Spontaneous Oxidation of Thiols and Thioether at the Air-Water Interface of a Sea Spray Microdroplet

The transport of dissolved organic sulfur, including thiols and thioethers, from the ocean surface to the atmosphere through sea spray aerosol (SSA) is of great importance for the global sulfur cycle. Thiol/thioether in SSA undergoes rapid oxidation that is historically linked to photochemical processes. Here, we report the discovery of a non-photochemical, spontaneous path of thiol/thioether oxidation in SSA. Among 10 investigated naturally abundant thiol/thioether, seven species displayed rapid oxidation in SSA, with disulfide, sulfoxide, and sulfone comprising the major products. We suggest that such spontaneous oxidation of thiol/thioether was mainly fueled by thiol/thioether enrichment at the air-water interface and generation of highly reactive radicals by the loss of an electron from ions (e.g., glutathionyl radical produced from ionization of deprotonated glutathione) at or near the surface of the water microdroplet. Our work sheds light on a ubiquitous but previously overlooked pathway of thiol/thioether oxidation, which could contribute to an accelerated sulfur cycle as well as related metal transformation (e.g., mercury) at ocean-atmosphere interfaces.