The 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 modelling 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 observed to be consistently lower for artificial seawater (AS) than buffered KI solutions. Natural seawater samples showed the strongest reduction of I2 emissions compared to equivalent [I-] artificial solutions, more pronounced over SML than over 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 seawater cannot be explained by chemical losses of I2 or HOI leading to VOI. An interfacial model explains this reduction by increased solubility of the product I2 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.