Ocean CO2 Measurements and Calculations

Timothy J. Lueker, Andrew G. Dickson, Charles D. Keeling
15 December 1999

The partial pressure of carbon dioxide in the ocean’s surface waters, precisely expressed as the fugacity (ƒCO2) is determined from dissolved inorganic carbon (DIC) and total alkalinity (TA) , and the first and second dissociation constants of carbonic acid (K1 and K2) The original measurements of K1 and K2 reported 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. Limnol. Oceanogr. 18, 897–907] are reformulated to give equations for pK1 and pK2 ( pK = -log 10 K) as a function of seawater temperature and salinity, consistent with the ‘‘total hydrogen ion’’ concentration scale:

pK1 = 3633.86/T – 61.2172 + 9.67770lnT – 0.011555 S + 0.0001152 S2

pK2 = 471.78/T + 25.9290 – 3.16967lnT – 0.01781 S + 0.0001122 S2

By equilibrating solutions of seawater with gas mixtures of known composition, we demonstrate that the above formulationsof K1 and K2 give calculated ƒCO2 values that agree with equilibrated values to 0.07 ± 0.50% (95% confidence interval, ƒCO2 up to 500 μatm) . Formulations of K1 and K2 based on other studies resulted in calculated ƒCO2 values approximatel- 10% lower than the measurements. E+uilibrations at ƒCO2 above 500 μatm yielded measured ƒCO2 values higher than calculated values by on average 3.35 ± 1.22% (95% confidence interval)) The cause for the ƒCO2 dependence of the results is not known.

The uncertainties in pK1 and pK2 were combined with the analytical uncertainties typical of contemporary measurements of DIC and TA to reveal the expected reliability of seawater ƒCO2 calculated from these parameters. For example, an uncertainty of 1.0 μmol kg-1 in DIC and 2 μmol kg-1 in TA (1 standard deviation (s.d.)) will result in uncertainty of the calculated ƒCO2 of 1% or ±3.5 μatm at 350 μatm (1 s.d.).

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