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 seawater 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 1⁻¹. Surface water isoprene concentrations showed a positive correlation with measured chlorophyll fluorescence, consistent with the biogenic origin of this non-methane hydrocarbon. Depth profiles showed a maximum in isoprene concentration similar to that of biological productivity as indicated by chlorophyll fluorescence. Daytime atmospheric mixing ratios of isoprene were never above 11 parts-per-trillion (pptv) and eight of eleven samples taken were below the ˜ 5 pptv detection limit. Using an estimated value of the Henry's law constant (K_H ˜ 3.1) a supersaturation of surface seawater of 2 to 3 orders of magnitude is estimated.Laboratory grown monocultures of several common phytoplankton species 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 photochemical box model confirmed that isoprene is rapidly consumed by its reaction with the OH radical in the marine atmosphere.