MEASUREMENT OF VERTICAL-DISTRIBUTION OF ISOPRENE IN SURFACE SEAWATER,ITS CHEMICAL FATE, AND ITS EMISSION FROM SEVERAL PHYTOPLANKTON MONOCULTURES

Concurrent measurements of isoprene (2-methyl-1,3-butadiene) in seawat er together with atmospheric concentrations in the Gulf Stream off the Florida coast were made. Florida Straits surface water concentrations of isoprene varied between 9.8 and 50.8 pmol l(-1). Surface water iso prene concentrations showed a positive correlation with measured chlor ophyll fluorescence, consistent with the biogenic origin of this non-m ethane hydrocarbon. Depth profiles showed a maximum in isoprene concen tration similar to that of biological productivity as indicated by chl orophyll fluorescence. Daytime atmospheric mixing ratios of isoprene w ere never above 11 parts-per-trillion (pptv) and eight of eleven sampl es taken were below the similar to 5 pptv detection limit. Using an es timated value of the Henry's law constant (K-H similar to 3.1) a super saturation of surface seawater of 2 to 3 orders of magnitude is estima ted. Laboratory grown monocultures of several common phytoplankton spe cies showed production of isoprene. Processes controlling the isoprene concentration in the photic zone of the ocean and the marine boundary layer are discussed. A calculation using a simple time dependent phot ochemical box model confirmed that isoprene is rapidly consumed by its reaction with the OH radical in the marine atmosphere.