Surface Ocean - Lower Atmosphere Study (SOLAS) Project Integration

Physical processes controlling air-sea exchange

Implementation Working Group two (IMP 2) focuses upon the following

• Transfer velocity (k)
• Dry/wet deposition

Transfer velocity (k)

Gas exchange across the air-sea interface can be calculated using concentrations in the ocean and atmosphere and the transfer velocity (k) across the interface (Liss and Merlivat, 1986). Typically, k has been scaled with wind speed (U₁₀); a recent parameterisation of this was developed by Nightingale et al. (2000):

k = 0.222(U₁₀)² + 0.333U₁₀

However, a modern measurement technique, eddy correlation, has been directly applied to dimethylsulphide (DMS). This produces an alternative parameterisation of k with wind speed (Blomquist et al., 2006):

k_DMS = (U₁₀)^1.3

Blomquist et al. (2006) attribute this variation to the differential effect of gas solubility within bubbles.  This highlights the important point that parameterising transfer velocities to wind speed alone is clearly simplistic and dependent upon the gas considered and the environmental conditions specific to when each transfer velocity measurement was made.  Amongst other things, transfer velocities can be affected by the impact of bubbles (e.g. Asher et al., 1996; Woolf, 1993), the sea-surface microlayer (e.g. Upstill-Goddard et al., 2003) and the impact of raindrops (e.g. Ho et al., 2000).  In a recent assessment, Woolf (2005) recommends that satellite-retrieved transfer velocities can be estimated more accurately from a combination of wind speed and significant wave height than by wind speed alone.

References

1. Asher, W.E. et al. (1996) The influence of bubble plumes on air-seawater gas transfer velocities. JGR 101: 12027-12041.
2. Blomquist, B.W. et al. (2006) DMS sea-air transfer velocity: Direct measurements by eddy covariance and parameterization based on the NOAA/COARE gas transfer model. GRL 33: art. no.-L07601.
3. Ho, D.T. et al. (2000) On mechanisms of rain-induced air-water gas exchange. JGR 105: 24045-24057.
4. Liss, P.S. and Merlivat, L., 1986. Air-sea gas exchange rates: introduction and synthesis. In: P. Buatmenard (Editor), The role of air-sea exchange in geochemical cycling. Reidel, pp. 113-127.
5. Nightingale, P.D. (2000) In situ evaluation of air-sea gas exchange parameterizations using novel conservative and volatile tracers. GBC 14: 373-387.
6. Upstill-Goddard, R.C. et al. (2003) Bacterioneuston control of air-water methane exchange determined with a laboratory gas exchange tank. GBC 17: art. no.-1108.
7. Woolf, D.K., 1993. Bubbles and the air-sea transfer velocity of gases. Atmosphere-Ocean, 31(4): 517-540.
8. Woolf, D.K., 2005. Parameterization of gas transfer velocities and sea-state-dependent wave breaking. Tellus Series B-Chemical and Physical Meteorology, 57(2): 87-94.

Dry/wet deposition

Dry and wet deposition of aerosol particles to the ocean surface is a major transport pathway of trace nutrients such as iron, nitrogen and phosphorous into the oceans. Currently, no satisfactory technique exists for direct estimation of dry deposition fluxes. At present, the best approach for estimating dry deposition of particles uses the model-based parameterisation developed by Slinn and Slinn (1980) or some more recent modification of it.

Wet deposition is easier to measure (i.e. precipitation rate and chemical composition), but sampling tends to be opportunistic on research cruises. Reliable multi-year studies do exist, but at relatively few marine locations (e.g. Moody et al., 1991; Galloway et al., 1993).

References

1. Galloway, J.N. et al. (1993) The temporal and spatial variability of scavenging ratios for nss sulfate, nitrate, methanesulfonate and sodium in the atmosphere over the North Atlantic Ocean. Atm Env 27: 235-250.
2. Moody, J.L. et al. (1991) Precipitation composition and its variability in the southern Indian Ocean – Amsterdam Island, 1980-1987. JGR 96: 20769-20786.
3. Slinn, S.A. and Slinn, W.G.N. (1980) Predictions for particle deposition on natural waters. Atm Env 14(9): 1013-1016.