Accuracy of CO2 System Calculations Improved with New Spectrophotometric K2 Model for Seawater

The carbonate dissociation constants, K₁ and K₂, describe the relative concentrations of CO₂ species (CO₂, HCO₃^-, and CO₃^2-) in terms of solution pH. Uncertainties in the current K₁ and K₂ parameterizations contribute significantly to the uncertainties in the CO₂ system calculations. Novel spectrophotometric pH methods were developed to experimentally determine K₂ based on the potentiometric procedures of Mehrbach et al. (1973). This work was used to generate a new pK₂ model for both open and coastal ocean conditions. The residuals generated using this model (experimental pK₂ observations – model pK₂ predictions) are substantially reduced relative to previous pK₂ parameterizations, affirming the enhanced precision of this new method.

The accuracy of this new model was assessed using large, repeat hydrography data sets where measurements of at least three CO₂ parameters (A_T, C_T, pH, pCO₂) for each water sample enabled comparisons of measured and calculated values. Using the K₂ model produced in our work, the mean differences between measured and calculated values of A_T, C_T, pH, and pCO₂ are significantly reduced compared to the K₂ model of Lueker et al. (2000). Comparisons of in situ pH and pCO₂ calculated with K₂ (this work) and K₂ (Lueker et al., 2000) show significant differences. Discrepancies are most pronounced at low temperatures, implying that deep ocean pH calculated from A_T and C_T can be substantially different depending on the K₁ and K₂ model used for calculations. Overall, the new pK₂ model, based on spectrophotometric pH methods, shows improved precision and more internally consistent CO₂ system calculations relative to previous parameterizations.