161
water, an empirical equation for Sc as a function of water temperature has been
proposed for various gases, including CO 2 (Jähne et al. 1987). The transfer velocity
for gases other than CO 2 (e.g., O 2 ) can also be calculated with Eq. 6.4.
Some problems have also been pointed out with the estimates of gas transfer
velocity. One problem is their reliability under strong-wind conditions. The thinfilm model should obviously not be applied under strong-wind conditions when
whitecaps occur on the water surface. The bulk formula method has been used under
such conditions because empirical observations have revealed a high correlation
between wind speed and gas transfer velocity. However, the correlation between the
transfer velocity and wind speed under extremely strong wind conditions, when the
field survey is difficult to be performed, has not been sufficiently evaluated, and the
gas transfer velocity estimates are unreliable because the gas flux is difficult to measure directly. Although such strong wind conditions occur infrequently, the CO 2 flux
under such conditions would be expected to have a significant influence on the
global CO 2 flux because the transfer velocity would be very high.
Another problem is that most empirical equations (e.g., Eq. 6.4) have been constructed by using data from observations made in the open ocean and large lakes.
Wind and current effects on the surfaces of coastal waters are likely to differ from
those on open-water surfaces (Tokoro et al. 2007, 2008).
6.3 Methods for CO 2 Flux Measurements
6.3.1 Bulk Formula Method
Estimates of air–water CO 2 fluxes can be made with the bulk formula method once
values have been assigned to the parameters in Eq. 6.3. Because pCO 2water can be
modeled with parameters such as water temperature and chlorophyll a concentrations that are easy to obtain for the open ocean, the bulk formula method has become
the standard way to estimate global CO 2 fluxes between the atmosphere and ocean
(Fig. 6.1, Takahashi et al. 2009). Although CO 2 fluxes are more difficult to estimate
in shallow coastal areas, the bulk formula method has also been used in many previous studies in such areas because of its simplicity and low cost. However, because
the bulk formula method can provide estimates only at fixed points and in the instant
of time, much effort is required to evaluate temporal and spatial variations. Unlike
direct measurement methods, the bulk formula method can be used to infer the factors that regulate air–water CO 2 fluxes because it uses an equation that includes
biochemical (pCO 2water ) and physical (transfer velocity) terms.
The value of pCO 2water , which is a key parameter in the bulk formula method, can
be determined from in situ measurements using sensors or from chemical analyses
of water samples (Fig. 6.4). The non-dispersive infrared (NDIR) method is the
infrared absorptiometry, and commonly used with sensors. With NDIR, the value of
pCO 2water is determined from the extent of infrared (IR) absorption of an equilibrated
6 Air–Water CO 2 Flux in Shallow Coastal Waters: Theory, Methods…
water, an empirical equation for Sc as a function of water temperature has been
proposed for various gases, including CO 2 (Jähne et al. 1987). The transfer velocity
for gases other than CO 2 (e.g., O 2 ) can also be calculated with Eq. 6.4.
Some problems have also been pointed out with the estimates of gas transfer
velocity. One problem is their reliability under strong-wind conditions. The thinfilm model should obviously not be applied under strong-wind conditions when
whitecaps occur on the water surface. The bulk formula method has been used under
such conditions because empirical observations have revealed a high correlation
between wind speed and gas transfer velocity. However, the correlation between the
transfer velocity and wind speed under extremely strong wind conditions, when the
field survey is difficult to be performed, has not been sufficiently evaluated, and the
gas transfer velocity estimates are unreliable because the gas flux is difficult to measure directly. Although such strong wind conditions occur infrequently, the CO 2 flux
under such conditions would be expected to have a significant influence on the
global CO 2 flux because the transfer velocity would be very high.
Another problem is that most empirical equations (e.g., Eq. 6.4) have been constructed by using data from observations made in the open ocean and large lakes.
Wind and current effects on the surfaces of coastal waters are likely to differ from
those on open-water surfaces (Tokoro et al. 2007, 2008).
6.3 Methods for CO 2 Flux Measurements
6.3.1 Bulk Formula Method
Estimates of air–water CO 2 fluxes can be made with the bulk formula method once
values have been assigned to the parameters in Eq. 6.3. Because pCO 2water can be
modeled with parameters such as water temperature and chlorophyll a concentrations that are easy to obtain for the open ocean, the bulk formula method has become
the standard way to estimate global CO 2 fluxes between the atmosphere and ocean
(Fig. 6.1, Takahashi et al. 2009). Although CO 2 fluxes are more difficult to estimate
in shallow coastal areas, the bulk formula method has also been used in many previous studies in such areas because of its simplicity and low cost. However, because
the bulk formula method can provide estimates only at fixed points and in the instant
of time, much effort is required to evaluate temporal and spatial variations. Unlike
direct measurement methods, the bulk formula method can be used to infer the factors that regulate air–water CO 2 fluxes because it uses an equation that includes
biochemical (pCO 2water ) and physical (transfer velocity) terms.
The value of pCO 2water , which is a key parameter in the bulk formula method, can
be determined from in situ measurements using sensors or from chemical analyses
of water samples (Fig. 6.4). The non-dispersive infrared (NDIR) method is the
infrared absorptiometry, and commonly used with sensors. With NDIR, the value of
pCO 2water is determined from the extent of infrared (IR) absorption of an equilibrated
6 Air–Water CO 2 Flux in Shallow Coastal Waters: Theory, Methods…