Experiments Investigating Gas Transfer and Momentum Flux across Air-Water Interfaces

Improved understanding of the physical mechanisms that control air-sea gas transfer will ultimately lead to more accurate models of climate change and global warming. The presence of surfactants on the sea surface influences the free surface behaviour, propagation characteristics of waves and alters the near-surface turbulent length and velocity scales. These effects are thought to inhibit surface renewal and therefore reduce the rate of air-water gas and heat transfer [Saylor et al. 2000]. A number of laboratory experiments in wind-wave tanks have shown that the presence of a surfactant results in significant reductions in the air-water gas transfer rate [e.g. Zappa et al. 2001]. In this study, we completed a series of experiments in a wind-wave flume at the University of Washington, Seattle. Measurements were made at wind speeds from 4 to 10 m/s with clean and surfactant contaminated water. Digital Particle Image Velocimetry (DPIV) was used to measure the two dimensional turbulent velocity fields beneath the air-water interface. Bulk gas transfer velocities were measured during these experiments for two gases, He and SF6. The wind waves were damped in the presence of a surfactant as expected. The DPIV data was used to study the characteristics of the wind drift layer and near-surface turbulence. The mean velocity profiles were observed to be logarithmic and this facilitated accurate calculation o