PHOTOCHEMICAL ALKENE FORMATION IN SEAWATER FROM DISSOLVED ORGANIC-CARBON - RESULTS FROM LABORATORY EXPERIMENTS

The production mechanism of light alkenes, alkanes, and Isoprene was i nvestigated in laboratory experiments by measuring their concentration s in natural seawater as a function of spectral range, exposure time a nd origin, and concentration of dissolved organic carbon (DOG). The pr oduction mechanism of alkanes and of isoprene could not be clarified. Ethene and propene are produced photochemically from DOG. The relevant spectral range is UV and short-wavelength visible light. Initial prod uction rates (up to day 10 of exposure) were in the range of several p mol L-1 h(-1) (mg DOC)(-1); the corresponding mean quantum yields for the spectral range of 300-420 nm were about 10(-8). Generally, the pro duction rates and the quantum yields for ethene were about 2 times tha t of propene. The key factors in the total column integrated oceanic a lkene production are the solar photon flux at sea surface, the penetra tion depth of the light into the ocean (especially the relation betwee n different light absorbers, i.e., the extinction due to absorption of DOG), and the wavelength-and DOG-dependent quantum yields. As a resul t of the high variability of these parameters, actual local alkene pro duction rates for a specific oceanic region may differ considerably fr om the globally averaged oceanic alkene production rates. The latter w ere estimated to be at most 5 Mt yr(-1).