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% yr 21 . 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% yr 21 , which is large enough to strongly impact calculations of air-sea oxygen fluxes.

Oxygen Optode Sensors: Principle, Characterization, Calibration, and Application in the Ocean

“…Johnson et al (2015) obtained both significant positive and negative drift for a fleet of 29 floats with a range of −0.9 to +1.3 % year −1 and an insignificant mean of +0.2 % year −1 , from which they concluded that optodes are stable in-situ on average. Finally, Bushinsky et al (2016) reported a significant optode drift for 10-12 out of 14 optodes with both positive and negative drift and a significant mean trend of −0.12 % year −1 .…”

“…Nicholson and Feen (2017), in fact, confirm a positive zero intercept based on field data. Furthermore, Bushinsky et al (2016), Drucker and Riser (2016), and Nicholson and Feen (2017) show that a slope correction alone might be insufficient at low O 2 levels, while it is appropriate near 100 % O 2 saturation.…”

“…As an alternative to O 2 at depth, optode in-air measurements were proposed by Körtzinger et al (2005) and first used for calibration by Fiedler et al (2013). Their operational potential to calibrate optodes was demonstrated subsequently by Bittig and Körtzinger (2015), Johnson et al (2015), and Bushinsky et al (2016) on floats and more recently also on gliders (Nicholson and Feen, 2017).…”

Insight into chemical, biological, and physical processes in coastal waters from dissolved oxygen and inert gas tracers

“…Precise and accurate measurements of O 2 concentration are currently achieved a variety of ways including with chemical sensors and traditional wet chemistry methods (titrations) (Carpenter, 1965;Johnson et al, 2015;Bushinsky et al, 2016). In recent years, investigators have improved the stability and calibration procedures for O 2 sensors so that they can operate autonomously for long deployments (Bushinsky and Emerson, 2013;Bittig and Körtzinger, 2015;Johnson et al, 2015;Bushinsky et al, 2016).…”

“…In recent years, investigators have improved the stability and calibration procedures for O 2 sensors so that they can operate autonomously for long deployments (Bushinsky and Emerson, 2013;Bittig and Körtzinger, 2015;Johnson et al, 2015;Bushinsky et al, 2016). However, regardless of the precision at which O 2 is measured, it is still necessary to separately quantify the biological and physical fluxes of O 2 in order to calculate the rate of net community O 2 production.…”