Air-Sea Gas Exchange - Research Projects:

   1.  Parameterization of CO2 Transfer Velocity Using Sea Surface Roughness (2002-2005, NOAA)

   2. Air-Sea Gas Transfer Velocity: Its Dependence on Small-Scale Surface Waves and Surface Films 

   3. Air-Sea Gas-Transfer in Coastal Waters

   4. NSF Young Investigator Award

   5. Influence of Biogenic Surfactants on Gas Exchange at the Air-Sea Interface

Air-Sea Gas Transfer Velocity: Its Dependence on Small-Scale Surface Waves and Surface Films

2000 - 2003


NOAA

PROJECT SUMMARY

The distribution of CO2 in surface waters is strongly influenced by the kinetics of exchange across the air-sea interface. Flux estimates based on air-sea exchange models are an important diagnostic tool and must be consistent with the long-term sequestering of CO2 predicted by various carbon cycle models. Uncertainty in the gas exchange rate and its dependence on major forcing factors is identified in the 1999 U. S. Carbon Cycle Science Plan as a limitation in understanding the ocean's role in absorbing anthropogenic CO2.  Recent work suggests the importance of the small-scale surface waves in controlling the kinetics of gas exchange at low to intermediate wind speeds [Bock et al, 1998]. Small gravity and gravity-capillary waves (wavenumbers > 25 rad/m) appear to play a dominant role. Laboratory and field experiments have suggested a significant statistical correlation between the mean square slope of these waves and gas transfer velocity.  Under varied oceanic conditions, surfactant films strongly impact small-scale waves, reducing their mean square slope and by inference, the gas transfer velocity [Frew, 1997; Frew et al., 1995, 2001]. This project focuses on the feasibility of parameterizing gas transfer velocity using mean square slope of small-scale waves.

Co-Investigators: Nelson M. Frew, Erik J. Bock.

Air-Sea Gas-Transfer in Coastal Waters

1994 - 2000


NSF

PROJECT SUMMARY

The goals of this program are to combine short time-scale measurements of the gas exchange rate with pertinent physical and chemical measurements to understand the mechanisms by which gas exchange at the air-sea interface is effected. We participated in two field experiments in 1995 (off Southern California) and 1997 (southeast of Cape Cod). Our measurements included capillary and capillary-gravity wave spectra, long wave spectra, surface chemical enrichment, wind stress, heat fluxes, and near-surface turbulent dissipation. At the same time the gas exchange rate was derived from a high resolution digital infrared imaging system by Drs. Bernd Jaehne and Horst Haussecker. The contribution of the PI has been mainly to obtain and analyze capillary and capillary-gravity wave spectra and long wave spectra. Main results from this project are: \item Our observations of gravity-capillary waves on clean water show a well-defined correlation with the wind friction velocity. However, the spectral values at higher wavenumbers are significantly higher than previous laboratory results. In the presence of surface films wave spectra may decrease by more than one order of magnitude at lower wind stresses. \item A new algorithm has been developed to estimate the directional surface wave spectrum from research catamaran, by combining the wave height measurement and the measurement of the platform motion. The results have been combined with the short wind-wave measurements to investigate the modulation of short wind waves by long waves.

Co-Investigators: Erik J. Bock, James B. Edson, Nelson M. Frew, Wade McGillis.

Publications resulting from this award:

Bock, E. J., et al., 1995. Description of the science plan for the April 1995 CoOP experiment, `Gas Transfer in Coastal Waters', performed from the research vessel New Horizon, in Air-Water Gas Transfer, edited by B. Jahne and E. C. Monahan, pp. 801-810, AEON Verlag & Studio, Hanau, Germany.

Hanson, K. A., T. Hara, E. J. Bock, and A. Karachintsev, 1997. Estimation of directional surface wave spectra from a towed research catamaran. J. Atmos. Oceanic Tech., 14(6), 1467-1482.

Hara, T., E. J. Bock, J. B. Edson, and W. R. McGillis, 1998. Observations of short wind waves in coastal waters. J. Phys. Oceanogr., 28(7), 1425-1438.

Hara, T., B. M. Uz, H. Wei, J. B. Edson, N. M. Frew et al., 2001. Surface wave and air-sea gas transfer during CoOP experiment, in Gas Transfer at Water Surfaces, edited by M. A. Donelan, W. M. Drennan, E. S. Saltzman and R. Wanninkhof, AGU.

