Characterization of a Time-Domain Dual Lifetime Referencing pCO2 Optode and Deployment as a High-Resolution Underway Sensor across the High Latitude North Atlantic Ocean

Abstract

The ocean is a major sink for anthropogenic carbon dioxide (CO 2 ), with the CO 2 uptake causing changes to ocean chemistry. To monitor these changes and provide a chemical background for biological and biogeochemical studies, high quality partial pressure of CO 2 (pCO 2 ) sensors are required, with suitable accuracy and precision for ocean measurements. Optodes have the potential to measure in situ pCO 2 without the need for wet chemicals or bulky gas equilibration chambers that are typically used in pCO 2 systems. However, optodes are still in an early developmental stage compared to more established equilibrator-based pCO 2 systems. In this study, we performed a laboratory-based characterization of a time-domain dual lifetime referencing pCO 2 optode system. The pCO 2 optode spot was illuminated with low intensity light (0.2 mA, 0.72 mW) to minimize spot photobleaching. The spot was calibrated using an experimental gas calibration rig prior to deployment, with a determined response time (τ 63 ) of 50 s at 25 • C. The pCO 2 optode was deployed as an autonomous shipboard underway system across the high latitude North Atlantic Ocean with a resolution of ca.10 measurements per hour. The optode data was validated with a secondary shipboard equilibrator-based infrared pCO 2 instrument, and pCO 2 calculated from discrete samples of dissolved inorganic carbon and total alkalinity. Further verification of the pCO 2 optode data was achieved using complimentary variables such as nutrients and dissolved oxygen. The shipboard precision of the pCO 2 sensor was 9.5 µatm determined both from repeat measurements of certified reference materials and from the standard deviation of seawater measurements while on station. Finally, the optode deployment data was used to evaluate the physical and biogeochemical controls on pCO 2 .

Fluorescence intensity and fluorescence lifetime measurements of various carbon dots as a function of pH

“…To get around this problem, architectures revolving around the use of sensor films containing multiple chemical species have been proposed. For example, the dual-lifetime referencing (DLR) method has a well-known pH-sensing architecture 35 – 38 . The DLR architecture combines a pH-sensitive fluorophore with a long-lived luminescent reference, where the pH-sensitive indicator’s fluorescence properties vary with pH.…”

The analysis of dissolved inorganic carbon in liquid using a microfluidic conductivity sensor with membrane separation of CO2

“…At present, CO 2 measurements are typically carried out by coulometry using large-volume sensors on research ships, according to Standard Operating Protocols (Dickson et al 2007). Although both pCO 2 and pH are very sensitive to temperature and pressure, they can be measured away from sampling depths under carefully controlled conditions, and while microfluidic miniaturisation approaches for in situ surface measurements are currently being developed, complementary autonomous measurements of DIC or A T would be preferred (Rérolle et al 2018;Clarke et al 2017). Given the technical challenges, however, such approaches have yet to be realised (Byrne 2014).…”

The Analysis of Dissolved Inorganic Carbon in Liquid Using a Microfluidic Conductivity Sensor with Membrane Separation of CO2

“…As both pCO2 and pH are very sensitive to temperature and pressure, they must be measured in-situ and while microfluidic miniaturisation approaches for in-situ surface measurements are currently being developed, autonomous measurements of DIC or AT would be preferred. 4,5 Given the technical challenges, however, such approaches have yet to be realised. 6 Several methods of Total CO2 (TCO2 or DIC) determination have been reported, e.g.…”