38
pH 7.6. Sustained observations at the Hawaii Ocean Time Series (HOT) and other
oceanic time series have already demonstrated a consistent drop of 0.04 pH in open
ocean surface waters over a period of 20 years (Dore et al. 2009).
The above numbers are modest at first sight due to the logarithmic transformation. Nevertheless, when transformed to absolute numbers, the observed decrease
from 8.2 to 8.1 represents an increase of 26% in acidity since the 1800s, while the
end-of-the-century forecast (to pH 7.8) represents an increase of 76% and the longterm projection to pH 7.6 represents a tripling of the amount of acid in ocean waters.
The latter changes have not been observed in the geological records throughout the
last 400,000 years (Sarmiento and Gruber 2006).
Chemical equilibrium is established between the atmospheric and gaseous
phases and the subsequent reaction products as follows:
CO
CO
g
a
2
2
( )
( )
=
(2.7)
where CO 2(g) is the gas in the atmosphere and CO 2(a) , the dissolved aqueous species.
The latter in turns reacts with water to form carbonate, bicarbonate, and H
+
ions:
CO
H O H HCO
a
2
2
3
( )
+
−
+
=
+
(2.8)
and
HCO
H CO
−
+
−
=
+
3
3
2
(2.9)
Stoichiometric constants for these reactions in seawater are
K H
g
a
pCO
pCO
=
( )
( )
2
2
/
(2.10)
where p denotes the partial pressure of carbon dioxide,
K ∗ = +
[ ]
+
−
1
3
2
H
HCO
CO
/
(2.11)
and
K ∗ =
+
] [
+
−
−
2
3
2
3
H
CO
HCO
/
(2.12)
where K* denotes apparent equilibria under the particular conditions of seawater T,
S, and P and the brackets denote the concentration in seawater of the chemical species (including dissolved complexes with other ions). Millero et al. (2002) have
developed empirical expressions for the derivative log transformed constants pK* 1
and pK* 2 based on experimental data for the environmental ranges of T, P, and S. At
S = 35 and T = 25 °C and P = 1 atm, these values are around 6.0 and 9.1. Since these
values bracket the current average pH value of about 8.1, seawater constitutes a
powerful acid-base buffer.
2 Electronic Sensors and Instruments for Coastal Ocean Observing
pH 7.6. Sustained observations at the Hawaii Ocean Time Series (HOT) and other
oceanic time series have already demonstrated a consistent drop of 0.04 pH in open
ocean surface waters over a period of 20 years (Dore et al. 2009).
The above numbers are modest at first sight due to the logarithmic transformation. Nevertheless, when transformed to absolute numbers, the observed decrease
from 8.2 to 8.1 represents an increase of 26% in acidity since the 1800s, while the
end-of-the-century forecast (to pH 7.8) represents an increase of 76% and the longterm projection to pH 7.6 represents a tripling of the amount of acid in ocean waters.
The latter changes have not been observed in the geological records throughout the
last 400,000 years (Sarmiento and Gruber 2006).
Chemical equilibrium is established between the atmospheric and gaseous
phases and the subsequent reaction products as follows:
CO
CO
g
a
2
2
( )
( )
=
(2.7)
where CO 2(g) is the gas in the atmosphere and CO 2(a) , the dissolved aqueous species.
The latter in turns reacts with water to form carbonate, bicarbonate, and H
+
ions:
CO
H O H HCO
a
2
2
3
( )
+
−
+
=
+
(2.8)
and
HCO
H CO
−
+
−
=
+
3
3
2
(2.9)
Stoichiometric constants for these reactions in seawater are
K H
g
a
pCO
pCO
=
( )
( )
2
2
/
(2.10)
where p denotes the partial pressure of carbon dioxide,
K ∗ = +
[ ]
+
−
1
3
2
H
HCO
CO
/
(2.11)
and
K ∗ =
+
] [
+
−
−
2
3
2
3
H
CO
HCO
/
(2.12)
where K* denotes apparent equilibria under the particular conditions of seawater T,
S, and P and the brackets denote the concentration in seawater of the chemical species (including dissolved complexes with other ions). Millero et al. (2002) have
developed empirical expressions for the derivative log transformed constants pK* 1
and pK* 2 based on experimental data for the environmental ranges of T, P, and S. At
S = 35 and T = 25 °C and P = 1 atm, these values are around 6.0 and 9.1. Since these
values bracket the current average pH value of about 8.1, seawater constitutes a
powerful acid-base buffer.
2 Electronic Sensors and Instruments for Coastal Ocean Observing