No abstract

New data obtained in the Mediterranean Sea and Pacific Ocean show that isoprene could be produced in sea water by biological processes, leading to concentrations in the ppb range (10−9 liter of gas per liter of water). Taking into account Henry's constant for isoprene in water and the very low concentrations measured in the marine atmosphere, the superficial sea water is supersaturated in isoprene by one or two orders of magnitude. From these observations, an oceanic flux of the order of 1.2 Mt per year of isoprene can be estimated. This is a small value, as compared with the marine fluxes of the other NMHC; it is also practically negligible in the global burden of isoprene. However, because of its short lifetime in air, isoprene in remote marine atmosphere, entirely originates from superficial seawater, it could be therefore an useful tracer of marine emissions of gaseous compounds.

Gaseous Rn-222, a daughter product of U-238, is injected into the atmosphere from the surface of continents. Its atmospheric cycle is particularly simple since it disappears only by radioactive decay (half-life 3.8 days). Radon measurements obtained over more than 15 years in remote stations in Antarctic and Subantarctic areas generally give concentrations as low as 0. I to 2 pCi.m-'. However, we have shown the existence of sharp increases of concentrations reaching 3 to 30 pCi'rn-' called "radonic storms". Owing to the negligible degassing of radon from the sea surface, such peaks are accounted for by long-range transport from remote continents (mainly South Africa) over Southern Indian and Antarctic oceans, with transit times ranging from 1.5 to 7 days and very low dilution factors of the order of 3 to 7. This airmass transport is related to warm sectors of cyclonic systems passing over South Africa and around the Antarctic continent.