National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 1999

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

Wanninkhof, R., E. Lewis, R.A. Feely, and F.J. Millero

Mar. Chem., 65(3–4), 291–301, doi: 10.1016/S0304-4203(99)00021-3 (1999)

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.

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