Solutions Out of Thin Air: Assessment of a Simplified Atmospheric Calibration of Oxygen Optodes for Spray Gliders

Advances in autonomous biogeochemical sensing and platform technology have increased our spatial and temporal data collection capabilities, but reliable and practical calibration of such sensors remains a crucial challenge. It has been demonstrated that oxygen optode sensors can be accurately calibrated in situ to several tenths of a percent by making measurements in air due to the virtually constant mole fraction of oxygen in the atmosphere. However, optodes that are in a pumped flow stream such as the SBE63 equipped on Spray Gliders cannot be exposed to the atmosphere while deployed. Alternatively, the optodes could be air-calibrated pre- and post-deployment. Here, we tested a simplified air-calibration protocol utilizing inexpensive components without temperature control and compared the results to those obtained from Winkler titrations. We conducted three experiments involving: 1) measurements in a laboratory-controlled environment, 2) an underway line during a week-long cruise, and 3) a Spray underwater glider deployment in Central California. In all cases, the simplified air calibration agreed with Winkler titrations to better than 1%: 0.46% in the laboratory, 0.23% and 0.64% for the underway, and 0.80% on the glider within the surface mixed layer (0 - 20 m). Larger errors were observed when there was active evaporation in the optode flow cell, causing either changes in humidity or air temperature during air calibration. This simple calibration approach can be applied to optodes equipped on a wide variety of platforms.