Besides turbulence, another important factor affecting the gas transfer across the air-water interface is the cleanliness of the interface. KL varies depending on the surface conditions because the damping effect of the turbulent velocity fluctuations by the surface is different between a clean and a dirty gas-liquid interface. A clean surface allows horizontal turbulence velocity fluctuations to persist up to the actual liquid surface because no large shear stresses are created at the interface. However, the vertical turbulence velocity fluctuations are damped due to the surface tension and gravity. When a gas-liquid interface is covered, for example by organic matters or chemical surfactants, greater tangential stress occurs and that increases the damping of the turbulence fluctuations near the interface. For severely contaminated interfaces, the hydrodynamical relations would be similar to those for solid-liquid mass exchange. Experimental results showed that the transfer velocity is indeed very sensitive to the cleanliness of the interface. The reaeration rate across a surface covered with monolayers ('very dirty surface') may be up to 6 times slower when compared to clean surface conditions. Moog and Jirka, for example, noted that their proposed equation (eqn ) may be of little help when nonhydraulic influences, such as methy-lene-blue-active substances (MBASs), must be taken into account. They recommended to perform MBAS measurements, especially for streams with slopes less than 0.002.
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