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Extending capabilities in atmospheric monitoring of methane stable isotope ratios
by Dr. Tim Arnold, Mr. Edward Chung, Dr. Chris Rennick, Dr. Emmal Safi, Mrs./Ms. Alice Drinkwater, Dr. Caroline Dylag, Mrs./Ms. Sara Defratyka, Dr. Volker Ebert, Dr. Javis Nwaboh


We present our current work and future plans for ground-based in situ monitoring of stable isotopologue ratios of methane (CH4) to advance the understanding of fluxes at both the global and regional scales. At regional scales the difficulty in making high-quality continuous measurements of the singly-substituted isotopologues hampers the routine take up of isotope ratios in emissions estimation frameworks. We will present the measurement protocols and results from ambient air sampling for both δ13C(CH4) and δ2H(CH4) using a laser spectrometer system. In 2021 the system was deployed to a tall tower monitoring site in the UK (a GAW regional station) to assess the utility of continuous isotope ratio measurements for country-scale emissions verification. Trends in global atmospheric CH4 – both mixing ratios and isotope ratios – are explained by various flux scenarios, from tropical wetland emission increases through to reductions in global hydroxyl. Looking beyond the singly-substituted isotopologues we have modelled the potential of the doubly-substituted isotopologues (isotopologues containing two or more rare-isotope substitutions) to understand the longer-term global atmospheric CH4 cycle (based on existing knowledge of signatures from sources and sinks for Δ13CH3D and Δ12CH2D2). Measurements of the doubly-substituted isotopologues (alongside a global network of δ13C and δ2H measurements) could provide constraints for understanding trends in the global total source and sink magnitudes. Finally, we will discuss the technical challenges faced in advancing this area of stable isotopic analysis for atmospheric CH4 and the work that is planned through a new EURAMET Partnership project called isoMET: ‘Metrology for European emissions verification on methane isotopes’ starting in 2022.


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Topic : Theme 2: Novel GHG concentration and flux methods and sensors.
Reference : T2-C11

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