Interface microlayer

Because of their hydrophobic nature, siUcones entering the aquatic environment should be significantly absorbed by sediment or migrate to the air—water interface. SiUcones have been measured in the aqueous surface microlayer at two estuarian locations and found to be comparable to levels measured in bulk (505). Volatile surface siloxanes become airborne by evaporation, and higher molecular weight species are dispersed as aerosols. 

An improved CHF model for low-quality flow The Weisman-Pei model was later improved by employing a mechanistic CHF model developed by Lee and Mu-dawwar (1988) based on the Helmholtz instability at the microlayer-vapor interface as a trigger condition for microlayer dryout (Fig. 5.21). The CHF can be expressed by the following equation due to the energy conservation of the microlayer (Lin et al. 1989) ... 

DOM also tends to be concentrated at boimdaries, such as the sediment-water and air-sea interfeces. The latter is also characterized by enrichments of POM, such as bacteria. Under calm conditions, the DOM and POM that collects at the air-sea interface forms a visible surfece slick or microlayer. On windy days, this organic matter can be whipped up into an emulsion that has the appearance of a very sturdy foam. DOM can also be transferred into the POM pool by adsorbing onto organic particles. 

Implicit in this model is the assumption that molecular diffusivity and Henry s Law constant are directly and inversely proportional, respectively, to the gas flux across the atmosphere-water interface. Molecular diffusion coefficients typically range from 1 x 10-5 to 4 x 10-5 cm2 s-1 and typically increase with temperature and decreasing molecular weight (table 5.3). Other factors such as thickness of the thin layer and wind also have important effects on gas flux. For example, wind creates shear that results in a decrease in the thickness of the thin layer. The sea surface microlayer has been shown to consist of films 50-100 pm in thickness (Libes, 1992). Other work has referred to this layer as the mass boundary layer (MBL) where a similar range of film thicknesses has been... 

Surface microlayer layer at the air-water interface that is composed of films 50 to 100 pm in thickness. 

Two of the key assumptions of the thin-film model (see Section 6.03.2.1.1) are that the main bodies of air and water are well mixed, i.e., that the concentration of gas at the interface between the thin film and the bulk fluid is the same as in the bulk fluid itself, and that any production or removal processes in the thin film are slow compared to transport across it. It is quite likely that there are near-surface gradients in concentrations of many photochemically active gases. Little research has been published, although the presence of near-surface gradients (10 cm to 2.5 m) in levels of CO during the summer in the Scheldt estuary has been reported (Law et al., 2002). Gradients may well exist for other compounds either produced or removed photochemically, e.g., di-iodomethane, nitric oxide, or carbonyl sulfide (COS). Hence, a key assumption made in most flux calculations that concentrations determined from a typical sampling depth of 4-8 m are the same as immediately below the microlayer may well often be incorrect. 

R. Conrad, W. Seiler (1988). Influence of the surface microlayer on the flux of nonconservative trace gases (CO, H2, CH4, N2O) across the ocean-atmosphere interface. J. Atmos. Chem., 6, 83-94. 

Fig. 5 (cont.). Visualisation of vortex ring interaction with various interfaces. Panel (c) is that for a stearic acid microlayer having concentration 3.5 x 10 10 mol cm 2 Taken from McKenna (1997)... 

Fig. 6. Variation of the near-surface radial component of velocity. Panel (a) is for a clean interface panel (b) is for the stearic acid microlayer. All variables are as defined in Fig. 5, u is the radial component of velocity, and U is the vertical free propagation speed of the vortex ring. Velocities are taken along a horizontal section at a depth d/D = 0.03 (0.125 cm). The temporal spacing between curves, At, is 0.15. Taken from McKenna (1997)...

Carlson DJ, Cantey LL, Cullen JJ (1988) Description of and results from a new surface microlayer sampling device. Deep-Sea Res 35 1205-1213 Cini R, Lombardini PP (1978) Damping effect of monolayers on surface wave motion in a liquid. J Colloid Interface Sci 65 387-389 Csanady GT (1990) The role of breaking wavelets in air-sea gas transfer. J Geo-phys Res 95 749-759... 

It is evident that many of the sampling devices described in the previous section in fact collect thin layers of water adjacent to and including the air/ sea interface itself. These type of surface film samples have become known as surface microlayers (Liss, 1975). Although such layers are in fact operationally defined by the devices used for their collection, there is the understanding that something happens to the properties of ordinary, bulk seawater at and near the air/sea interface which is contained in such microlayer samples. In this way, the microlayer becomes a real phenomenon in much the same way that the particulate state, understandable in a common-sense sort of way, is also usually defined by operational methods such as membrane filtration. 

Thus, the presence of a discontinuity in properties near or at the air/sea interface can be inferred from a difference in properties between the bulk seawater and the microlayer sample which contains, the interfacial region. For an organic surfactant adsorbed at or near the sea surface, for example, this will manifest itself by a higher concentration or enrichment in the microlayer sample. There is, nevertheless, the vexed question of how much of a screen microlayer sample (of typical thickness —200 pm) consists of a real concentration anomaly near the interface and how much consists of seawater of bulk composition. Since there have been a number of differing and intractable opinions voiced in this regard, it is timely to examine closely what can be said, in terms of known surface chemistry, about the structure of the microlayer. 

To some authors, it seems to be apparent that the microlayer consists of a surface monolayer of adsorbed organic matter of thickness —20 A diluted by a vast excess, 10 —10 times as much, subsurface seawater. It should be noted that a monolayer thickness of this magnitude applies mostly to mono-layers of simple surfactants such as fatty lipids. Water-soluble surfactants of the wet variety (MacIntyre, 1974) can form monolayer films of much greater thickness, with hydrophobic parts of the molecule attached to the interface and hydrophilic parts extending by as much as —1 pm into the aqueous phase. The results of Baler et al. (1974), to be discussed shortly, show that films in a dry state on germanium prisms used for the Blodgett (1934, 1935) type of sampling method have thicknesses determined by ellip-... 

Many of the photochemically active species present at the sea surface will be transported into the atmosphere in marine aerosols via the sea surface interface. Blough recently reviewed the photochemistry of the sea surface microlayer and the compounds which reside in the thin layer of material at the sea surface... 

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