A new method for continuous measurements of O2 in surface water in combination with pCO2 measurements: Implications for gas phase equilibration

An autonomous system for the recording of the surface water pCO2 in the Baltic Sea was complemented by a module for continuous O2 measurements. An optical O2 sensor was used and in analogy to the determination of the pCO2, the pO2 was determined in the headspace of an equilibrator. By this procedure the automatic periodic calibration was considerably facilitated and biofouling was avoided. Uncertainties were associated with the use of a vented equilibrator. Due to the continuous pressure equalization with the ambient atmosphere, the headspace partial pressure of any gas (e.g. pO2, pCO2) reflects a steady state that may deviate substantially from the equilibrium. To calculate the true equilibrium pO2 and thus the O2 concentration in the water sample, we derived an equation that is based on the measurement of the headspace O2 mol fraction, xO2, and on the assumption that the surface water was saturated with atmospheric N2 and Ar. Model calculations indicated that deviations from this assumption are less then 2% for most of the year. The combined pO2/pCO2 measurement system was deployed in July 2005 on a cargo ship that commutes regularly between the southwestern Baltic Sea and the Gulf of Finland. First results showed a reasonable agreement with O2 data obtained at a measurement platform in the vicinity of the shipʹs track. The spatial resolution of the O2 measurements was similar to that of the pCO2 system and allowed the identification of small scale (< 1 nautical mile) biogeochemical and hydrographical inhomogeneities.