pC02 is the partial pressure of CO2 in a liquid or gas. In oceanography, it is used to investigate carbon dioxide (CO2) dissolved in the surface layers of the ocean in order to better understand changes in ocean carbon chemistry and ocean acidification due to enhanced atmospheric CO2.

The RV Celtic Explorer has a pCO2 Underway System. pC02 data was collected as part of the CE17017 Irish Groundfish Survey Leg 2 survey on board the RV Celtic Explorer in 2017 by the Marine Institute (MI) in the North East Atlantic Ocean.

Instrument information: - SST (± 0.001 deg C): SST was measured with a thermosalinograph from Seabird (SBE21) with an external SBE38 which was installed close to the seawater intake. Both instruments were calibrated approximately every 12 months.

-Equilibrator temp (±0.05 °C): Equilibrator temperature is measured by a Fluke Hart 1523, wich is known to be stable over a longer period. It was calibrated in 2016.

-SSS (± 0.5 PSU): SSS was measured with a thermosalinograph from Seabird (SBE21) which was calibrated approximately every 12 months.

-Atmospheric press. (± 0.1 mbar): Atmospheric pressure was measured using a Druck barometer with a precision of 0.2%. The sensor is installed at approximately 5m height and pressure is corrected to sea level.

-Equilibrator press (± 2 mbar): The equilibrator is open to the atmosphere. Since no absolute pressure sensor was installed the pressure from the atmospheric pressure sensor was used. A SETRA difference pressure sensor is attached to the equilibrator.

-pCO2/fCO2 (±2µatm):
Accuracy of pCO2/fCO2 data was recalculated to be better than 2 µatm. IR sensor: Licor 7000, calibrated with 3 non-zero standard gases. Standard gases: Manufactued - Air Products, Calibrated - Mace Head Research Station (198.29 399.28 606.52]) Water flow rate: 2-3 L min-1 Gas flow rate: 80 - 180 mL min-1 (equilibrator and standard gas flow was 100 mL min-1, atm flow rate was 180 mL min-1)

A General Oceanics pCO2 system (model 8050) was used for pCO2 measurements. The system is described in detail in Pierrot et al.(2009). Surface water is pumped continuously from the intake to the equilibrator. The equilibrator contains a water spray head, and as the water flows through it the dissolved CO2 equilibrates with the headspace. The headspace is dried and xCO2 is determined by an infrared sensor. Calculations were performed following Pierrot (2009) and are described in detail in Steinhoff (2010).

Remarks during data reduction: - using computer time instead of GPS time. Difference below 2 seconds - condenser didn't work properly. Temperature was between 10 and 14 degC - using deck box pressure for atm pressure and lab pressure - 500 min running mean for atm xCO2 and fCO2 - 12060 data points