FY 1998

Discrete water column measurements of CO₂ fugacity and pH_T 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 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 the 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.