Geology Reference
In-Depth Information
If C
w
is the actual concentration of the dissolved gas in the surface
seawater and
C
w
¼
C
(equ)
the system is at equilibrium and no net transfer occurs. If, however,
there is a concentration difference, DC, where
DC
¼
C
a
H
1
C
w
there will be a net flux. If
C
a
H
1
4 C
w
the water is sub-saturated with regard to the trace gas and transfer
occurs from air to water. Conversely, gas transfers from supersaturated
water to the atmosphere if
C
a
H
1
o
C
w
The rate at which gas transfers occurs is expressed by
F
¼
K
(T)w
DC
where K
(T)w
is termed the total transfer velocity. This can be broken
down into component parts as follows:
1
K
ð
T
Þ
w
¼
1
ak
w
þ
1
Hk
a
¼
r
w
þ
r
a
where k
a
and k
w
are the individual transfer velocities for chemically
unreactive gases in air and water phases, respectively and a (
¼
k
reactive
/
k
inert
) is a factor that quantifies any enhancement of gas transfer in the
water due to chemical reaction. The terms r
w
and r
a
are the resistances to
transfer in the water and air phases, respectively, and are directly
analogous to the resistance terms in Equation (7.8). For chemically
reactive gases, usually r
a
cr
w
and atmospheric transfer limits the overall
flux. For less reactive gases the inverse is true and K
(T)w
R k
w
; the
resistance in the water is the dominant term.
Much research has gone into evaluating k
w
and K
(T)w
, both in theo-
retical models, and in wind tunnel and field studies. The results are
highly wind speed dependent due to the influence of wind upon the
surface state of the sea. The results of some theoretical predictions and
experimental studies
16
for CO
2
(a gas for which k
w
is dominant) are
shown in Figure 6.
In addition to dry deposition, trace gases and particles are also
removed from the atmosphere by rainfall and other forms of precipita-
tion (snow, hail, etc.), entering land and seas as a consequence. Wet
deposition may be simply described in two ways. First,
