Effect of gas transfer velocity on CO2 fluxes

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The ApCO2 maps are combined with solubility (s) in sea water and the kinetic forcing function, the gas transfer velocity (k), to produce the flux equation:

F = ksApCO2

where the gas transfer velocity, k, is controlled by near-surface turbulence in the liquid boundary layer. Laboratory studies in wind-wave tanks have shown that k is a strong but non-unique function of wind speed (Wanninkhof et al., 2002). Results from various wind-wave tank investigations and field studies indicate that factors such as fetch, wave direction, atmospheric boundary layer stability and bubble entrainment influence the rate of gas transfer. Moreover, surfactants can inhibit gas exchange through their damping effect on waves. The commonly used gas transfer parameterizations have been based solely on wind speed, in large part because k is strongly dependent on wind, global and regional wind speed data are readily available and effects other than wind speed have not been well quantified (Wanninkhof et al., 2002). Table 3.1 shows the regional variations of the climatological sea-air exchange fluxes.

Using an alternative gas exchange for-mulization, however, can suggest a different distribution of fluxes. For example, Wanninkhof and McGillis (1999) have suggested a cubic relationship to wind speed instead of the quadratic relationship of Wanninkhof (1992). The cubic relationship gives an uptake that is 45% larger than the quadratic relationship (Table 3.2). This primarily results from a larger CO2 uptake in the high-latitude sink regions because of the stronger impact of the higher winds on the gas exchange (Feely et al., 2001). More studies of gas exchange processes at high wind speed regimes are required before determining whether the quadratic, cubic or some other newly developed relationship is appropriate for high wind speeds.

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