Discrete Water Column Measurements of CO₂ Fugacity and pH_T in Seawater: A Comparison of Direct Measurements and Thermodynamic Calculations

Authors

S. McElligott, University of South Florida
R. H. Byrne, University of South FloridaFollow
K. Lee, University of Miami
R. Wanninkhof, Atlantic Oceanographic and Meteorological Laboratories, Miami
F. J. Millero, University of Miami
R. A. Feely, Pacific Marine Environmental Laboratory, NOAA

Document Type

Article

Publication Date

1998

Keywords

Seawater, Carbon dioxide, Fugacity, Spectrophotometric pH, Measurements, Thermodynamic calculations

Digital Object Identifier (DOI)

https://doi.org/10.1016/S0304-4203(97)00080-7

Abstract

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 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.

Was this content written or created while at USF?

Yes

Citation / Publisher Attribution

Marine Chemistry, v. 60, issue 1-2, p. 63-73

Scholar Commons Citation

McElligott, S.; Byrne, R. H.; Lee, K.; Wanninkhof, R.; Millero, F. J.; and Feely, R. A., "Discrete Water Column Measurements of CO₂ Fugacity and pH_T in Seawater: A Comparison of Direct Measurements and Thermodynamic Calculations" (1998). Marine Science Faculty Publications. 1717.
https://digitalcommons.usf.edu/msc_facpub/1717