DISCRETE WATER COLUMN MEASUREMENTS OF CO2 FUGACITY AND PH(T) 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 meas urements and calculations involving CO2 fugacity and pH(T). This asses sment represents the first large scale field comparison of pH(T) and f (CO2) data. Comparisons of direct discrete CO2 fugacity (f(CO2)) measu rements 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 paramete r 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 th an 1%. Comparisons of measured fugacity, f(CO2) (measured), and CO2 fu gacity calculated from C-T and pH(T), f(CO2) (C-T, pH(T)), indicate th at the precision of f(CO2) calculations is good relative to direct mea surements. 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), C O2 fugacity, as well as [H+](T), calculated from C-T and A(T) are very imprecise and render comparisons with direct measurements of little u se. Consequently, precise calculations of f(CO2) require the use of di rect pH(T) measurements. (C) 1998 Elsevier Science B.V.