The fugacity of carbon dioxide (fCO2) in surface seawater is measured using a General Oceanics Inc. automated system (Model 8050; figure 1, Pierrot et al 2009). Seawater is sprayed into an equilibration chamber and CO2 in the headspace gas equilibrates with the seawater. The headspace gas is pumped through a thermoelectric condenser and a nafion drying tube, before flowing through a Licor 7000 non-dispersive infrared gas analyser that is used to measure the CO2 mole fraction (XCO2) of the dried air.

The gas flow is stopped temporarily for the CO2 measurements, which are made at atmospheric pressure. A set of four reference gases that include zero-CO2 air and three CO2 –in-air gases that bracket the range of CO2 values expected in the ocean are analysed about every three hours to calibrate the gas analyser response. The standard gas concentrations are on the WMO-X2007 mole fraction scale. Atmospheric CO2 is measured after each set of the reference gas measurements by pumping clean air from outside the ship.

Sea surface salinity is measured using a thermosalinograph (Seabird Electronics SBE21 or Seabird Electronics SBE45) located next to the CO2 system (figure 2). A remote temperature sensor (Seabird Electronics SBE 38) located at the intake is used to measure sea surface temperature . The travel time between the intake and CO2 system is typically about 1 minute, with warming between the intake and instrumentation usually less than 0.3ºC. Meteorological data, salinity, SST, ships position and time are taken from the ships logging system.