pOCO – the Tunable Laser Diode Absorption Spectroscopy (TLDAS)-based pCO2 Instrument

Understanding the ocean carbon system is critical to the prediction of its response to increased anthropogenic carbon input. Underway carbon system measurements can increase empirical observations of in situ carbon system parameters by orders of magnitude over bottle samples alone. We present the development of pOCO: a new underway partial pressure of carbon dioxide (pCO₂) instrument using Tunable Laser Diode Absorption Spectroscopy (TLDAS). pOCO measures pCO₂ by monitoring the CO₂ concentration of dissolved gases extracted from seawater flow via a Teflon AF membrane inlet. The TLDAS technique has several advantages over nondispersive infrared (NDIR) or indicator-based pCO₂ instruments. TLDAS uses a single absorption line of CO₂, so it is immune to water vapor fluctuations, eliminating the need for drying or humidity compensation, as is required in NDIR-based pCO₂ instrumentation. No consumable reagents or indicators are used, enabling future long-term deployments or deep-sea applications. Laser spectroscopic measurements of CO₂ are made using a 2 µm tunable laser in a 10.4m Herriott cell held at 2 kPa absolute pressure. Nonlinear curve fitting is used to extract the gas concentration from raw laser tuning data. The background signal is inferred in real time from non-absorbing portions of the tuning data, eliminating reliance on purging the optical cell with N₂ for background signal characterization. Field data from an August 2019 deployment on the R/V Thomas G. Thompson in the seawater flow-through system off the coast of New England will be presented.