Characterisation of pH and pCO2 optodes towards high resolution in situ ocean deployment

Abstract

Anthropogenic emissions of carbon dioxide (CO2) into the atmosphere results in climate change and perturbations of the oceanic carbonate system. Atmospheric CO2 concentrations have increased from pre-industrial times from land use changes, fossil fuel combustion and increasing cement production. The ocean is currently a major sink for this anthropogenic CO2, increasing pCO2 and dissolved inorganic carbon concentrations and decreasing pH and carbonate ion concentrations. This has potential biogeochemical and ecosystem consequences. To monitor the ocean’s uptake of CO2, identify regions of enhanced carbonate system changes, and observe the effectiveness of CO2 emission mitigation strategies, high quality pCO2 and pH measurements with good temporal and spatial coverage are required. This thesis presents the characterisation of novel pH and pCO2 sensing spots which use fluorescent detection techniques, and evaluates their potential for in situ seawater deployments.

The optode sensing spots were illuminated with low intensity light (0.2 mA, 0.72 mWatt), and the time-domain dual lifetime referencing analysis technique was applied to maximise the lifetime of the spot and reduce overall power consumption of the measurement (1.8 Watt). The same hardware was used for both the pH and pCO2 spot interrogation, thereby demonstrating the versatility of the optode. After initial calibrations over typical seawater ranges for pH and pCO2, (pH range 7.6 – 8.2 and pCO2 range 280-1000 ?atm) the temperature and salinity dependence of the spots was evaluated. The pH displayed both a temperature (- 0.046 pH / °C from 5-25 °C) and salinity dependency (-0.01 pH /psu over 5-35), while the pCO2 sensor showed only a temperature dependence (-39 ?atm °C-1). Precision of the repeated measurement of certified reference material was 0.0074 pH and 0.8 ?atm for the pH and pCO2 optodes, respectively. The optodes were deployed as shipboard autonomous underway systems in the North Atlantic and Southern Ocean, both important CO2 sink regions. The successful pCO2 data deployment produced good quality data that has been used to quantify the spatial controls on carbonate chemistry and air-sea CO2 fluxes in the North Atlantic, and evaluate the effect of sampling rate and interpolation method on the determined flux. This thesis demonstrates the potential of these small sensors, with the ultimate aim to deploy them on ARGO floats and gliders.

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