In‐air one‐point calibration of oxygen optodes in underway systems

Meike Becker,Are Olsen,Gilles Reverdin

doi:10.1002/lom3.10423

Abstract

AbstractShips of opportunity are a frequently used platform in surface ocean carbon observations and estimating the annual ocean carbon sink. For understanding the drivers behind changes in the ocean carbon system, oxygen measurements alongside the carbon dioxide measurements can be a valuable tool. We developed an in‐air calibration system for oxygen optodes in underway systems. The regular measurements of atmospheric oxygen enable us to correct for sensor drift and biofouling. This new system can help to obtain reliable oxygen data from underway applications, especially if the vessel is not easily accessible and a frequent recalibration of the optode is not feasible.

Highlights

The solution for high precision measurements of carbon dioxide (CO2) is a rather large system based on the absorption in the infrared part of the radiation spectrum by a sample of headspace gas that has been brought into equilibration with a large volume of seawater (Takahashi 1961), returning readings of the partial pressure of CO2 in water
In situ air calibration routines were developed for float-mounted optodes, resulting in an accuracy of 0.2%
We developed a system for in-air measurements of optodesin underway mode for use on Ships of opportunity (SOOP) vessels that operate a pressure of CO2 (pCO2) system or have similar installations

Summary

Introduction

The solution for high precision measurements of carbon dioxide (CO2) is a rather large system based on the absorption in the infrared part of the radiation spectrum by a sample of headspace gas that has been brought into equilibration with a large volume of seawater (Takahashi 1961), returning readings of the partial pressure of CO2 (pCO2) in water. Optodes are much smaller than the pCO2 systems, have less power consumption, do not use external standards, and are widely used for profiling applications on floats or on gliders (Körtzinger et al 2005; Bushinsky et al 2016; Johnson et al 2017) These oxygen measurements are an important component of the biogeochemical Argo program (Gruber et al 2010; Roemmich et al 2019). While the predeployment drift rate can be as high as several percent per year (D’Asaro and McNeil 2013), the sensor drift in many factory calibrated optodes in float applications was found to be about 0.6% yr−1 (Bittig et al 2018a) With this drift, recalibrating the sensor on an annular basis will not be enough to reach a target precision as within the Argo program of 0.5% (Gruber et al 2010), especially if a potentially nonlinear drift caused by biofouling is added upon the sensor drift.

Methods

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Discussion

Conclusion