Surface microlayers, chemical processes

There are very few measurements of the DMS concentration in the sea surface microlayer. The first report (44) indicated an enrichment of 5 times relative to underlying water samples. Other reports indicate that no enrichment was observed in tne microlayer DMS concentration (23.47.511. These differences may be related to the sampling techniques used. It is possible that chemical and biological processes in tne sea surface microlayer may affect the transfer of DMS from the bulk ocean to the atmosphere. However, at present, very little is known about the processes affecting the chemistry of DMS in the microlayer. 

This chapter considers the oxidation of iodide in seawater by natural oxidants (02, H202, and 03). The oxidation of iodide to iodate is considered slow, yet the six-electron T-IOj redox couple normally used to represent the process (or predict stability) is thermodynamically favorable (2). We will discuss both one- and two-electron-transfer processes with these oxidants, focusing on the first step of electron transfer and using the frontier molecular orbital theory approach in conjunction with available thermodynamic and kinetic data. The analysis shows that the chemical oxidation of I to I03 is not a very important process in seawater, except perhaps at the surface microlayer. 

I does not appear to be oxidized to I03 to any significant degree in the ohotic zone or the surface microlayer by chemical reactions. However, iodide oxidation may occur by biological mechanisms as found with macroalgae (3). Kennedy and Elderfield (36, 37) provide evidence for iodide oxidation (presumably bacterial) in sediments, but biologically mediated oxidation has not been shown yet as a significant process in the photic zone. 

Surface microlayers are Implicated in many chemical processes. They are exposed to the full solar spectrum of light arriving at the surface, and are often Inqiortant In photochemlcally mediated reactions. The microlayer, associated with most natural waters. Is considerably different In chemical composition from the underlying water column (26). Hence, the photochemlcally mediated reactions that take place In this layer may differ substantially from those In the water column. The differences In the reactions may be one of kinetics (rates of the reaction), or maybe mechanistic In nature and the reactlon(s) proceed(s) via different pathways resulting In different reaction products. 

TWO types of physical loss processes should be considered In the external removal of a chemical species. First, the species of Interest may be lost to the atmosphere through water-air exchange. This process depends on the Henry s Law constant for the chemical species, as well as the atmospheric concentration and the structure of the surface microlayers (. Wind stress and turbulence of the water body surface have a pronounced effect, especially for surface-active materials for which bubble scavenging and surface film ejection as aerosol takes place. Transfer rates at the air-water interface are complex problems In themselves and are not dealt with In this Chapter. 

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