Abstract
Abstract. Oxygen optode measurements on floats and gliders suffer from a slow time response and various sources of drift in the calibration coefficients. Based on two dual-O2 Argo floats, we show how to post-correct for the effect of the optode's time response and give an update on optode in situ drift stability and in-air calibration. Both floats are equipped with an unpumped Aanderaa 4330 optode and a pumped Sea-Bird SBE63 optode. Response times for the pumped SBE63 were derived following Bittig et al. (2014) and the same methods were used to correct the time response bias. Using both optodes on each float, the time response regime of the unpumped Aanderaa optode was characterized more accurately than previously possible. Response times for the pumped SBE63 on profiling floats are in the range of 25–40 s, while they are between 60 and 95 s for the unpumped 4330 optode. Our parameterization can be employed to post-correct the slow optode time response on floats and gliders. After correction, both sensors agree to within 2–3 µmol kg−1 (median difference) in the strongest gradients (120 µmol kg−1 change over 8 min or 20 dbar) and better elsewhere. However, time response correction is only possible if measurement times are known, i.e., provided by the platform as well as transmitted and stored with the data. The O2 in-air measurements show a significant in situ optode drift of −0.40 and −0.27 % yr−1 over the available 2 and 3 years of deployment, respectively. Optode in-air measurements are systematically biased high during midday surfacings compared to dusk, dawn, and nighttime. While preference can be given to nighttime surfacings to avoid this in-air calibration bias, we suggest a parameterization of the daytime effect as a function of the Sun's elevation to be able to use all data and to better constrain the result. Taking all effects into account, calibration factors have an uncertainty of 0.1 %. In addition, in-air calibration factors vary by 0.1–0.2 % when using different reanalysis models as a reference. The overall accuracy that can be achieved following the proposed correction routines is better than 1 µmol kg−1.
Highlights
While oceanic oxygen measurements of the last century were mostly based on Winkler titrations of discrete water samples or profiles acquired with CTD-mounted electrochemical oxygen sensors, such observations rely increasingly on O2 optode sensors (Tengberg et al, 2006) deployed on autonomous platforms
We want to refine the findings of Bittig et al (2014) on the optode time response and of Bittig and Körtzinger (2015) on in-air measurements and in situ drift stability
Shipboard CTD–O2 casts with optodes, as well as glider and float data in different ocean regions, served Bittig et al (2014) to characterize the response time τ as well as the flow / lL regime on these profiling platforms
Summary
While oceanic oxygen measurements of the last century were mostly based on Winkler titrations of discrete water samples or profiles acquired with CTD-mounted electrochemical oxygen sensors, such observations rely increasingly on O2 optode sensors (Tengberg et al, 2006) deployed on autonomous platforms. Körtzinger: Update on response times, in-air measurements, and in situ drift careful pre-/post-deployment calibrations for short deployments, e.g., of gliders. It is timely and useful to revisit the foundations of both pumped and unpumped O2 optode behavior on autonomous, profiling platforms, to ensure optimal data postprocessing and best data quality. In this technical note, we want to refine the findings of Bittig et al (2014) on the optode time response and of Bittig and Körtzinger (2015) on in-air measurements and in situ drift stability
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