pCO2 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. pCO2 data was collected as part of the CE18007 INFOMAR Seabed Mapping survey of the Celtic Sea on board the RV Celtic Explorer in April"May 2018. 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 - 220 mL min-1 (equilibrator gas flow was 220 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). Collect pC02 data (partial pressure of CO2 in a liquid or gas) to investigate carbon dioxide (CO2) dissolved in the surface layers of the ocean which contributes towards our understand of changes in ocean carbon chemistry and ocean acidification due to enhanced atmospheric CO2.