C~-C 6 Nonmethane hydrocarbon (NMHC) concentrations in the atmospheric boundary layer and in surface seawater were simultaneously measured during an oceanographic cruise in the intertropical Indian Ocean. NMHC were found to be mainly C~-C 4 alkenes and C2-C 3 alkanes. Their concentrations ranged from 1 to 30ร 10 -9 1/1 in the seawater and 0.1 to 15 ppbv in the atmosphere. Seawater appeared to be a source because the C2-C 6 NMHC were supersaturated with respect to the atmosphere by 2 or 3 orders of magnitude.
After a selection of the pure marine atmospheric samples, performed with the help of stable and radioactive continental tracers, we found an identical composition in NMHC of surface air and seawater. This observation enabled us to establish that the gas transfer between sea and air occurred according to nonsteady state processes, and that the fluxes cannot be deduced only from atmospheric measurements. An order of magnitude value of the oceanic source for the different NMHC is however derived from the comparison of their sea water concentrations to that of propane and an independent evaluation of the marine source of this last compound. Key words. Nonmethane hydrocarbons, sea-air exchanges. ยฉ * If ko H and ko~ are the kinetic constants of oxidation of the NMHC by OH and 03; it is given by: it = kOH.(OH)+ ko3.(O3) , where (OH) and (O3) are the atmospheric concentrations of OH radicals and ozone.