A method for in-situ calibration of Aanderaa oxygen sensors on surface moorings

Abstract

Oxygen-based mass balance estimates of net biological production can be calculated using accurate in-situ O2 measurements. All presently available oxygen sensors drift if deployed for periods of months in the sun-lit surface ocean and must be calibrated to maintain accuracy. We have developed an in-situ calibration system in which Aanderaa optode oxygen sensors periodically determine the pO2 of air. In the moored application described here, the optode is housed in a subsurface chamber on a surface mooring where it alternately measures the pO2 of the water and air. During calibration intervals, atmospheric air is pumped into the optode housing, displacing the water. Subsequent oxygen measurements of the air inside the housing provide a measurement of the atmospheric pO2, which can then be compared to expected atmospheric pO2 based on measured pressure and temperature. We present calibrations for one optode in both water (against Winkler titrations) and air (against atmospheric oxygen). The optode response in lab experiments was different in air than in water, measuring low and exhibiting temperature dependence (−1% at 20°C and −2.3% at 3°C). Initial field tests in a freshwater inlet confirmed this offset and showed a drift of −0.5% over 3months of deployments.