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

The NOAA Eq. Pac. CO2 system data set (similar or equal to 2500 water samples) has been evaluated to assess the internal consistency of measurements and calculations involving CO2 fugacity and pH(T). This assessment represents the first large scale field comparison of pH(T) and f(CO2) data. Comparisons of direct discrete CO2 fugacity (f(CO2)) measurements with CO2 fugacity calculated from total inorganic carbon (C-T), total alkalinity (A(T)) and spectrophotometric pH (pH(T) = -log[H+](T)) indicate that a variety of improvements are needed in the parameter measurements and thermodynamic relationships used to relate f(CO2), C-T, A(T) and pH(T) in seawater. CO2 fugacity calculated from C-T and pH(T) or A(T) and pH(T) agree with direct measurements to no better than 1%. Comparisons of measured fugacity, f(CO2) (measured), and CO2 fugacity calculated from C-T and pH(T), f(CO2) (C-T, pH(T)), indicate that the precision of f(CO2) calculations is good relative to direct measurements. In contrast, due to the extreme sensitivity of f(CO2) and [H+](T) calculations to relatively small errors in both C-T and A(T), CO2 fugacity, as well as [H+](T), calculated from C-T and A(T) are very imprecise and render comparisons with direct measurements of little use. Consequently, precise calculations of f(CO2) require the use of direct pH(T) measurements. (C) 1998 Elsevier Science B.V.