A novel Laser-based technique for in-situ automated measurements of atmospheric CH4 isotopologues and implications for current and future emission scenarios

Abstract

Methane (CH4) is the second strongest anthropogenic greenhouse gas in terms of radiative forcing after carbon dioxide (CO2). It has a relatively short atmospheric lifetime for a greenhouse gas (~9 years)and so is an attractive target for near-term climate mitigation strategies. While the total amount fraction of atmospheric CH4 is currently measured by atmospheric monitoring stations to calculate global and regional budgets, it is not currently possible to distinguish the total amount of CH4 emitted by individual sectors. Different sources emit CH4 with unique isotope ratios, and so continuous isotope ratio measurements have the potential to provide the data needed to disaggregate emissions sources.   We have developed Boreas, an automated field deployable CH4 preconcentrator coupled to a laser spectrometer able to provide continuous, high frequency measurements of both δ 13C (CH4) and δ 2H (CH4) and the total CH4 amount fraction in ambient air. Boreas was deployed to a UK atmospheric monitoring site located in Heathfield, East Sussex, in May 2021.   We compare atmospheric measurements of δ13C(CH4) and δ2H(CH4) made from Spring 2021 to Spring 2023 with equivalent model output and analyse similarities and differences in the context of currently known isotopic source signatures in Europe. We also consider potential future CH4 emission scenarios and show how these measurements could be used under future emission reduction strategies.