Emissions of Light Nonmethane Hydrocarbons from the Atlantic into the Atmosphere

During two Atlantic cruises of the German research vessel Polarstern, 1988 and 1989, the concentrations of light nonmethane hydrocarbons (NMHC) in seawater were measured. On the basis of a simple budget analysis, the oceanic mixed layer represents a NMHC reservoir with an internal production and a major loss by emission into the atmosphere. As a consequence, the concentrations of NMHC depend on the rates of ocean-atmosphere exchange: high exchange rates reduce the concentrations and vice versa. With the prevailing transfer velocities the emission rates were calculated according to ocean-atmosphere exchange models. The regional averages of the alkene emission rates vary by 1 order of magnitude. For ethene the maximum value was 5 × 108 molecules cm−2 s−1. The emissions of the various alkanes were generally below 1 × 108 molecules cm−2s−1. The total C2-C4 hydrocarbon emissions during both cruises average 6 × 108 molecules cm−2 s−1, 70 % of which are alkene emissions, with ethene alone contributing 42 % to the total. No indications for enhanced emissions of NMHC at high phytoplankton concentrations or in the proximity to coastlines were observed. Thus we regard the emissions as representative for the mid-Atlantic and the season of the investigations, August to October. The calculated emission rates of the shortlived alkenes are validated by comparison with atmospheric measurements of NMHC. The emission rates are substantially lower than the majority of reported oceanic emission estimates by up to about 2 orders of magnitude.