks to FlexFoil bags and measured at the University of Utrecht.