237
Dissolved Gases Other than CO 2
Redfield (1948) used oxygen data in the Gulf of Maine to evaluate atmospheric exchange
under natural conditions. In the spring, he found that O 2 was transferred from the sea to
the atmosphere as a result of photosynthesis. In the summer, the warming of the waters
produced further evolution of O 2 . In the winter, O 2 was regained by the colder waters
(40% to oxidize organic matter and 60% adsorbed). He determined the exchange coefficient, E = D/ k f/ k, was 3 × 10 6 cm 3 month –1 atm –1 from the oceans in the spring and
l3 × 10 6 cm 3 month –1 atm –1 to the ocean in the winter.
The effect of wind speed on the exchange of O 2 across the air–water interface is shown
in Figure 6.4. The exit coefficient is constant below a wind speed of 3 m s –1 (5 cm above
the liquid). The coefficient is greatly increased above 3 cm s –1 , suggesting turbulent flow.
The wind speeds of 5 cm are about half the value they are at the standard meteorological
height of 10 cm. Although these early studies have been improved on, they do provide
insight into some of the factors controlling gas transfer in natural waters. Broecker and
Peng (1982) have compared wind tunnel measurements with calculated values using 14 C
and radon measurements. The measured results (Figure 6.4) are in good agreement.
Air bubbles can also affect the exchange of gases across the air–sea interface. Hydrostatic
pressure can affect the rate of solution and composition of air bubbles. The radius of the
dissolving bubbles decreases linearly with time and depth. The composition of the air
bubble becomes enriched with N 2 because of the larger Henry’s law coefficient for O 2 compared to N 2 . Free bubbles exhibited a diffusion coefficient twice as large as that for stationary bubbles. This difference is probably related to differences in the interfacial boundary
between stationary and free bubbles and the liquid.
Air bubbles are carried to 20 m below the sea surface during a storm. The partial pressure of the gases in bubbles at this depth is three times the values at the sea surface. Gas
exchange between a bubble and seawater is larger than across the air–sea interface because
the laminar layer is thinner and the partial pressures are increased. Two populations of
bubbles have been found: (a) those less than 40 μ in radius associated with atmospheric
particles deposited on the sea surface and (b) those about 100 μ in radius formed at the sea
surface by collapsing waves. Air bubbles can change the magnitude of the flux of a gas as
much as barometric pressure changes of 2% in 12 h (the passage of a storm). The rate of
exchange by bubbles could be several orders of magnitude greater than seasonal fluxes;
thus, bubbles are an important mechanism for gas exchange.
At low film thickness (high wind speeds), the chemical effects are negligible, but
waves can influence the gas transfer. The transfer rate is proportional to the mean square
slope of the wave. For capillary waves, the enhancement was predicted to be a factor of
nine. Waves produced in wave tunnels only show an enhancement of 10%. The wind
tunnel results showed that the enhancement of the transfer rate varied as a function of
the mean square slope of the waves divided by the friction velocity in the water. More
work is needed to elucidate the many factors that control the transfer of gases across the
air–sea interface.
6.5 Nonreactive Gases
Nitrogen and the noble gases are regarded as nonreactive. The distribution of nonreactive
gases is affected by physical processes and the effects of temperature and salinity on solubility. Studies of the distribution of the nonreactive gases can be useful in separating the
Dissolved Gases Other than CO 2
Redfield (1948) used oxygen data in the Gulf of Maine to evaluate atmospheric exchange
under natural conditions. In the spring, he found that O 2 was transferred from the sea to
the atmosphere as a result of photosynthesis. In the summer, the warming of the waters
produced further evolution of O 2 . In the winter, O 2 was regained by the colder waters
(40% to oxidize organic matter and 60% adsorbed). He determined the exchange coefficient, E = D/ k f/ k, was 3 × 10 6 cm 3 month –1 atm –1 from the oceans in the spring and
l3 × 10 6 cm 3 month –1 atm –1 to the ocean in the winter.
The effect of wind speed on the exchange of O 2 across the air–water interface is shown
in Figure 6.4. The exit coefficient is constant below a wind speed of 3 m s –1 (5 cm above
the liquid). The coefficient is greatly increased above 3 cm s –1 , suggesting turbulent flow.
The wind speeds of 5 cm are about half the value they are at the standard meteorological
height of 10 cm. Although these early studies have been improved on, they do provide
insight into some of the factors controlling gas transfer in natural waters. Broecker and
Peng (1982) have compared wind tunnel measurements with calculated values using 14 C
and radon measurements. The measured results (Figure 6.4) are in good agreement.
Air bubbles can also affect the exchange of gases across the air–sea interface. Hydrostatic
pressure can affect the rate of solution and composition of air bubbles. The radius of the
dissolving bubbles decreases linearly with time and depth. The composition of the air
bubble becomes enriched with N 2 because of the larger Henry’s law coefficient for O 2 compared to N 2 . Free bubbles exhibited a diffusion coefficient twice as large as that for stationary bubbles. This difference is probably related to differences in the interfacial boundary
between stationary and free bubbles and the liquid.
Air bubbles are carried to 20 m below the sea surface during a storm. The partial pressure of the gases in bubbles at this depth is three times the values at the sea surface. Gas
exchange between a bubble and seawater is larger than across the air–sea interface because
the laminar layer is thinner and the partial pressures are increased. Two populations of
bubbles have been found: (a) those less than 40 μ in radius associated with atmospheric
particles deposited on the sea surface and (b) those about 100 μ in radius formed at the sea
surface by collapsing waves. Air bubbles can change the magnitude of the flux of a gas as
much as barometric pressure changes of 2% in 12 h (the passage of a storm). The rate of
exchange by bubbles could be several orders of magnitude greater than seasonal fluxes;
thus, bubbles are an important mechanism for gas exchange.
At low film thickness (high wind speeds), the chemical effects are negligible, but
waves can influence the gas transfer. The transfer rate is proportional to the mean square
slope of the wave. For capillary waves, the enhancement was predicted to be a factor of
nine. Waves produced in wave tunnels only show an enhancement of 10%. The wind
tunnel results showed that the enhancement of the transfer rate varied as a function of
the mean square slope of the waves divided by the friction velocity in the water. More
work is needed to elucidate the many factors that control the transfer of gases across the
air–sea interface.
6.5 Nonreactive Gases
Nitrogen and the noble gases are regarded as nonreactive. The distribution of nonreactive
gases is affected by physical processes and the effects of temperature and salinity on solubility. Studies of the distribution of the nonreactive gases can be useful in separating the