Environmental Engineering Reference
In-Depth Information
and performing some algebraic manipulations, one obtains (see Butler, 1982 for
details)
∂P
CO
2
P
CO
2
[
∂
H
+
≈
(6.164)
H
+
]
[
Alk
]
and
∂
[
CO
2
−
3
CO
2
]
tot
∂
H
+
≈
[
CO
2
]
aq
+[
]
H
+
.
(6.165)
[
Alk
]
Hence,
[
CO
2
]
tot
R
B
.
(6.166)
CO
2
−
3
[
CO
2
]
aq
+[
]
For typical seawater samples that have an average pH of 8, we can approxi-
mate
HCO
3
]=
10
−
2.7
and[CO
2
]
tot
≈[
HCO
3
]=
10
−
2.7
.Hence,[CO
2
]
[
Alk
]≈[
=
10
−
pH
/K
a1
HCO
2
3
10
−
4.7
,
CO
2
3
=
HCO
3
K
a2
/
10
−
pH
=
10
−
3.8
, and
=
R
B
⊕
9.5 at 298 K (Butler, 1982).
Theequationfor
R
B
givesthechangeinpartialpressureofCO
2
requiredtoproduce
a specified change in [CO
2
] in seawater if [Alk] is constant. Thus if
R
B
⊕
11, compared with an experimental value of
≈
10, Butler
(1982) estimated that about a 10% change in atmospheric concentration is required
to bring about a total change of 1% in seawater CO
2
concentration. He also estimated
that the pre-industrial era (1750-1800) must have had
145 mol CO
2
per m
2
of ocean
surface area and the mixed layer of ocean surface water must have had
∼
900 mol/m
2
.
∼
15 mol/m
2
)
should correspond to only a 1%
A 10% increase in atmospheric CO
2
∼
9 mol/m
2
)
increase in CO
2
in surface water. Extrapolating, we can conclude that
in about two and one-half years, the oceans can absorb about 50% of the increased
CO
2
in the atmosphere. Thus, there is a significant lag in the CO
2
absorption by the
world's oceans. This feed-forward mechanism tends to increase the atmospheric CO
2
concentration.
Sophisticated and complex climate models have been developed to forecast the
CO
2
increase, and how it could affect the climate and crops in different regions of
the world. We shall not discuss these topics here; the student is referred to the recent
document by IPCC (2007) for further details.
Asalreadystated,thelargestexchangeofcarbon(asCO
2
orcarbonatesandorganic
molecules) occurs between the ocean and the atmosphere. The exchange between the
biota and the atmosphere is equally important (see Figure 6.42).The average time that
aCO
2
molecule remains free in the air is
(
∼
4 years before it is taken up by the biota or
ocean. However, the adjustment time, that is, the time taken by atmospheric CO
2
level
to reach a new equilibrium, if either the source or sink is disturbed, is
∼
50-200 years.
The net flux into or out of oceans depends both on the partial pressure of CO
2
in the
atmosphere and on the concentration of total carbon in surface waters.
One can construct a simple model to relate the changes in atmospheric CO
2
to
increased global production of CO
2
. The following example is an illustration of such
a model.
∼
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