Long-term variations of the mole fraction and carbon isotope ratio of atmospheric methane observed at Ny-Ålesund, Svalbard from 1996 to 2013

Abstract We have conducted systematic observations of the CH4 mole fraction and its carbon isotope ratio (δ13C) at Ny-Ålesund, Svalbard (78°55′N, 11°56′E) using air samples collected weekly since 1991 and 1996, respectively. The CH4 mole fraction showed long-term increase until 1999, stagnation between 2000 and 2006, followed by an increase after 2006. On the other hand, δ13C showed monotonous increase until 2006 and decrease after 2006. By comparing the rates of change in the CH4 mole fraction and δ13C under the assumption that the atmospheric CH4 lifetime is constant, it is suggested that the temporal pause of the CH4 mole fraction observed at Ny-Ålesund is attributed to reductions of CH4 release from the microbial and fossil fuel sectors. On the other hand, the increase in CH4 after 2006 could be ascribed to an increase in microbial CH4 release. The CH4 and δ13C data presented in this paper would be useful for clarifying their temporal variations in the Arctic atmosphere, as well as providing additional constraints on the global CH4 budget.