Solubility of gases

The atmosphere is almost entirely a mixture of gases. These gases can enter the ocean across the air-sea interface. If no molecules of a particular gas were in solution in the upper ocean then this transfer would act as a drain on the atmospheric store of the gas in question (see §3.2). Once sufficient molecules from the atmosphere accumulate in the water for as many to leave the sea as enter in a given time, equilibrium is reached and the water is said to be saturated with respect to the gas. Atmospheric gases are generally close to a state of saturation in surface waters of the ocean. Indeed, their concentrations are often slightly above saturation because of the additional input to the ocean through the dissolution of bubbles from breaking waves.

All water in the sea has at one time been exposed to the atmosphere at the ocean surface. Therefore, it would be expected that atmospheric gases which

Table 3.1. Saturation concentration and solubility for atmospheric gases at 1 atmosphere pressure in sea water of salinity 35 psu

Gas

(atmos.)

Oceanic saturation

0°C 24° C

0°C 24° C

Nitrogen

28

0.781

14.3

9.2

18.3

11.8

(N2)

Oxygen

32

0.209

8.1

5.0

38.7

23.7

(O2)

Argon

40

9.3 x 10-3

0.39

0.24

42.1

26.0

(Ar)

Carbon dioxide

44

3.54 x 10-4

0.51

0.24

1437

666

(CO2)

Neon

20

1.8 x 10-5

1.8 x 10-4

1.5 x

10-4

10.1

8.6

(Ne)

Helium

4

5.2 x 10-6

4.1 x 10-5

3.8 x

10-5

7.8

7.4

(He)

9.4 x 10-5

10-5

Krypton

84

1.1 x 10-6

5.1 x

85.6

46.2

(Kr)

Nitrous oxide

44

3.0 x 10-7

3.2 x 10-4

1.4 x

10-4

1071

476

(N2O)

* A more consistent unit would be mol/kg, but as the values are several orders of magnitude smaller for these units I use the more traditional value for ease of comparison.

* A more consistent unit would be mol/kg, but as the values are several orders of magnitude smaller for these units I use the more traditional value for ease of comparison.

are unreactive, such as nitrogen and the noble gases, would have similar concentrations throughout the world oceans. This has been found to be the case for all except a few gases. Helium has additional sources from hydrothermal activity and the radioactive decay of a product of nuclear bomb tests, tritium,3 H. Tritium enters the ocean through its incorporation into water molecules in the atmosphere (3H1HO) and their subsequent precipitation or surface exchange. The oceanic concentration of helium therefore tells us more about the currents transporting water away from mid-ocean ridges, or diffusion into the interior of the ocean from the surface input of tritium, than its atmospheric content (§3.7).

Some atmospheric gases have been introduced by man. They can be entirely new additions to the environment, such as many of the chlorofluorocarbons, or supplement natural sources, such as carbon dioxide. The distribution of their oceanic concentration helps to trace the movement of water from the surface into the deeper ocean.

The ocean will be a sink for those gases purely produced by man, such as the chlorofluorocarbons, or CFCs. Due to the chemical inertness of CFCs, these gases are emitted into the atmosphere at much greater rates than their slow absorption by the ocean. The size of the ocean sink for such gases is therefore rather small, and not considered further.

Some atmospheric gases, such as carbon dioxide, are very reactive in water and so the ocean can absorb rather more of these gases than their molecular weight and atmospheric composition would suggest. In Table 3.1 the ocean surface saturation concentrations for a selection of atmospheric gases at their

Fig. 3.1. Solubility of various atmospheric gases in sea water as a function of temperature. The temperature range covers most of the observed range in oceanic waters. Units are millilitres of gas contained in a litre of sea water of salinity 35 psu, assuming an overlying atmosphere purely of each gas. [Data from Broecker and Peng, 1982.]

Fig. 3.1. Solubility of various atmospheric gases in sea water as a function of temperature. The temperature range covers most of the observed range in oceanic waters. Units are millilitres of gas contained in a litre of sea water of salinity 35 psu, assuming an overlying atmosphere purely of each gas. [Data from Broecker and Peng, 1982.]

present atmospheric partial pressures are given for two different temperatures. Table 3.1 also gives the solubility of these gases. This is defined as the saturation concentration that would be found if the entire atmosphere was composed of the particular gas. The solubility is an absolute measure with which to compare different gases, while the saturation value, which is just the product of the solubility and the atmospheric partial pressure, includes the effect of the actual atmospheric composition.

From Table 3.1 we can see several interesting properties of gas solubility. For inert gases it is a function of molecular weight. There is also, particularly for the heavier gases, a striking inverse dependence on temperature. This is because higher temperatures mean the molecules have a greater energy, and this, in turn, implies a higher rate of exchange of molecules between the air and water, so that the equilibrium situation occurs with fewer molecules of gas within the liquid. This dependence on temperature is illustrated for a number of gases in Fig. 3.1.

Some atmospheric gases have anomalously high solubilities, for their molecular weight. These include carbon dioxide, methane, and nitrous oxide; all of these are greenhouse gases. Their high solubility occurs because the gases are involved in reactions with water which convert much of the gas to soluble ions. Such gases also show an enhanced temperature-dependence of their solubility (compare the variation of solubility of CO2 and O2 with temperature in Fig. 3.1). This enhancement occurs because the rates of the chemical reactions that are responsible for allowing more gas to be absorbed are also temperature-dependent. If these chemical reactions can soak up gas faster than it can enter

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the ocean then saturation will not be reached. It can also be found that in particular locations excessive super-saturation occurs due to chemical or physical input of gas. Fig. 3.2 shows the partial pressure of CO2 in the surface waters of the ocean. In §3.3 we will discuss in detail the uptake of carbon dioxide but note for now the excess near the equator and the depletion in sub-polar and higher latitudes.

Some gases which are present in only very small quantities in the atmosphere are products of biological activity within the sea. For these the atmosphere is a sink, rather than a source, region. Such gases, which can interact with the climate through contributing to the greenhouse effect (for example, N2O and CH4) or cloud processes (for example, dimethyl sulphide), will be discussed in Chapter 4.

Solubility is also weakly dependent on salinity, with higher salinity decreasing solubility. Pressure is also important. Deeper in the ocean, where the pressure is higher, a higher concentration of gas can be sustained. We will see how this is important for air-sea exchanges in §3.3.2.

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