1,3,3,1,2- 1Max Planck Institute for Chemistry, Atmospheric Chemistry Dept., Mainz, Germany (sergey.gromov@mpic.de)
- 2Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, The Netherlands
- 3Department of Chemistry, University of Copenhagen, Copenhagen, Denmark
Recent theoretical and laboratory studies [1, 2] provide new and updated estimates of carbon and hydrogen kinetic isotope effects in the gas-phase reactions removing atmospheric methane (CH4). In particular, new temperature dependence of kinetic fractionation in reactions with hydroxyl (OH) and chlorine (Cl) radicals is determined for a range of isotopomers, including the multiply substituted (clumped) ones. In this study, we use the ECHAM/MESSy Atmospheric Chemistry (EMAC) model in a comprehensive chemistry-inclusive hind-cast setup [3] to obtain projections of the nominal and effective sink fractionation in atmospheric CH4 throughout the 1960–2020 period, based on the new isotope kinetic data. Use of EMAC allows to obtain realistic spatiotemporal distribution of fractionation magnitudes resulting from convolution of temperature and sink rate distributions, further modified by atmospheric transport and mixing.
Our simulations yield a range of significantly different projections for most of the isotopomers, compared either to the literature values or to theoretical approach/laboratory data considered. The laboratory and more advanced theoretical approaches yield larger fractionations for both 13CH4 and CH3D in reactions with OH. The opposite is obtained for the reactions with Cl, however with more advanced theory being closer to the laboratory data-based estimates. For clumped isotopomers, comparison to available literature data yields no systematic relationships.
The obtained time series witness a significant (up to 1.5‰) increase in the total nominal 13CH4 sink fractionation during 1960–1990 due to the changes in the stratospheric Cl sink distribution, following the onset of anthropogenic chlorofluorocarbons (CFC) emissions. After the global ban on CFCs, a reverse gradual decrease on the order of 0.1‰/decade is projected. A similar, though much smaller in relative magnitude, evolution is estimated for CH3D. Whilst the mean OH the sink rate-weighted atmospheric temperature exhibits a slight positive trend, the Cl and O(1D) sink rates-weighted temperatures witness larger decreases, in line with tropospheric warming and stratospheric cooling occurring in the last decades. We discuss the implications and uncertainties of our findings for isotope-inclusive efforts to improve past and present CH4 atmospheric budget estimates.
References
1. M. K. Mikkelsen, et al., Kinetic isotope effects in methane oxidation reactions: temperature dependence of the OH and Cl KIEs for 13CH4, CDH3, 13CDH3, CD2H2, CD3H, and CD4 from 100 to 500 K, AGU Fall Meeting, B13N-1736, 2025. https://agu.confex.com/agu/agu25/meetingapp.cgi/Paper/1911885
2. C.-C. Chen, C. van der Veen, G. Adnew, T. Röckmann, Comparative analysis of 13CKIE and DKIE in CH4-OH reaction, AGU Fall Meeting, A21M-2178, 2025. https://agu.confex.com/agu/agu25/meetingapp.cgi/Paper/1944364
3. P. Jöckel, et al., RD1SD: EMAC CCMI-2022 hindcast simulations with specified dynamics, ERA-5, 1979-2019. World Data Center for Climate (WDCC) at DKRZ. Deposited 18 June 2024. https://doi.org/10.26050/WDCC/ESCiMo2_RD1SD
How to cite: Gromov, S., Chen, C.-C., Johnson, M. S., Mikkelsen, M. K., Pozzer, A., and Röckmann, T.: Atmospheric methane sink isotope fractionation throughout last six decades: Projections using new kinetic data and implications for CH4 budget, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18527, https://doi.org/10.5194/egusphere-egu26-18527, 2026.
