Accurate oxygen measurements on modified argo floats using in situ air calibrations

Abstract

Oxygen is an important tracer for biological processes in the ocean. Measuring changes in oxygen over annual cycles provides information about photosynthesis and respiration and their impact on the carbon cycle. Long-term, accurate oxygen measurements over wide areas are needed to determine changes in ocean oxygen content and oxygen deficient zones. Oxygen sensors have been increasingly mounted on Argo floats that profile between 2000 m and the surface. Most of these measurements are currently too inaccurate to calculate the air-sea gas flux, which is the dominant flux of oxygen in the surface ocean and typically driven by surface oxygen supersaturation states of only several percent. In this study, we present data from 17 Aanderaa oxygen optodes mounted on 11 Argo floats modified to make atmospheric measurements for calibration. Optodes measure oxygen equally well in air and water, allowing the use of atmospheric oxygen to perform on-going, in situ calibrations throughout the float lifetime. We find that it is necessary to make atmospheric measurements at night, that raising optodes higher into the air reduces variance in measurements, and that multiple measurements each time a float surfaces provide the best calibration data. Initial optode calibration on deployment has an average uncertainty of 6 0.1% (1 r) and drift can be calculated to 6 0.1% yr21. Measurable drift was determined in 10–12 optodes out of the 14 that were deployed for ∽ 2 yr. The maximum drift rate measured was 20.5% yr21, which is large enough to strongly impact calculations of air-sea oxygen fluxes.