National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 1998

Discrete water column measurements of CO2 fugacity and pHT in seawater: A comparison of direct measurements and thermodynamic calculations

McElligott, S., R.H. Byrne, K. Lee, R. Wanninkhof, F.J. Millero, and R.A. Feely

Mar. Chem., 60(1-2), 63–73, doi: 10.1016/S0304-4203(97)00080-7 (1998)

The NOAA Eq. Pac. CO2 system data set (≃ 2500 water samples) has been evaluated to assess the internal consistency of measurements and calculations involving CO2 fugacity and pHT. This assessment represents the first large scale field comparison of pHT and fCO2 data. Comparisons of direct discrete CO2 fugacity (fCO2) measurements with CO2 fugacity calculated from total inorganic carbon (CT), total alkalinity (AT) and spectrophotometric pH (pHT = –log[H+]T) indicate that a variety of improvements are needed in the parameter measurements and thermodynamic relationships used to relate fCO2, CT, AT and pHT in seawater. CO2 fugacity calculated from CT and pHT or AT and pHT agree with direct measurements to no better than 1%. Comparisons of measured fugacity, fCO2 (measured), and CO2 fugacity calculated from CT and pHT, fCO2(CT, pHT), indicate the the precision of fCO2 calculations is good relative to direct measurements. In contrast, due to the extreme sensitivity of fCO2 and [H+]T calculations to relatively small errors in both CT and AT, CO2 fugacity, as well as [H+]T, calculated from CT and AT are very imprecise and render comparisons with direct measurements of little use. Consequently, precise calculations of fCO2 require the use of direct pHT measurements.

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