Gas exchange across the airsea interface

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The solubility of a gas determines the relative ease with which it can be absorbed into the ocean if there were no other gases present in the atmosphere, and if an equilibrium state was achieved. The troposphere, however, is a mixture of many gases and the basic driving force for exchange of gas across the air-sea interface is the difference in gas concentration, or partial pressure, between the two media. The flux, F, of a gas across the interface into the ocean is often written as

Fig. 3.2. Distribution of the partial pressure of CO2, in microatmospheres, in the surface waters of the global ocean during February, relative to the atmospheric partial pressure at the time of measurement. Negative values indicate sub-saturation (i.e. the surface water has a lower partial pressure than the atmosphere); positive values show super-saturation. Dotted lines indicate estimated rather than observed values. [Data supplied by the Pacific Marine Environmental Laboratory.]

Fig. 3.3. Variation of the gas transfer velocity with wind speed, u. Note the difference in wind dependence between the two gases O2 and CO2. The transfer of oxygen effectively obeys the breaking wave regime at a wind speed some 2 ms-1 less than for carbon dioxide. The units of transfer velocity are equivalent to the cm of air column entering the water per hour.

Fig. 3.3. Variation of the gas transfer velocity with wind speed, u. Note the difference in wind dependence between the two gases O2 and CO2. The transfer of oxygen effectively obeys the breaking wave regime at a wind speed some 2 ms-1 less than for carbon dioxide. The units of transfer velocity are equivalent to the cm of air column entering the water per hour.

F = kr(Pa - Ps), (3.1)

where Pa and Ps are the partial pressures of the gas in question in the surface atmosphere and ocean respectively and kT is the transfer velocity.

As its name suggests kT has the units of a velocity; it represents the variability of the rate of exchange due to the sea state and atmospheric stability. A calm sea and stable air will allow only slow exchange because the surface air mass is renewed infrequently and there is little bubble entrainment in these conditions (see §2.8.2). By contrast, rough seas and strong winds allow frequent renewal of the surface air from above and also actively bypass molecular diffusion across the interface by copious bubble production. Intermediate conditions show moderate transfer. There is often an abrupt change in sea state, and transfer velocity, when the wind becomes strong enough for breaking waves. The wind speed is thus taken as an indicator of these physical states and the variation in kT, and thus gas exchange rate, is a strong function of this variable, as shown in Fig. 3.3.

There are also other effects which the transfer velocity must take into account. The large solubility differences between different gases, due partially to molecular weight but mostly to a gas's chemical reactivity in water, have important consequences for the exchange rate at low wind speeds. At high wind speeds the supply of gas is regulated by the physical mechanisms associated with breaking waves rather than chemical uptake. Although not clear from Fig. 3.3, at wind speeds below 4-5 ms-1 chemically reactive carbon dioxide has a 50% higher transfer velocity than oxygen. However, in the breaking wave regime inert gases are pumped into the sea at a greater rate than chemically active ones, because the air immediately above the water surface, which is entrained during the wave breaking, is enriched in the inert species.

Another mechanism which affects the air-sea flux rate is the heat transfer between the two media. If the air is humid and the latent heat flux is directed

Table 3.2. Annual global fluxes of gases between the ocean and atmosphere. A positive value is into the ocean

Flux (gigatonnes/yr)

Carbon dioxide

Methane

Nitrous oxide

Sulphur gases (e.g. DMS)

Non-methane hydrocarbons

-0.010 ± 0.005 -0.003 ± 0.002 -0.025 ± 0.011 -0.055 ± 0.030

towards the ocean, producing condensation on the water surface, then gas flux into the ocean is inhibited. It has been shown that the reverse is true for sensible heat. Essentially, the solubility increase with decreasing temperature establishes a chemical potential gradient towards the ocean, if the sea surface temperature is less than the air temperature. The resulting flux is then where Pa is atmospheric partial pressure, Ps the ocean surface partial pressure, Pm = 0.5(Pa + Ps), Tm = 0.5(Ta + Ts), R is the ideal gas constant, and Qsol is the energy released when a mole of the gas is dissolved in sea water, termed the enthalpy of solution. Equation (3.2) reduces to (3.1) if the air and sea temperature are identical. Much of the ocean equatorwards of 45°, for much of the year, is warmer than the air above it. The chemical gradient decreases the flux of gas into the sea in these circumstances by about 10% on average. However, over polar latitudes, particularly in summer, over regions of strong upwelling, and off the east coast of heated continents during summer, the sensible heat gradient acts to enhance gas flux into the ocean.

The transfer velocity is often termed the piston velocity because it can be thought of as the size of a column of gas pumped into, or out of, the ocean by the gas partial pressure difference across the air-sea interface. This is an appealing physical association for kT. There is a danger, however, of this association gaining too strong a hold on the scientific imagination. The preceding discussion clearly shows that the transfer velocity is dependent on a large number of physical parameters. The applicability of present gas exchange theory is thus hedged with a multitude of special cases. The transfer of gases, particularly carbon dioxide, across the air-sea boundary is an important climatic process. Accurate estimates of the spatial and temporal fluxes of gases are thus vital for well-based predictions of the future climate. Table 3.2 shows present estimates of the fluxes of climatically important gases between the ocean and atmosphere. Water vapour has been neglected because of the essential balance between precipitation and evaporation over the global ocean. As a comparison, the evaporation of water vapour from the ocean each year is 4.25 x 105 gigatonnes.

Forecasts of future change assume partitioning of carbon dioxide between the air and sea will remain consistent with current behaviour. While estimates of the transfer velocity are available in a number of controlled situations the complexity of the processes involved are such that global estimates of the flux

of CO2 into the oceans are uncertain by a factor of 25% at least. The physical and chemical processes involved in such fluxes need to be better understood in combination. This would ultimately lead to a more useful theory of gas exchange, bringing the quantum step in accuracy of our flux estimates required for confidence in predictions of climatic change.

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