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

Measurements of δ 13 C 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 ∼ 6 · 10³ 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.