Photochemical production and air-sea exchange of carbonyl sulfide in the eastern Mediterranean Sea

During the EGAMES (Evasion of GAses from the MEditerranean Sea) expedition in the Aegean and eastern Ionian Sea in July 1993, we measured atmospheric and sea surface concentrations of carbonyl sulfide (COS). The surface waters were found to be supersaturated with respect to the atmosphere at all times with an average saturation ratio of 2.6 in open ocean areas and 5.2 in coastal areas. Using air-sea exchange coefficients based on the empirical model by Liss and Merlivat (1986), we estimated an average COS flux of 55 nmol m−2 day−1 from the ocean into the atmosphere. Although the supersaturation of COS was twice as high in coastal areas, during our cruise the flux was not different from open ocean areas because of extremely low windspeeds encountered at coastal areas. We observed a clear diel cycle for coastal area averages and open ocean averages of COS. This cycle could be described using an empirical model including terms for photoproduction, dark production, hydrolysis and air-sea exchange. The in situ COS photoproduction rates were found to be 0.12 fmol l−1 s−1 W−1 m2 for coastal areas and 0.03 fmol 1−1 s−1 W−1 m2 for open ocean areas. After correction to surface light intensities, the coastal and open ocean photoproduction rates were 0.99 and 0.08 fmol l−1 s−1 W−1 m2. Our model provides a satisfactory fit to the open ocean field measurements only if a dark production term is included. The dark production rates were 0.38 and 0.55 fmol l−1 s−1, for coastal and open ocean areas, respectively. From incubation experiments with open ocean waters carried out on board the R.V. Aegaio we found average photoproduction rates of 0.09 fmol l−1 s−1 W−1 m2 and dark production rates of 0.42 fmol 1−1 s−1, which are very close to the values found in sea surface open ocean waters. Finally, we observed photo decomposition of methyl mercaptane (MeSH) at a rate between 0.41 and 1.7 fmol l−1 s−1 W−1 m2 suggesting MeSH as a potential precursor of COS.