Global stratospheric measurements of the isotopologues of methane from the Atmospheric Chemistry Experiment Fourier transform spectrometer

Abstract. This paper presents an analysis of observations of methane and its two major isotopologues, CH₃D and ¹³CH₄, from the Atmospheric Chemistry Experiment (ACE) satellite between 2004 and 2013. Additionally, atmospheric methane chemistry is modeled using the Whole Atmospheric Community Climate Model (WACCM). ACE retrievals of methane extend from 6 km for all isotopologues to 75 km for ¹²CH₄, 35 km for CH₃D, and 50 km for ¹³CH₄. While total methane concentrations retrieved from ACE agree well with the model, values of δD–CH₄ and δ¹³C–CH₄ show a bias toward higher δ compared to the model and balloon-based measurements. Errors in spectroscopic constants used during the retrieval process are the primary source of this disagreement. Calibrating δD and δ¹³C from ACE using WACCM in the troposphere gives improved agreement in δD in the stratosphere with the balloon measurements, but values of δ¹³C still disagree. A model analysis of methane's atmospheric sinks is also performed.