THE INFLUENCE OF BUBBLES ON AIR-SEA GAS EXCHANGE
Principal Investigator:
Rik Wanninkhof
Collaborating
scientist(s):
In collaboration with:
Dr. William Asher, Applied Physics Laboratory, University of Washington
Dr. Edward Monahan, U. Connecticut
Objective:
Determine the influence of solubility and diffusivity on bubble mediated
air-sea gas exchange
Description:
Gas transfer under rough and windy conditions, when whitecaps are present,
contributes disproportionately to the
global flux of environmentally important gases such as carbon dioxide
(CO2, carbonyl sulfide (COS),
and dimethyl sulfide (DMS). To quantify gas transfer under these
conditions exchange through bubbles must be fully understood.
Method:
A field study has been performed to investigate bubble mediated gas
exchange. The study was performed in a wave pool where breaking waves
could be created in a controlled and reproducible fashion.
Accomplishment:
Key pragmatic issues are how to parameterize gas exchange in presence of
bubble plumes, what the absolute magnitude of
gas transfer is, and how the relative rates of exchange of gases differ in
presence of bubbles compared to exchange over an
unbroken surface. This study, in which the exchange rates of helium (He),
sulfur hexafluoride (SF6),
nitrous oxide (N2O) are
compared in a freshwater surf pool, addresses the difference in relative
exchange rates of gases with different physical properties.
The relative rates of gas transfer through bubbles of N2O,
CO2, He, and SF6 for the experiment (Figure 1) are compared with models developed
by Woolf [1995] and Keeling [1993]. The model of Woolf
shows reasonable agreement with the observed results while that of Keeling
overpredicts the exchange of N2O.
Key reference:
Asher, W., and R. Wanninkhof, The effects of breaking waves on
dual-tracer gas exchange experiments, in Air-Water Gas Transfer:
Selected papers from the Third International Symposium on air-water
gas transfer, edited by B. Jaehne, and E. Monahan,
pp. 517-529, Aeow Verlag, Hanau, Germany.
Keeling, R.F., On the role of large bubbles in air-sea gas exchange and
supersaturation in the ocean, J. Mar. Res., 51, pp. 237-271,
1993.
Wanninkhof, R., W. Asher, and E. Monahan, The influence of large
bubbles on air-water gas exchange: Results from the gas transfer
experiments during WABEX-93, in Air-Water Gas Transfer: Selected papers from the Third
International Symposium on air-water gas transfer, edited by B. Jaehne, and E. Monahan,
pp. 239-255, Aeow Verlag, Hanau, Germany, 1995.
Woolf, D.K., Bubbles and their role in gas exchange, The sea surface and
global change, submitted, 1995.
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