Geocheminar spring-2016

Constructing an Atmospheric Methane Budget using 13CH3D and CH2D2

We develop a theoretical model using relative abundances and fractionations of 13CH3D and CH2D2, the doubly substituted mass-18 isotopologues of methane, to quantitatively track the sources and the sinks of atmospheric methane. The goal is a better determination of the methane budget in the atmosphere. Atmospheric methane has been increasing over recent centuries. From 1999 to 2006, however it approached a steady state, with emissions roughly in balance with the sum of sinks (Dlugokenchy et al., 2003; Khalil et al., 2007). Since 2007 a renewed increase in atmospheric CH4 is observed. The reasons for this evolution are not yet known.

Different methane sources have different isotope ratios because of variations in substrates, formation reactions, and temperatures. Isotope ratio measurements will provide useful constraints on source components and sink processes. However, bulk isotope ratios alone are unlikely to be diagnostic because of mixing of sources.

Using recently published budgets (Whiticar and Schaefer 2007) and estimates of equilibration temperatures of various methane sources (Stolper et al., 2014; Wang et al., 2015), along with ?13CH3D, and ?CH2D2 measured in biogenic methane sources in Ed Young’s lab using the new Panorama gas-source mass spectrometer, we estimate the source flux of singly- and doubly-substituted isotopologues to the air, in terms of both bulk ratios and deviations from the stochastic distributions of multiply-substituted species. The composition of atmospheric methane is also influenced by sink reactions. The main sink reactions with OH• and Cl• are modeled with first-principles transition state theory. At steady state conditions, our model predicts that the main sink reactions in the atmosphere generate a distinct signature of higher ?CH2D2 relative to the source composition, while at the same time increasing ?13CH3D and ?CH2D2. Finally in order to mimic recent changes in atmospheric methane concentration with the use of its rare isotopes as the tracker, we construct a one-box model with exercising different scenarios. For this purpose, initially a steady state is considered and then in our dynamic model we apply the non-steady state conditions by inducing changes in sources and sinks.