Ocean Outgassing of Methyl Chloroform as an Underestimated Source of Emission

Abstract Methyl chloroform (MCF) is an ozone‐depleting substance used as an industrial solvent. Its primary sink is oxidation by the hydroxyl radical (OH), making it a key tracer for estimating atmospheric oxidative capacity. Following Montreal Protocol regulations, MCF emissions declined rapidly after the 1990s. However, the recent atmospheric MCF decay suggests persistent emissions and/or declining OH (contradicting chemistry‐climate models projecting increasing or stable OH). The air‐sea exchange of MCF has been poorly constrained due to limited observations and simplified ocean representations. We simulate oceanic MCF fluxes using a modern ocean reanalysis and validate with depth‐resolved observations. Results suggest the ocean has shifted from a net sink to a net source around 2005, outgassing 0.5 Gg yr −1 in the 2010s (up to 30% of inferred MCF emissions). This ocean outgassing is an order of magnitude larger than previous estimates, and accounts for up to a third of the model‐observation discrepancy in OH.