|The mechanisms that control the mass transfer of gases across the air-water interface are not well understood. Current models for air and water gas transfer have limited effectiveness because they are based on large scale bulk conditions and empirical estimates, yet the mechanisms that control the transfer are small scale. The Surface Laser Induced Fluorescence (SLIF) technique was developed and it provided measurement of small scale variation in mass transfer across the surface of the liquid. The SLIF technique was applied to study the relationship between atmospheric humidity and oxygen mass transfer across the air-water interface.
The Surface Laser Induced Fluorescence technique is based on the premise that variation of surface-imaged fluorescence intensity of a dissolved fluorescent probe corresponds to variation in the oxygen concentration below the surface because the oxygen acts as a quencher of the probe. A tank system was developed to create a known difference in the concentration gradient within the same SLIF image to demonstrate that the SLIF system detects variation in fluorescence related solely to variations in oxygen concentration. Image processing procedures were developed to remove uneven incident radiation. The SLIF system demonstrated large fluorescence intensity differences between water saturated with dissolved oxygen and water with low levels of dissolved oxygen.
The SLIF technique was applied to study oxygen mass transfer across flat air-water interfaces. The SLIF images were transformed into mapped estimates of the relative mass transfer rates and boundary layer thicknesses with high spatial resolution. The Surface Laser Induced Fluorescence technique identified mass transfer variations related to Rayleigh-Bénard downwelling within flat air-water interface images and maps.
The effect of relative humidity on mass transfer across flat air-water interfaces was examined using the SLIF technique. It was shown that the relative humidity had an effect upon the presence, size, speed, and relative thickness of Rayleigh-Bénard downwelling zones under flat air-water interfaces. It is theorized that Rayleigh-Bénard circulations affect the mass transfer of gases across the interface by convective transport of oxygen rich liquid to downwelling zones from the surrounding liquid just below the air-water interface. The transport of oxygen rich liquid from this near-surface zone reduces the resistance to mass transfer across the interface for renewed areas but increases the mass transfer resistance across the downwelling zones.