Surface microlayers, chemical

Fig. 2. Variations in surface microlayer chemical composition as shown by DEI-MS spectra for three microlayers (SepPak eluates) collected along a cruise track from Delaware Bay south-eastward across the continental shelf. Spectral intensities are normalised to the most abundant m/z intensity. The spectrum of the sample in the upper panel, which was collected within the Delaware River plume, is dominated by the presence of humic compounds that characteristically pyrolyse to produce mass fragments at every m/z value. The contribution from humic materials decreases markedly with distance from the estuary...

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. 

Selection of the environmental matrices to be analysed should take into consideration both the general objective of the monitoring and also the opportunity to study the distribution within and the transport between the most important environmental components with respect to the observed chemical substances. Thus a suggested list of important matrices is as follows sea water (with particular reference to the surface microlayer), marine particulate and sediments, marine ice, marine organisms (especially krill), aerosols, superficial snow and soil and... 

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. 

Liss, P.S., 1975. Chemistry of the sea surface microlayer. In J.P. Riley and G. Skirrow (Editors), Chemical Oceanography, 2. Academic Press, London, pp. 193—243. 

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. 

The sea-surface microlayer not only enriches more chemicals in this interface but also displays more distinct biological activity relative to the underlying water. As a biological sulfur compound, the source and sink of DMS in the... 

N.M. Frew and R.K. Nelson, Scaling of marine microlayer film surface pressure-area isotherms using chemical attributes, J. Geophys. Res. 97 (1992) 5291-5300. 

Frew, NM, Nelson RK (1992a) Isolation of marine microlayer film surfactants for ex situ study of their surface physical and chemical properties. J Geophys Res 97 5281-5290... 

At the upper ocean microlayer and at the boundary layer confining the air bubbles, among other various physical and chemical effects, organic materials significantly alter the surface tension of water, y. In slick covered sea surfaces, depressions of y exceeding 20 mN m"1 have been observed. 

In aquatic environments, organotin concentrations were elevated in sediments, biota, and surface water microlayers collected near marinas, aquaculture rearing pens, and other facilities where organotin-based antifouling paints were used. In some cases, organotin concentrations in the water column were sufficiently high to pose a substantial risk to sensitive species. Data are limited on concentrations of organotins in environmental samples, especially in samples from terrestrial ecosystems, and this may be attributed, in part, to limitations in routine chemical analytical capabilities. 

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