Active chlorine in the remote marine boundary layer: Modeling anomalous measurements of δ13C in methane

Measurements of delta C-13 in methane in the marine boundary layer (MBL) of the extratropical Southern Hemisphere imply a kinetic isotope fractionation much larger than would be expected if the hydroxyl radical were the only tropospheric methane sink. We use a simple chemical box model to show that the assumption of a MBL active chlorine (Cl-.) sink can explain these anomalous observations provided there is a seasonal cycle in the Cl-. sink with a summer-winter concentration difference similar to 6 . 10(3) cm(-3). The required summer maximum and yearly mean Cl-. concentrations are plausible, and imply a global Cl-. sink strength for methane of < 5 Tg yr(-1). Choice of a Cl-. sink seasonal cycle linked to the nonsinusoidal dimethyl sulfide seasonal cycle gives the observed fractionation with a smaller yearly mean Cl-. concentration than equivalent sinusoidal Cl-. cycles.