Influence of the Sea Surface Microlayer on Oceanic Iodine Emissions

The influence of organic compounds on iodine (I₂) emissions from the O₃ + I^- reaction at the sea surface was investigated in laboratory and modeling studies using artificial solutions, natural subsurface seawater (SSW), and, for the first time, samples of the surface microlayer (SML). Gas-phase I₂ was measured directly above the surface of liquid samples using broadband cavity enhanced absorption spectroscopy. I₂ emissions were consistently lower for artificial seawater (AS) than buffered potassium iodide (KI) solutions. Natural seawater samples showed the strongest reduction of I₂ emissions compared to artificial solutions with equivalent [I^-], and the reduction was more pronounced over SML than SSW. Emissions of volatile organic iodine (VOI) were highest from SML samples but remained a negligible fraction (<1%) of the total iodine flux. Therefore, reduced iodine emissions from natural seawater cannot be explained by chemical losses of I₂ or hypoiodous acid (HOI), leading to VOI. An interfacial model explains this reduction by increased solubility of the I₂ product in the organic-rich interfacial layer of seawater. Our results highlight the importance of using environmentally representative concentrations in studies of the O₃ + I^- reaction and demonstrate the influence the SML exerts on emissions of iodine and potentially other volatile species.