Abstract. The production mechanism of light alkenes, alkanes, and isoprene was investigated in laboratory experiments by measuring their concentrations in natural seawater as a function of spectral range, exposure time and origin, and concentration of dissolved organic carbon (DOC). The production mechanism of alkanes and of isoprene could not be clarified. Ethene and propene are produced photochemically from DOC. The relevant spectral range is UV and short-wavelength visible light. Initial production rates (up to day 10 of exposure) were in the range of several pmol L -• h -• (mg DOC)-i; the corresponding mean quantum yields for the spectral range of 300-420 nm were about 10 -s. Generally, the production rates and the quantum yields for ethene were about 2 times that of propene. The key factors in the total column integrated oceanic alkene production are the solar photon flux at sea surface, the penetration depth of the light into the ocean (especially the relation between different light absorbers, i.e., the extinction due to absorption of DOC), and the wavelength-and DOC-dependent quantum yields. As a result of the high variability of these parameters, actual local alkene production rates for a specific oceanic region may differ considerably from the globally averaged oceanic alkene production rates. The latter were estimated to be at most 5 Mt yr -•. We carried out several experiments in the laboratory and studied the formation of NMHC, including isoprene, in natural seawater. We investigated several parameters affecting the NMHC production, especially the wavelength dependence and the role of DOC. We will focus on C2-to C3-hydrocarbons, which represent more than 80% of total NMHC emissions from the oceans [Plass-Dalmer et al., 1995].

The production mechanism of light nonmethane hydrocarbons (NMHC) in seawater was investigated during the North Atlantic atmospheric chemistry program (NATAC) in April and May 1991 in the European coastal seas and the North Atlantic. A significant alkene production occurred in the presence of light only. Under conditions of negligible NMHC emissions (low wind velocity) increasing hydrocarbon concentrations were observed during daytime, whereas the concentrations remained constant during night. NMHC formation experiments were carried out with seawater filled in quartz glass bottles and showed the same dependence of light. Experiments with differently pretreated seawater samples indicated that the presence of dissolved organic material (DOM) is also necessary for alkene production. We suggest a two‐step production mechanism for alkenes: first DOM is released, probably from algae, then part of this material is photochemically transformed into alkenes. The production rates in the quartz glass bottles were comparable to the production rates in the ocean surface. This indicates that the processes occurring in the experimental setups represent the processes occurring in the field. Since the production ‐ and emission rates were in the same range it can be concluded that the budget of light alkenes in the remote marine environment is determined by the production in seawater as the dominant source and the flux into the atmosphere as the main loss process.

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.

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