Light nonmethane hydrocarbons in seawater

Abstract

A database of dissolved C2‐C4 hydrocarbons in the surface water of the oceans is compiled based on more than 1000 measurements. Hydrocarbon emission rates are calculated using a diffusive microlayer approach and climatologic wind data. This database is used to calculate averages and ranges of variation, and an attempt is made to identify the environmental factors which have an impact on the hydrocarbons dissolved in seawater. The paper focuses on data obtained in situ since other techniques generally contain larger uncertainties. Mean concentrations are 134 pmol/L for ethene, 59 pmol/L for propene, and 37 pmol/L for 1‐butene. Alkane concentrations are lower with an average value of 22 pmol/L for ethane and less than 14 pmol/L for the other alkanes and acetylene. Ninety percent of the concentrations of an individual compound generally ranges within an order of magnitude. Ethene concentrations are significantly anticorrelated with the transfer velocities of the sea‐air exchange (r=−0.49; r0.01=0.29). Ethene concentrations are not correlated with the solar radiation, chlorophyll a, and the water temperature. Averaged emissions of C2‐C4 hydrocarbons extrapolated to the global ocean of 2.1 × 1012 g/yr are calculated, with ethene alone contributing about 40% to the total. Thus the oceanic source is on the low side of previous estimates and plays a minor role in global budgets compared to continental sources.