ISAMO (Iron Salt Atmospheric Methane Oxidation)

Methane is a well-mixed greenhouse gas responsible for >1/3 of global warming since pre-industrial times whose atmospheric burden continues to increase with a new record set in 2022. Active chlorine in the atmosphere is poorly constrained and so is its role in the oxidation of methane. This uncertainty propagates into methane source budgets through isotope-constrained top-down models, in which the observed abundance of ¹³C in tropospheric methane (commonly expressed as δ¹³C-CH₄) is used to constrain the sources of methane using their characteristic δ¹³C-CH₄ values. These models need to account for the change in the observed δ¹³C-CH₄ by the Cl and OH sinks, which shift the observed isotope towards higher δ¹³C-CH₄ values of fossil fuel sources, and away from ¹³C depleted biological sources. The ISAMO project focuses on the hypothesis that Cl atoms are produced naturally by the action of sunlight on particles containing iron and chloride and these chlorine atoms oxidize atmospheric methane. To study this, we use the sensitive and selective indirect quantification of the concentration of atomic Cl through the strong carbon kinetic isotope effect (KIE) in the CH₄ + Cl reaction, which leaves the remaining CH₄ enriched in ¹³C, and producing extremely ¹³C-depleted CO. We will present field and laboratory observations and global modelling, including CO isotope measurement from flasks samples across the North Atlantic. We show how this mechanism affects ¹³C depletion in atmospheric CO and how the corresponding ¹³C enrichment in CH₄ affects global methane emission estimates.