Graduation Year

2024

Document Type

Thesis

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Marine Science

Major Professor

Robert H. Byrne, Ph.D.

Co-Major Professor

Don Chambers, Ph.D.

Committee Member

Kimberly Yates, Ph.D.

Keywords

dissociation constants, CO2 system, modeling, spectrophotometry

Abstract

pH is typically measured at a constant temperature and, as needed, converted to in-situ temperature using a system of thermodynamic equations. Using this equilibrium approach, adjustment of pH to in situ conditions requires two measured CO2 system parameters (pH, total alkalinity, dissolved inorganic carbon, or CO2 fugacity). Currently, when pH is the only measured CO2 system parameter, linear models are used to adjust the pH to in-situ temperature. Although many studies have attempted to optimize CO2 equilibrium constants (K1 and K2) for the CO2 system over a range of conditions, very little prior work has experimentally examined which CO2 system models best predict changes in pH with changing temperature (∆pH/∆t). In the present work, theoretical predictions of ∆pH/∆t are examined using CO2sys. Calculations involved the use of five different sets of dissociation constants to determine how the ∆pH/∆t varies with salinity (10–35) and temperature (0–40°C), for seawater whose pH measured at 25°C (pH25) ranged between 7.2 and 8.2. It is shown that, in a closed system, the dependence of ∆pH/∆t on temperature is approximately linear, and that the dependence on salinity for open-ocean conditions is relatively weak. Notably, ∆pH/∆t is strongly dependent on the ratio of total alkalinity (AT) to total inorganic carbon (CT). The pH of a sample measured at constant temperature, such as 25°C, serves as an excellent proxy for AT/CT whereupon ∆pH/∆t can be well-modeled solely in terms of pH25 (i.e., pH at 25°C), temperature and salinity. It is, furthermore, demonstrated that ∆pH/∆t modeled in this manner is substantially independent of AT when 1800 < AT < 2400 µmol kg¬-1. In addition to theoretical models of ∆pH/∆t behavior, direct experimental determinations of ∆pH/∆t were conducted spectrophotometrically that allow assessment of the consistency between CO2sys models and measured values over a range of salinity (10–36¬), temperature (15–40°C) and pH at 25°C (7.2–8.2). Finally, an improved model of ∆pH/∆t is presented that requires inputs of only salinity, temperature, and pH25.

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