Tackling Oxygen Optode Drift: Near-Surface and In-Air Oxygen Optode Measurements on a Float Provide an Accurate in Situ Reference

Abstract

A yet unexplained drift of (some) oxygen optodes during storage/transport and thus significant deviations from factory/laboratory calibrations have been a major handicap for autonomous oxygen observations. Optode drift appears to be systematic and is predominantly a slope effect due to reduced oxygen sensitivity. A small contribution comes from a reduced luminophore lifetime, which causes a small positive offset. A reliable in situ reference is essential to correct such a drift. Traditionally, this called for a ship-based reference cast, which poses some challenges for opportunistic float deployments. This study presents an easily implemented alternative using near-surface/in-air measurements of an Aanderaa optode on a 10-cm stalk and compares it to the more traditional approaches (factory, laboratory, and in situ deployment calibration). In-air samples show a systematic bias depending on the water saturation, which is likely caused by occasional submersions of the standardheight stalk optode. Linear regression of measured in-air supersaturation against in-water supersaturation (using ancillary meteorological data to define the saturation level) robustly removes this bias and thus provides a precise (0.2%) and accurate (1%) in situ correction that is available throughout the entire instrument's lifetime.

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

“…A linear O 2 dependence of the observed drift was first shown with repeated optode laboratory calibrations in Bittig et al (2012) and later refined in Bittig and Körtzinger (2015). Bittig and Körtzinger (2015) also proposed a mechanistic explanation for the O 2 response drift, being mainly (1) due to reduced quenching of the luminophore (i.e., a reduced O 2 sensitivity), and (2) a counteracting, destabilizing effect on the luminophore itself.…”

“…Bittig and Körtzinger (2015) also proposed a mechanistic explanation for the O 2 response drift, being mainly (1) due to reduced quenching of the luminophore (i.e., a reduced O 2 sensitivity), and (2) a counteracting, destabilizing effect on the luminophore itself. The first expressed itself as a factor on O 2 , whereas the second manifested itself as a (positive) oxygen intercept at zero O 2 .…”

“…This is linearly related with the oxygen content (Equations 2, 3). It can be caused by a degraded O 2 accessibility to the luminophore (e.g., due to migration of the luminophore) or a decreased O 2 diffusivity within the sensing foil, which reduces the likelihood of collision between O 2 and luminophore molecules and thus the quenching (Bittig and Körtzinger, 2015). In parallel, the lifetime 0 of the luminophore becomes smaller.…”

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

“…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.…”

“…The optode was not calibrated because equipment for Winkler titrations was not available in the field and the optodes drift during storage/transport (Bittig and Körtzinger, 2015;Johnson et al, 2015). Although the profiling CTD had an O 2 sensor, it was malfunctioning during the time-series and the data from it could not be used.…”