Technical Note: Oxygen Optodes on Profiling Platforms: Update on Response Times, In-Air Measurements, and In-Situ Drift

Abstract. Oxygen optode measurements on floats and gliders suffer from a slow time response and sensors that are or eventually run out of calibration. 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 after Bittig et al. (2014). Also their 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 are in the range of 25–40 s, while they are between 60–95 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 minutes 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 −0.4 % yr−1 and −0.2 % yr−1, respectively. Optode in-air measurements are systematically biased high during mid-day 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 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 reference. The overall accuracy that can be achieved following the proposed correction routines is <1 μmol kg−1.