Modeling the variation of delta C-13 in atmospheric methane: Phase ellipses and the kinetic isotope effect

We use the TM2 three-dimensional atmospheric tracer model with a methane so urce-sink budget based on existing literature to simulate small spatial and temporal variations in the C-13/C-12 ratio Of atmospheric methane. The res ults show that delta C-13 varies markedly with wind direction everywhere ou tside the extratropical Southern Hemisphere (ETSH). Within the ETSH, both m ethane mixing ratio and delta C-13 have regular seasonal cycles with differ ing and latitude-dependent phases. Phase diagrams constructed from these se asonal cycles, showing changes in delta C-13 versus changes in mixing ratio , have elliptical shapes. The slope of the major axis of these ellipses is determined by the kinetic isotope effect (KIE) of the single atmospheric me thane removal process used in the model. The ellipse eccentricity is determ ined by seasonal variation in the source delta (CH4)-C-13, which is dominat ed by the biomass burning source because of its isotopic enrichment relativ e to other sources. Comparison of the model results, for a KIE based on CH4 + OH oxidation, with observations in the South Pacific region shows signif icant discrepancies in both the ellipse major axis slopes and eccentricitie s. We suggest that this is an indicator of an additional sink process that discriminates strongly against (CH4)-C-13. Such a,Sink could be active chlo rine in the marine boundary layer.