Ocean pCO2 calculated from dissolved inorganic carbon, alkalinity, and equations for K1 and K2: validation based on laboratory measurements of CO2 in gas and seawater at equilibrium

Abstract

Abstract 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.).