160
both DIC and TA. Because the influence of calcification on pCO 2 is very complicated, we elaborate on calcification effects in Sect. 6.4.3.
Salinity is closely correlated with DIC and TA and thus has a strong indirect
effect on pCO 2water . The correlation reflects the fact that bicarbonate, which is the
major contributor to both DIC and TA, is among the major anions in seawater.
Freshwater with low concentrations of calcium and magnesium cations is characterized as soft water and, in order to maintain charge neutrality, contains low concentrations of bicarbonate and carbonate anions. Hence, DIC and TA of soft water
arelow. Conversely, DIC and TA of hard water are high, because hard water contains high concentrations of calcium and magnesium ions. In shallow coastal
waters where freshwater and seawater are mixed, changes of salinity and hence
pCO 2water reflect dynamic environmental factors such as tides and river discharge.
The pCO 2water in shallow coastal waters is also affected by temporal and spatial
variations of both DIC and TA of freshwater.
In summary, pCO 2water is determined by complex interactions involving the carbonate system, which is affected by parameters such as water temperature and salinity as well as by processes such as photosynthesis, respiration, calcification,
terrestrial runoff, and tidal exchange. Because it is very difficult to precisely determine pCO 2water via indirect methods, pCO 2water is generally measured directly with
specialized sensors or determined on the basis of chemical analyses of water samples (see Sect. 6.3).
6.2.3 Transfer Velocity
The transfer velocity is the ratio of the CO 2 molecular diffusion coefficient to the
film thickness in the thin-film model. The correlation between the transfer velocity
and the wind speed at the water surface has been investigated by using direct measurements of CO 2 fluxes in the ocean and in large lakes such as the Great Lakes. The
following is the most commonly used empirical equation for the transfer velocity as
a function of wind speed (Wanninkhof 1992).
k
U
Sc
=
´
´ ( )
-
0 39
660
10
2
0 5
.
.
(6.4)
In Eq. 6.4, U 10 is the wind speed at an altitude of 10 m above the water surface.
Because the vertical profile of wind speed near the water surface can be approximated by a logarithmic relationship (e.g., Kondo 2000), U 10 can be calculated with
observational data recorded by an anemometer at the site or obtained from a public
database. The parameter Sc, called the “Schmidt number”, is defined by the relationship between the turbulence near the water surface and the thickness of the
boundary layer of solute concentrations. In Eq. 6.4, the gas transfer velocity is
expressed under the assumption that the temperature of seawater is 20 °C, at which
the Sc is 660, before the correction by on-site Sc. In the case of freshwater and seaT. Tokoro et al.
both DIC and TA. Because the influence of calcification on pCO 2 is very complicated, we elaborate on calcification effects in Sect. 6.4.3.
Salinity is closely correlated with DIC and TA and thus has a strong indirect
effect on pCO 2water . The correlation reflects the fact that bicarbonate, which is the
major contributor to both DIC and TA, is among the major anions in seawater.
Freshwater with low concentrations of calcium and magnesium cations is characterized as soft water and, in order to maintain charge neutrality, contains low concentrations of bicarbonate and carbonate anions. Hence, DIC and TA of soft water
arelow. Conversely, DIC and TA of hard water are high, because hard water contains high concentrations of calcium and magnesium ions. In shallow coastal
waters where freshwater and seawater are mixed, changes of salinity and hence
pCO 2water reflect dynamic environmental factors such as tides and river discharge.
The pCO 2water in shallow coastal waters is also affected by temporal and spatial
variations of both DIC and TA of freshwater.
In summary, pCO 2water is determined by complex interactions involving the carbonate system, which is affected by parameters such as water temperature and salinity as well as by processes such as photosynthesis, respiration, calcification,
terrestrial runoff, and tidal exchange. Because it is very difficult to precisely determine pCO 2water via indirect methods, pCO 2water is generally measured directly with
specialized sensors or determined on the basis of chemical analyses of water samples (see Sect. 6.3).
6.2.3 Transfer Velocity
The transfer velocity is the ratio of the CO 2 molecular diffusion coefficient to the
film thickness in the thin-film model. The correlation between the transfer velocity
and the wind speed at the water surface has been investigated by using direct measurements of CO 2 fluxes in the ocean and in large lakes such as the Great Lakes. The
following is the most commonly used empirical equation for the transfer velocity as
a function of wind speed (Wanninkhof 1992).
k
U
Sc
=
´
´ ( )
-
0 39
660
10
2
0 5
.
.
(6.4)
In Eq. 6.4, U 10 is the wind speed at an altitude of 10 m above the water surface.
Because the vertical profile of wind speed near the water surface can be approximated by a logarithmic relationship (e.g., Kondo 2000), U 10 can be calculated with
observational data recorded by an anemometer at the site or obtained from a public
database. The parameter Sc, called the “Schmidt number”, is defined by the relationship between the turbulence near the water surface and the thickness of the
boundary layer of solute concentrations. In Eq. 6.4, the gas transfer velocity is
expressed under the assumption that the temperature of seawater is 20 °C, at which
the Sc is 660, before the correction by on-site Sc. In the case of freshwater and seaT. Tokoro et al.