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The partial pressure of carbon dioxide in the ocean's surface waters, precisely expressed as the fugacity ( f CO 2 ) is determined from dissolved inorganic carbon (DIC) and total alkalinity (TA), and the first and second dissociation constants of carbonic acid, K 1 and K 2 . The original measurements of K 1 and K 2 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 p K 1 and p K 2 (p K =−log 10 K ) as a function of seawater temperature and salinity, consistent with the “total hydrogen ion” concentration scale: p K 1 =3633.86/T−61.2172+9.67770 ln T−0.011555 S+0.0001152 S 2 p K 2 =471.78/T+25.9290−3.16967 ln T−0.01781 S+0.0001122 S 2 By equilibrating solutions of seawater with gas mixtures of known composition, we demonstrate that the above formulations of K 1 and K 2 give calculated f CO 2 values that agree with equilibrated values to 0.07±0.50% (95% confidence interval, f CO 2 up to 500 μatm). Formulations of K 1 and K 2 based on other studies resulted in calculated f CO 2 values approximately 10% lower than the measurements. Equilibrations at f CO 2 above 500 μatm yielded measured f CO 2 values higher than calculated values by on average 3.35±1.22% (95% confidence interval). The cause for the f CO 2 dependence of the results is not known. The uncertainties in p K 1 and p K 2 were combined with the analytical uncertainties typical of contemporary measurements of DIC and TA to reveal the expected reliability of seawater f CO 2 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 f CO 2 of 1% or ±3.5 μatm at 350 μatm (1 s.d.). |
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