Surface partial pressure of carbon dioxide in sea water
The world's oceans readily exchange carbon dioxide (CO2) with the atmosphere. The CO2 dissolves in water to an extent determined by its partial pressure and the chemical reactions of the dissolved carbon dioxide with other solutes. The partial pressure of carbon dioxide (PCO2) is the gas phase pressure (i.e. in the air above a waterway) of carbon dioxide which would be in equilibrium with the dissolved carbon dioxide.