CO2SYS

CO2SYS is a widely used program for calculating the state of the carbonate system in seawater and freshwater. It uses a set of thermodynamic equations to interrelate measurable parameters such as pH, total alkalinity (TA), dissolved inorganic carbon (DIC), and partial pressure of carbon dioxide (pCO2), allowing researchers to compute the full suite of carbonate chemistry variables from any two input parameters, along with temperature, salinity, and other relevant constants.

Overview:

CO2SYS is fundamentally a Fortran program but is often accessed through user-friendly interfaces in languages like MATLAB, Python, and R. It allows users to input measured or estimated values for two carbonate system parameters, along with ancillary information like temperature, salinity, silicate and phosphate concentrations, and relevant pressure. Based on these inputs, CO2SYS calculates other carbonate system variables, including:

• pH (expressed on various scales, such as total, seawater, and free pH scales)
• pCO2 (partial pressure of carbon dioxide)
• fCO2 (fugacity of carbon dioxide)
• Dissolved inorganic carbon (DIC or TCO2)
• Total alkalinity (TA)
• Concentrations of carbonate, bicarbonate, and carbonic acid ions
• Saturation states of calcium carbonate minerals (aragonite and calcite)

Functionality:

The program relies on a set of equilibrium constants for the reactions governing the carbonate system. Users can select from a variety of published constants, each derived from different experimental datasets and potentially more applicable to specific environmental conditions. These constants relate to the dissociation of carbonic acid, boric acid, silicic acid, phosphoric acid, and the solubility products of aragonite and calcite. CO2SYS also accounts for the effects of pressure on these constants, allowing calculations to be performed at different depths in the ocean. The program also utilizes equations to determine the concentration of borate as a function of salinity.

Applications:

CO2SYS is used extensively in a variety of fields, including:

• Oceanography: To study ocean acidification, carbon cycling, and the effects of changing CO2 levels on marine organisms.
• Limnology: To investigate the carbonate chemistry of lakes and rivers.
• Climate Science: To model the exchange of CO2 between the atmosphere and the oceans.
• Aquaculture: To monitor and control water quality in aquaculture systems.
• Environmental Science: To assess the impacts of pollution on aquatic ecosystems.

Limitations:

While CO2SYS is a powerful tool, it is essential to recognize its limitations:

• Accuracy of Input Data: The accuracy of the calculated results depends heavily on the accuracy of the input parameters. Errors in the measurement of pH, TA, or DIC can propagate and affect the values of other calculated variables.
• Choice of Constants: The selection of appropriate equilibrium constants is crucial. Different constants may yield different results, especially under extreme conditions (e.g., high salinity or temperature). Understanding the provenance and applicability of each set of constants is essential.
• Assumptions: The model relies on a number of simplifying assumptions, such as the assumption that the system is in equilibrium. In reality, biological processes and mixing can create disequilibrium conditions.
• Trace Elements: CO2SYS primarily focuses on the major components of the carbonate system and generally does not account for the influence of trace elements or organic matter on alkalinity.

Further Development:

CO2SYS continues to be refined and updated to incorporate new data and improve its accuracy. Researchers are actively working to develop more comprehensive models that account for the complexities of natural aquatic systems.