| dc.description | The most interesting scientific problems tend to occur at interfaces, both physical and disciplinary. Study of gas exchange processes across the air-sea interface requires the tools of atmospheric scientists, chemical engineers, and chemical and physical oceanographers. Gas exchange is least well understood under the dynamic conditions of wave breaking and bubble injection, when fluxes are expected to be large. Results from studies under such conditions have led to controversial results, e.g. high local fluxes that are difficult to reconcile with global budgets. The largely unresolved differences await improved instrumentation, new approaches and more data. A new method based on measurement of total dissolved gas pressure ('gas tension' for succinctness) is described. In experiments the state of gaseous equilibrium of a water parcel in a closed tank is continuously monitored using a simple 'gas tension device' (GTD) developed for this purpose. Measurements of the dissolved gas content of the water, and response to changes in applied gas-phase pressure, allows calculation of gas exchange velocities and steady-state supersaturations. Exchange rates of argon, nitrogen, helium and carbon dioxide are determined in this work in single-gas experiments. Effort is concentrated on the generally neglected experimental configuration of gas invasion with bubble injection. An intermittent waterfall provides bubble injection in a type of simulated breaking-wave. Photographic examination of the bubble populations shows many small bubbles in sea water compared with fewer and larger bubbles in fresh water. The results show that the bubble population in sea water contributes to higher gas exchange velocities than the bubble population generated by the same mechanism in fresh water. A model developed as a tool for interpreting the new gas tension measurements incorporates a simple depth-dependent bubble gas flux model and allows a total gas exchange velocity to be further separated into components for the free-surface exchange velocity and the bubble exchange velocity. The gas tension method provides a sensitive, robust, and versatile technique for gas-exchange studies. With simple modifications the GTD will be a useful field instrument. | en_US |
