First retrievals of methane isotopologues from FTIR ground-based observations in the High Arctic

Atmospheric methane concentrations have reached a new high at 1845 ± 2 ppb, accounting for an increase of 256 % since pre-industrial times (WMO, 2016). In the last ten years, methane has been on the rise again at rates of ~0.3%/year (e.g., Bader et al., 2016), after a period of stabilization of about 5 years. This recent increase is not fully understood due to remaining uncertainties in the methane budget, influenced by numerous anthropogenic and natural emission sources. In order to examine the cause(s) of this increase, we focus on the two main methane isotopologues, i.e. CH3D and 13CH4. Both isotopologues are emitted in the atmosphere with a different ratio depending on the emission processes involved. As heavier isotopologues will react more slowly than 12CH4, each isotopologue will be depleted from the atmosphere at a specific rate depending on the removal process. Methane isotopologues are therefore good tracers of the methane budget. In this contribution, the first development and optimization of the retrieval strategy of CH3D as well as the preliminary tests for 13CH4 will be presented and discussed, using FTIR (Fourier Transform infrared) solar spectra collected at the Polar Environment Atmospheric Research Laboratory, located at Eureka, Nunavut (80.05 °N, -86.42 °E, 610 m a.s.l.). Mixing ratio vertical profiles from a Whole Atmosphere Community Climate Model (WACCM v.4, Marsh et al., 2013) simulation developed by Buzan et al. (2016) are used as a priori inputs. The uncertainties affecting the retrieved columns, as well as an evaluation of the information content, will be discussed in order to assess the best strategy to be employed based on the altitude sensitivity range and complete error budget