Information on the sensors - CO2 and pH

Sensors fitted to the moorings are calibrated before and after deployments and include:

Surface Battelle mapCO2 sensor to measure the partial pressure of CO2 in air and water in remote ocean environments. This is the only autonomous sensor that analyses standard gas, air and seawater CO2, allowing for rigorous quality checks on field data. The equilibrator design also helps minimise problems due to biofouling. Before deployment the sensor is compared in a laboratory seawater test tank against a General Oceanics model 8050 pCO2 measurement system. The sensors are checked to ensure agreement with the General Oceanics system of better than 2 µmol/mol over ranges of temperatures and CO2 values expected at the mooring location. The General Oceanics system is calibrated against four CO2-in-air standard gases (0, 260, 370, 450 µmol/mol) referenced to the X2007 molar scale. From June 2012 the LI-COR 820 infrared detector for each mapCO2 system has also been tested directly in the laboratory before and after deployment using the same four WMO X2007 referenced gas standards to ensure an accuracy of < 2 µmol/mol.

Aanderra Oxygen Optode to measure dissolved oxygen concentration. The oxygen sensors are calibrated using at CSIRO, Hobart, across a range of temperature and oxygen concentrations to determine calibration coefficients for the Stern-Volmer equation (Uchida et al., 2008). The dissolved oxygen values in the calibration tank are verified using modified winkler titrations. The percent saturation is calculated from salinity and temperature using Garcia and Gordon (1992, 1993).

Isfet-pH sensor to measure pH at the total scale. The sensors is calibrated using discrete measurements of total dissolved alkalinity and total dissolved carbon dioxide.

Surface Seabird 16plus V2, for salinity and temperature measurements, designed for long-duration, fixed-site deployments. The sensors are calibrated at a NATA test facility for temperature

All sensors are calibrated before and after deployments.

From these measurements, an evaluation of the temporal variability in air-sea CO2 fluxes can be performed which can be used to assist in identifying the associated air-sea exchange mechanisms.

Other CO2 system measurements from National Reference Stations provide additional information to characterise pH and carbonate chemistry:

  • Total dissolved CO2 (TCO2) μmol/kg
  • Total alkalinity (TA)  μmol/kg
  • Salinity
  • pH – DIC calculated from TCO2 and TA