PRODUCTION MECHANISM OF C2-C4 HYDROCARBONS IN SEAWATER - FIELD-MEASUREMENTS AND EXPERIMENTS

The production mechanism of light nonmethane hydrocarbons (NMHC) in se awater was investigated during the North Atlantic atmospheric chemistr y program (NATAC) in April and May 1991 in the European coastal seas a nd 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 observ ed during daytime, whereas the concentrations remained constant during night. NMHC formation experiments were carried out with seawater fill ed in quartz glass bottles and showed the same dependence of light. Ex periments 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 alk enes: first DOM is released, probably from algae, then part of this ma terial is photochemically transformed into alkenes. The production rat es 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 domina nt source and the flux into the atmosphere as the main loss process.