The optimal carbonate dissociation constants for determining surface water pCO2 from alkalinity and total inorganic carbon

Rik Wanninkhof, Ernie Lewis, Richard A. Feely, Frank J Millero

Research output: Contribution to journalArticle

78 Citations (Scopus)

Abstract

In many numerical ocean chemistry models, total dissolved inorganic carbon, DIC and total alkalinity, TA are transported between subsurface boxes. The partial pressure pCO2 is subsequently calculated from TA and DIC in the surface box in order to account for air-sea exchange of carbon dioxide. The conversion is commonly performed by solving the thermodynamic relationships for equilibria between carbonate, bicarbonate, and aqueous CO2 using carbonate dissociation constants. Four independent determinations of the constants have been made for seawater in the past 50 years. These results have been corrected, refit, and combined by others creating a virtual cottage industry of laboratory and field verification, and cross-checks. Here, we show that, based on field observations in three ocean basins, the calculated surface water pCO2 from TA and DIC corresponds best with the measured pCO2 if the constants proposed by Mehrbach et al. [Mehrbach, C., Culberson, C.H., Hawley, J.E, Pytkowicz, R.M., 1973. Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure Limnology and Oceanography 18, 897-907] as refit by Dickson and Millero [Dickson, A.G., Millero, F.J, 1987. A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Res. 34, 1733-1743] are used. This suggests that the K1/K2 ratio of Mehrbach et al. for a pCO2 range of 280-450 μatm is correct but it does not necessarily imply that the absolute values of K1 and K2 are correct or that these constants work as well at higher pCO2. An independent cross-check with pCO2 measurements made at constant temperature (20°C) is in agreement with these conclusions. The differences in calculated pCO2 can be as great as 30 μatm depending which constants are used. If the models are forced with a prescribed atmospheric pCO2 they will end up having surface concentration of TA that are up to 20 μeq kg-1 too high or DIC concentration up to 20 μmol kg-1 too low if constants other than those of Mehrbach et al. are used. This makes comparisons between observation and models problematic.

Original languageEnglish (US)
Pages (from-to)291-301
Number of pages11
JournalMarine Chemistry
Volume65
Issue number3-4
DOIs
StatePublished - Jun 1999

Fingerprint

Dacarbazine
Carbonates
inorganic carbon
Alkalinity
Surface waters
alkalinity
Carbon
Carbonic Acid
Seawater
surface water
carbonate
Limnology
Oceanography
Equilibrium constants
Bicarbonates
seawater
Carbon Dioxide
Partial pressure
Catchments
Atmospheric pressure

Keywords

  • Alkalinity
  • Carbon dioxide
  • Dissociation
  • Oceanography

ASJC Scopus subject areas

  • Chemistry(all)
  • Oceanography

Cite this

The optimal carbonate dissociation constants for determining surface water pCO2 from alkalinity and total inorganic carbon. / Wanninkhof, Rik; Lewis, Ernie; Feely, Richard A.; Millero, Frank J.

In: Marine Chemistry, Vol. 65, No. 3-4, 06.1999, p. 291-301.

Research output: Contribution to journalArticle

Wanninkhof, Rik ; Lewis, Ernie ; Feely, Richard A. ; Millero, Frank J. / The optimal carbonate dissociation constants for determining surface water pCO2 from alkalinity and total inorganic carbon. In: Marine Chemistry. 1999 ; Vol. 65, No. 3-4. pp. 291-301.
@article{907e9f5ed0b947d3beb434ae950df57f,
title = "The optimal carbonate dissociation constants for determining surface water pCO2 from alkalinity and total inorganic carbon",
abstract = "In many numerical ocean chemistry models, total dissolved inorganic carbon, DIC and total alkalinity, TA are transported between subsurface boxes. The partial pressure pCO2 is subsequently calculated from TA and DIC in the surface box in order to account for air-sea exchange of carbon dioxide. The conversion is commonly performed by solving the thermodynamic relationships for equilibria between carbonate, bicarbonate, and aqueous CO2 using carbonate dissociation constants. Four independent determinations of the constants have been made for seawater in the past 50 years. These results have been corrected, refit, and combined by others creating a virtual cottage industry of laboratory and field verification, and cross-checks. Here, we show that, based on field observations in three ocean basins, the calculated surface water pCO2 from TA and DIC corresponds best with the measured pCO2 if the constants proposed by Mehrbach et al. [Mehrbach, C., Culberson, C.H., Hawley, J.E, Pytkowicz, R.M., 1973. Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure Limnology and Oceanography 18, 897-907] as refit by Dickson and Millero [Dickson, A.G., Millero, F.J, 1987. A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Res. 34, 1733-1743] are used. This suggests that the K1/K2 ratio of Mehrbach et al. for a pCO2 range of 280-450 μatm is correct but it does not necessarily imply that the absolute values of K1 and K2 are correct or that these constants work as well at higher pCO2. An independent cross-check with pCO2 measurements made at constant temperature (20°C) is in agreement with these conclusions. The differences in calculated pCO2 can be as great as 30 μatm depending which constants are used. If the models are forced with a prescribed atmospheric pCO2 they will end up having surface concentration of TA that are up to 20 μeq kg-1 too high or DIC concentration up to 20 μmol kg-1 too low if constants other than those of Mehrbach et al. are used. This makes comparisons between observation and models problematic.",
keywords = "Alkalinity, Carbon dioxide, Dissociation, Oceanography",
author = "Rik Wanninkhof and Ernie Lewis and Feely, {Richard A.} and Millero, {Frank J}",
year = "1999",
month = "6",
doi = "10.1016/S0304-4203(99)00021-3",
language = "English (US)",
volume = "65",
pages = "291--301",
journal = "Marine Chemistry",
issn = "0304-4203",
publisher = "Elsevier",
number = "3-4",