Frew, N. M.,  R. K. Nelson, W. R. McGillis, J. B. Edson, E. J. Bock and T. Hara, 2001. Spatial variations in surface microlayer surfactants and their role in modulating air-sea exchange, in Gas Transfer at Water Surfaces, edited by M. A. Donelan, W. M. Drennan, E. S. Saltzman and R. Wanninkhof, AGU.

Bock, E. J., J. B. Edson, N. M. Frew, T. Hara, H. Haussecker et al., 2001. Overview of the CoOP experiments: physical and chemical measurements parameterizing air-sea gas exchange, in Gas Transfer at Water Surfaces, edited by M. A. Donelan, W. M. Drennan, E. S. Saltzman and R. Wanninkhof, AGU.

Hara, T., K. A.Hanson, E. J. Bock, and B. M. Uz, 2001. Observation of hydrodynamic modulation of gravity-capillary waves by dominant gravity waves. J. Geophys. Res., Submitted.

Zuykov, D. G. and T. Hara, 2000. Small scale Langmuir circulations. In preparation.

Influence of Biogenic Surfactants on Gas Exchange at the Air-Sea Interface

1993 - 1996


NSF

PROJECT SUMMARY

Surfactants produced by marine phytoplankton were postulated to modulate gas exchange rates across the air-sea interface. This laboratory study examined the links between gas transfer velocity and wind stress, detailed small-scale wave spectra, surface viscoelastic modulus and turbulence in order to improve the parameterization of the gas transfer velocity in the presence of surfactants.  Circular wind-wave flumes of different scales at Woods Hole and Heidelberg were instrumented to measure gas transfer rates using multiple gas tracers, wind stress, small-scale wave characteristics, and turbulence.  Wind stress and surfactant concentrations were varied systematically to quantify their effect on transfer velocity and the wave field. The wave field was characterized using point and scanning laser slope gauges. Surface properties (wave damping ratio and viscoelastic modulus) of the surfactant solutions were characterized using mechanically generated capillary wave packets and a dispersion relation including dilational viscolelastic effects. The principal findings were as follows: Gas transfer velocity varies linearly with water-side friction velocity only in the case of surfactant-free waters.  Very low concentrations of surfactants (<1 ppb) are effective in reducing the rate of gas transfer, which becomes strongly nonlinear with wind stress.  At constant wind stress, the gas transfer rate exhibits a power law dependence on bulk surfactant concentration, which is in dynamic equilibrium with the surface concentration via diffusional exchange.  Mass transfer decreases more steeply than momentum transfer with increasing surfactant concentration. Measurements of wave damping ratios indicate that a strong inverse relationship exists between gas transfer velocity and enhanced viscoelastic damping of short wind-waves in the presence of surfactants.  Gas transfer velocity and elastic modulus also are inversely correlated.  Wave field characteristics are strongly dependent on surfactant concentration.  Longer gravity waves (<12 rad/m) are not significantly affected. At intermediate wave numbers (20-60 rad/m), B(k) (degree of saturation) does not simply correlate with wind friction velocity and is moderately reduced by surfactants. At higher wave numbers (>100 rad/m), surfactants strongly reduce B(k), often completely eliminating waves above 200-300 rad/m, whereas B(k) remains high up to the maximum resolvable wave number (1200 rad/m) for clean water.  The relationship between the gas transfer velocity and wave degree of saturation and mean square slope of short wind waves (wave numbers above 25 rad/m) is approximately linear both for clean and surfactant-influenced interfaces. A poor correlation with transfer velocity is observed below 25 rad/m, suggesting that longer waves do not contribute strongly to gas exchange.

Co-Investigators: Nelson M. Frew, Erik J. Bock

Publications resulting from this award:

Frew, N. M., E. J. Bock, W. R. McGillis, A. V. Karachintsev, T. Hara, T. Muensterer,  and B. Jaehne, 1995. Variation of air-water gas transfer with wind stress and surface viscoelasticity, in Air-Water Gas Transfer, B. Jaehne and E. C. Monahan (eds.), AEON Verlag & Studio, Hanau, pp. 529-541.

Hara, T., E. J. Bock, N. M. Frew, and W. R. McGillis, 1995. Relationship between air-sea gas transfer velocity and surface roughness, in Air-Water Gas Transfer, B. Jaehne and E. C. Monahan (eds.), AEON Verlag & Studio, Hanau, pp. 611-616.

Bock, E. J., T. Hara, N. M. Frew, and W. R. McGillis, 1999. Relationship between air-sea gas transfer and short wind waves, J. Geophys. Res., 104(C11), 25,821-25,831.