Use of multiple atmospheric chemistry transport models to interpret high frequency methane isotope ratio measurements at independently managed sites

Greenhouse gas (GHG) emissions require accurate quantification for the development of effective mitigation strategies. Top-down methods to estimate GHG emissions combine ambient GHG measurements, atmospheric chemical transport models (ACTMs), and prior independent information on what is understood of fluxes (including isotopic source signatures where applicable). High frequency stable isotope ratio measurements of δ13C-CH4 and δH-CH4 have potential to help differentiate changes in the sources of CH4 emissions at regional scales. While independent efforts to make in situ, high frequency observations are being made at multiple locations, currently there is no network of harmonised measurements across Europe (with each site using different calibration and traceability strategies tailored to their specific analytical setup). In this work we study CH4 isotope ratio measurements made at independently managed sites (Heathfield, UK; Heidelberg, Germany; and Zeppelin, Norway) using three different combinations of ACTMs and associated meteorology: NAME with the UK Met Office Unified Model; FLEXPART with ECMWF IFS analysis and short-term forecasts; and STILT with ECMWF IFS analysis and short-term forecasts. The use of multiple models aims to investigate the magnitude of simulated differences to evaluate model uncertainty in this system. We will demonstrate the extent to which model simulations can be used to investigate analytical problems (e.g. measurement offsets between sites) as well as provide initial results on the potential for a network of high-frequency in situ isotope ratio measurements to understand changes in European CH4 emissions over the coming decades.