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New pH Sensor for Monitoring Ocean Acidification

By Dr. Melchor González-Dávila • Dr. J. Magdalena Santana-Casiano • Hervé Prêcheur-Massieu

Aerial picture of the VULCANO buoy with atmospheric and communication sensors.
The increase in the production and emission of anthropogenic CO2 and its absorption by the oceans leads to a reduction in oceanic pH, a process referred to as ocean acidification, which affects many other physicochemical processes. Atmospheric CO2 is expected to continue its increase, and consequently the chemical changes will likely continue well into the future, affecting the ocean biogeochemical cycling. In order to characterize the ocean's chemical and ecosystem-related changes, examination of CO2 system parameters over a wide range of temporal and spatial scales is necessary. Shipboard analyses in oceanic time series conducted during irregular ocean expeditions, which lasted between a fortnight and a month, have provided most of our understanding of recent trends in the oceanic CO2 system. However, our ability to make frequent autonomous measurements over a broad range of spatial scales would greatly augment the current suite of open-ocean and coastal observations.

The carbon dioxide system in natural waters is defined by the measurement of two or more carbonate parameters: pH, carbon dioxide fugacity (fCO2), total alkalinity (TA) and total dissolved inorganic carbon (DIC). The ocean carbon and ocean acidification research communities have long been craving robust methodologies and in-situ sensor technology for precise carbonate variable measurements. In-situ sensors for pH, pCO2, DIC and TA of seawater have been developed based in spectrophotometric techniques that in most cases require accurate dye additions and consideration of the effects of aging. The QUIMA group at the University of Las Palmas de Gran Canaria and SensorLab (Las Palmas, Spain) have developed a new family of rugged and extremely stable spectrophotometric pH sensors for both lab-based research and buoy monitoring, specifically designed for unattended operation independently of dye and aging effects in surface waters.

Sensor History
The development of the first lab-based pH sensor prototype was completed by the QUIMA group in 2007. The experience from this first prototype enabled the development of the first submarine pH sensor, completed in 2009. Integrating a spectrophotometric pH system in a standalone, submarine unit was not a straightforward process. Many challenges had to be addressed. Achieving high accuracy in open water proved to be quite challenging, as the pressure changes due to the swell caused small, short-term pH measurement errors. A pressure compensation system was designed and implemented so the undesirable effects of pressure variations fell below the measurement uncertainty. The presence of fouling was a big challenge as well. In addition to the typical obstruction problems, the gas exchange created by the fouling close to the sample intake modified the pH readings. Different anti-fouling coatings and techniques were tested, but most of them interfered with the pH readings. The chosen solution was using ETFE tubing with the sample intake placed at least 1 foot away from the sensor body, which helped to avoid the fouling interference. This proved to be very effective for long-term deployments. All the prototypes developed are still being used today by the QUIMA group and have been used in various scientific publications.

Sensorlab pH Sensors
SensorLab was established in 2011, and QUIMA group prototypes gave birth to the first commercial submarine sensor, the SP100-SM. The experience acquired with this sensor led to the new SP101-SM, packing several improvements over the previous generation of sensors. These improvements included a 40 percent reduction in the sensor power consumption, thanks to a redesigned higher efficiency electronic controller, and a new low-power LED light source. The corrosion resistance has also been improved with the addition of an epoxy coating, plus polyurethane finishing on top of the 6060 hard anodized aluminum housing. To continue this article please click here.

Dr. Melchor González-Dávila has a Ph.D. in chemistry from the University of La Laguna and is a full professor in marine chemistry. He is the director of the QUIMA research group, with more than 70 of his research papers published in high impact factor index journals. Since February 2012, he has been dean of the University of Las Palmas de Gran Canaria faculty of marine sciences.

Dr. J. Magdalena Santana-Casiano studied marine sciences at the University of Las Palmas de Gran Canaria. She graduated in 1988 and received her Ph.D. in 1991. Since July 2010, she has been vice dean of graduate studies. In May 2011, she became a full professor in chemical oceanography. Currently, she is the principal investigator of the ECOFEMA project and the European CARBOCHANGE project.

Hervé Prêcheur-Massieu is the general manager of SensorLab. He has been developing instrumentation in different fields for more than 10 years. He holds a bachelor's degree in electronic engineering.

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