Photochemical Formation of Hydroxyl Radical, Hydrogen Peroxide and Fe(II) in the Sea Surface Microlayer (SML) Collected in Okinawa, Japan

Abstract:

The sea surface microlayer (SML) covers upper 1 to 1000 µm deep boundary layer of the ocean, where important biogeochemical processes take place. Photo-chemical reactions are activated by sunlight, so it is assumed that more photo-chemical reactions occur in SML than underlying bulk seawater (bulk). We initiated a study to understand chemical changes occurring in the SML by studying photochemical formation of oxidants such as hydrogen peroxide and hydroxyl radical (OH), both of which react with various organic substances and determine their lifetimes. Since OH can be formed by reaction between hydrogen peroxide and Fe(II), Fe(II) photoformation was also studied. We collected SML samples using a widely-used glass plate method and bulk samples by using a polyethylene bottles near the coast of Okinawa Island, Japan. Results showed that dissolved organic carbon (DOC) concentrations in the SML were about twice those of bulk seawater samples. Hydrogen peroxide formation in the SML samples was ca. 2.8 times faster than the bulk seawater samples. On the other hand, Fe(II) and OH photoformation kinetics was similar for both SML and bulk samples. Although it was predicted that more OH could be formed from reaction between hydrogen peroxide and Fe(II), OH formation kinetics was similar in both SML and bulk, suggesting that either Fe(II) did not react with hydrogen peroxide or reaction is very slow, possibly by forming a complex with organic compounds in the SML and bulk.