Evaluation of a seawater equilibrator for shipboard analysis of dissolved oceanic trace gases Original Research Article

Equilibrators have been used for years to obtain gas phase samples for measurement of the partial pressure of dissolved trace gases in near surface seawater. For air–sea exchange studies these measurements assume (1) complete and instantaneous equilibration and (2) that the water sampled is representative of the water in contact with the atmosphere. We present a simple mathematical model to evaluate the factors that determine the equilibration time constant for any gas of known solubility and to determine the deviation from equilibrium that arises from any vent flow of air into the equilibrator headspace. Both these expressions are functionally dependent on a dimensionless equilibrator coefficient ε. The model predictions were tested using measurements under field conditions utilizing two gases of widely differing solubility, carbon dioxide and carbon monoxide. For both these gases we show that the value of ε for our particular equilibrator is about 0.3–0.4. We also present a depth profile of CO in the top 5 m of the open ocean which demonstrates that seawater sampled through a shipʹs water inlet at 5 m depth is representative of surface water.