Velocity gas transfer

The piston velocity, or gas transfer velocity, is a function of wind speed. There are large differences in the relationships between piston velocity and wind speed, especially at liigher wind speeds (e.g., Liss and Merlivat, 1986 Wannin-khof, 1992). This is the limiting factor for these calculations. 

Figure 2 Variation of the gas transfer velocity with wind speed. The units of transfer velocity are equivalent to the number of cm of the overlying air column entering the water per hour (Taken from Bigg,28 with permission of Cambridge University Press)...

Table 5.5 Average gas transfer velocities corrected to a Schmidt number of 600 ( goo) for rivers and estuaries. 

Borges, A.V., Delille, B., Schiettecatte, L., Gazeau, F., Abril, G, and Frankignoulle, M. (2004) Gas transfer velocities of CO2 in three European estuaries (Randers Fjord, Scheldt, and Thames). Limnol. Oceanogr. 49, 1630-1641. 

Crusius, J., and Wanninkhof, R. (2003) Gas transfer velocities measured at low wind speed over a lake. Limnol. Oceanogr. 48, 1010-1017. 

Zappa, C J., Raymond, P.A., Terray, E.A., and McGillis, W.R. (2003) Variation in surface turbulence and the gas transfer velocity over a tidal cycle in a macro-tidal estuary. Estuaries 26, 1401-1415. 

Asher W., Wang Q., Monahan E. C., and Smith P. M. (1998) Estimation of air-sea gas transfer velocities from apparent microwave brightness temperature. Mar. Tech. Soc. J. 32, 32-40. 

Glover D. M., Frew N. M., McCue S. J., and BockE. J. (2002) A multi-year rime series of global gas transfer velocity from the TOPEX dual frequency, normalised radar backscatter algorithm. In Gas Transfer at Water Surfaces, Geophysical Monographs (eds. M. A. Donelan, W. M. Drennan, E. S. Saltzman, and R. Wanninkhof). AGU, Washington, DC, vol. 127, pp. 325-333. 

The first term on the right side represents the interface exchange that is the product of a gas transfer velocity, G, and the concentration difference between that measured in the ocean s surface, [A ], and that expected at saturation equihhrium,... 

The gas transfer velocity is proportional to wind speed and can be estimated if this is known. The bubble terms, however, must be determined from a model of bubble processes (explained in Chapter 10) and the concentrations of the inert gases Ar, N2 and Ne. 

Traditional turbulence-diffusion models (based on the boundary layer adjoining a solid wall) imply that n = 2/3, but a value n = 1/2 is appropriate for a boundary layer adjoining a free surface (Jahne and Haussecker 1998). The appropriate value of n depends on the wind stress and the surfactant loading of the surface. Soloviev and Schluessel (1994) have described a procedure for estimating gas transfer velocities from measurements of heat transport, assuming that transport is adequately described by the classical (Danckwerts) surface renewal model. The key relationship can be written in the form ... 

In summary, large footprint measurements permit an estimate of gas transfer velocities, but these estimates are sensitive to model assumptions. Furthermore, large footprint measurements give no direct information on the characteristics of surface renewal. Also, the method is extremely sensitive to drift in the calibration of either the surface or the bulk temperature measurement. 

Atmane MA, Asher WE, Jessup AT (2004) On the use of the active infrared technique to infer heat and gas transfer velocities at the air-water free surface. J GeophysRes 109 C08S14... 

Schmidt number scaling of the heat transfer velocities according to Eq. 1.2 suggests nearly an order of magnitude reduction in the gas transfer velocity due to the slick (5.1 cm h 1 and 0.64 cm h"1 outside and inside the slick respectively) at a wind speed of 4 m s 1. 

Studies of the air-water gas exchange process by infrared imaging techniques enable local and fast measurements of the heat transfer velocity. Applying Schmidt number scaling, the obtained gas transfer velocities are consistent with mass balance methods and are in good agreement for the... 

Using heat as a proxy tracer for gases not only delivers the transfer coefficient at high spatial and temporal resolution but also gives direct insight into the spatial structure of near surface turbulence enabling detailed investigations of the influence of surface chemical enrichments on the gas transfer velocity and the turbulent transport mechanisms. 

Figure 15. Relationship between gas transfer velocity k (normalized to a Schmidt number of 600) and wind speed u (normalized to 10-m height). Data from SFs/ He experiment (circles) are shown in comparison to the relationships proposed by Liss and Merlivat (1986, solid line), Wanninkhof (1992, dotted) and Smethie et al. (1985, dashed). From Wanninkhof et al. (1993).

For this second relation, dual tracer experimenters have used a power law dependence of gas transfer velocities on Schmidt number (the ratio of kinematic viscosity of water to the diffusivity of the gas) ... 

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