Pj. Milne et al., MEASUREMENT OF VERTICAL-DISTRIBUTION OF ISOPRENE IN SURFACE SEAWATER,ITS CHEMICAL FATE, AND ITS EMISSION FROM SEVERAL PHYTOPLANKTON MONOCULTURES, Marine chemistry, 48(3-4), 1995, pp. 237-244
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.