Volatile organic compounds emission from light-induced reactions at the sea surface microlayer Raluca Ciuraru, François Bernard, Ludovic Fine, Christian George CNRS-IRCELYON, Université Lyon 1, 69629 Villeurbanne, France The sea surface microlayer is the organic-enriched layer present at the air-sea interface which has different physical and chemical properties compared to subsurface waters1. The chemical analysis of this microlayer is of great interest for many reasons including its major influence to reduce air–sea gas exchange. Surface seawater contains a variety of substances which act as photosensitizers. They include components of the dissolved organic matter known also as humic acids. The sea surface microlayer is the primary recipient of the solar energy and since it is enriched in chemicals and biota, a number of processes are likely to be more effective here than in the water column2. The focus of this study is to determine if the organic film acts as a hydrophobic barrier for the air-sea gas exchange and to identify and characterize the volatile organic compounds emissions due to the photochemical processing of the sea surface microlayer. Synthetic salt solutions containing a photosensitizer (humic acids) and an organic surfactant (nonanoic acid) have been irradiated by a Xe lamp, the VOCs being further identified and analyzed by a High Resolution PTR-ToFMS. It has been observed that the presence of a thick organic film on the salt solutions reduces the transfer from the aqueous solution to the gas phase. The formation of certain saturated and unsaturated aldehydes, acids and a series of alkenes and dienes have been observed. All these compounds were confirmed by GC/MS analysis. An isoprene formation was also observed under irradiation. The isoprene is formed only in the presence of the organic surfactant with the need for the photosensitizer. The dependence of the isoprene concentration with the surfactant concentration and its surface tension is shown and discussed. 1. Donaldson, D. J. & George, C., Environ. Sci. Technol. 46, 10385–10389, 2012. 2. Liss, P. S. & Duce, R. A. Cambridge University Press, Cambridge, 1997.