Development and use of an optical pCO2 sensor in marine studies

Abstract

Partial pressure of CO2 (pCO2) is one of the most important parameters, which are measured in the global ocean in conjunction with ocean acidification studies. It is also a parameter of great interest to aquaculture and fish industries since CO2 in large amounts is highly toxic for animals. The requirements for pCO2 measuring systems, e.g. long-term stability, accuracy, high sampling frequency, easy maintenance, low-power consumption, are based on the demands from the industry and scientific community, and the current trends according to global sensor development initiatives.

A newly developed fluorescence lifetime based optical sensor for measuring pCO2 in water was evaluated and described (Paper 1). The advantages and drawbacks of this new technology in comparison to existing methods were discussed and compared in a number of in situ field deployments (Papers 1-3). A cross-sensitivity of the pCO2 optode to the most commonly co-existing substances in water and seawater was evaluated (Papers 1 and 3). A number of parameters, which influence the response of the pCO2 sensor, were thoroughly investigated in a specially designed experiment and assessed using a multivariate data analysis approach (Paper 1, 4). We have especially focused on describing the influence of salinity change and hydrostatic pressure on the sensor response in separate laboratory tests (Paper 4). A simplified calibration procedure for narrow ranges of pCO2 was proposed and a practically usable mathematical calibration model was elaborated and verified (Paper 4). As a result of sensor development efforts in achieving stability and accuracy of the sensor, pre-conditioning and single-point referencing procedures were proposed (Paper 1).

The developed pCO2 sensor was successfully used in a number of biogeochemical studies (Paper 2), for monitoring pCO2 levels in fish tanks (Paper 1) and for detecting CO2 leakages out of a simulated sub-seabed Carbon Capture Storage site (CCS) (Paper 3). Long-term high-temporarily resolved pCO2 data in combination with other sensor data (especially oxygen), gave us a deep insight into the governing processes (such as air-sea exchange, vertical mixing, primary production, organic mater degradation), which drive seasonal changes in the carbonate system of the Koljo Fjord, western Sweden. In the same study, stability of the sensor over a year of continuous measurements was confirmed. Due to its excellent performance, the pCO2 sensor has found its well-deserved place as a part of warning/monitoring systems in the proposed strategy for detection of CO2 leakage out of CCS sites.

The range of potential applications for the pCO2 sensor is obviously wide. This challenge stimulates further development and improvements of the sensor, especially in accuracy and tolerance to salinity changes. These tasks are being addressed and investigations are in progress.