}

TY - JOUR

T1 - The optimal carbonate dissociation constants for determining surface water pCO2 from alkalinity and total inorganic carbon

AU - Wanninkhof, Rik

AU - Lewis, Ernie

AU - Feely, Richard A.

AU - Millero, Frank J

PY - 1999/6

Y1 - 1999/6

N2 - In many numerical ocean chemistry models, total dissolved inorganic carbon, DIC and total alkalinity, TA are transported between subsurface boxes. The partial pressure pCO2 is subsequently calculated from TA and DIC in the surface box in order to account for air-sea exchange of carbon dioxide. The conversion is commonly performed by solving the thermodynamic relationships for equilibria between carbonate, bicarbonate, and aqueous CO2 using carbonate dissociation constants. Four independent determinations of the constants have been made for seawater in the past 50 years. These results have been corrected, refit, and combined by others creating a virtual cottage industry of laboratory and field verification, and cross-checks. Here, we show that, based on field observations in three ocean basins, the calculated surface water pCO2 from TA and DIC corresponds best with the measured pCO2 if the constants proposed by Mehrbach et al. [Mehrbach, C., Culberson, C.H., Hawley, J.E, Pytkowicz, R.M., 1973. Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure Limnology and Oceanography 18, 897-907] as refit by Dickson and Millero [Dickson, A.G., Millero, F.J, 1987. A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Res. 34, 1733-1743] are used. This suggests that the K1/K2 ratio of Mehrbach et al. for a pCO2 range of 280-450 μatm is correct but it does not necessarily imply that the absolute values of K1 and K2 are correct or that these constants work as well at higher pCO2. An independent cross-check with pCO2 measurements made at constant temperature (20°C) is in agreement with these conclusions. The differences in calculated pCO2 can be as great as 30 μatm depending which constants are used. If the models are forced with a prescribed atmospheric pCO2 they will end up having surface concentration of TA that are up to 20 μeq kg-1 too high or DIC concentration up to 20 μmol kg-1 too low if constants other than those of Mehrbach et al. are used. This makes comparisons between observation and models problematic.

AB - In many numerical ocean chemistry models, total dissolved inorganic carbon, DIC and total alkalinity, TA are transported between subsurface boxes. The partial pressure pCO2 is subsequently calculated from TA and DIC in the surface box in order to account for air-sea exchange of carbon dioxide. The conversion is commonly performed by solving the thermodynamic relationships for equilibria between carbonate, bicarbonate, and aqueous CO2 using carbonate dissociation constants. Four independent determinations of the constants have been made for seawater in the past 50 years. These results have been corrected, refit, and combined by others creating a virtual cottage industry of laboratory and field verification, and cross-checks. Here, we show that, based on field observations in three ocean basins, the calculated surface water pCO2 from TA and DIC corresponds best with the measured pCO2 if the constants proposed by Mehrbach et al. [Mehrbach, C., Culberson, C.H., Hawley, J.E, Pytkowicz, R.M., 1973. Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure Limnology and Oceanography 18, 897-907] as refit by Dickson and Millero [Dickson, A.G., Millero, F.J, 1987. A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Res. 34, 1733-1743] are used. This suggests that the K1/K2 ratio of Mehrbach et al. for a pCO2 range of 280-450 μatm is correct but it does not necessarily imply that the absolute values of K1 and K2 are correct or that these constants work as well at higher pCO2. An independent cross-check with pCO2 measurements made at constant temperature (20°C) is in agreement with these conclusions. The differences in calculated pCO2 can be as great as 30 μatm depending which constants are used. If the models are forced with a prescribed atmospheric pCO2 they will end up having surface concentration of TA that are up to 20 μeq kg-1 too high or DIC concentration up to 20 μmol kg-1 too low if constants other than those of Mehrbach et al. are used. This makes comparisons between observation and models problematic.

KW - Alkalinity

KW - Carbon dioxide

KW - Dissociation

KW - Oceanography

UR - http://www.scopus.com/inward/record.url?scp=0032966789&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0032966789&partnerID=8YFLogxK

U2 - 10.1016/S0304-4203(99)00021-3

DO - 10.1016/S0304-4203(99)00021-3

M3 - Article

VL - 65

SP - 291

EP - 301

JO - Marine Chemistry

JF - Marine Chemistry

SN - 0304-4203

IS - 3-4

ER -