Measuring Acidification in the Arctic Ocean – MACAO

In order to be able to be deployed on different oceanic platforms, the pH sensor must be light, robust, and precise with a fast response time. Each type of existing pH sensor has advantages and disadvantages. The electrode has the advantage of being easily miniaturized, it consumes little energy and has a fast response time. On the other hand, it drifts and requires frequent calibrations which are impossible to perform on drifting buoys. The electrodes therefore cannot be deployed alone to measure ocean acidification without human intervention. The spectrophotometric method consumes more energy, has a rather slow response time but allows stable pH measurements over time. The objective of the hybrid pH sensor is to combine the advantages of these two methods. Spectrophotometric pH measurements are used to calibrate the electrode. The hybrid pH sensor comprises an electrode for potentiometric pH measurement and a spectrophotometer based on the pH variations of a colored indicator, m-cresol purple.

A hybrid pH sensor comprising an electrode and a spectrophotometer has been developed and tested in the laboratory, on a surface buoy and underway during a research cruise.
The hybrid surface pH sensor was deployed during the PIRATA 2023 campaign with the continuous pCO2 measurement system for comparison.

The development of the sensor technology will be continued within the framework of the “Grands Fonds marins” project.

The intermediate surface version the hybrid sensor (in electrode and colorimeter in a bowx outside) was presented at a conference (Rérolle et al., 2019). Results show that the potentiometric part of the sensor is capable to operate in real ocean pressure and temperature conditions. In the rest of the project, a commercial sensor is used. The in situ version was presented during the Oceanology 2020 conference. The colorimetric part provides a stable reference to perform periodic recalibrations and remove drift.

Ocean acidification is a direct consequence of the CO2 increase and a threat for marine ecosystems. The Arctic Ocean is the most vulnerable region with the strongest pH decrease compared to other regions of the ocean. However, there are very few direct pH measurements due to the lack of a system for continuous measurements although efforts are being made to develop such a system. As a consequence, long-term trends in surface pH are often calculated using two other measured ocean carbon system parameters. The oceanographic community is currently in need of pH sensor technology that will affordably, accurately and efficiently measure ocean chemistry from its shallowest to its deepest waters.

MACAO focuses on the development and testing of accurate pH sensors capable of long-term monitoring of the water column while deployed on different platforms (buoys, profilers, ships). The sensor development effort will implement a novel hybrid approach, utilizing two different and complementary measurement techniques (the colorimetric method and the potentiometric method) to generate temporally dense and highly accurate data. The Arctic is a key area for pH monitoring but very hard to access. Autonomous sensors are particularly needed in this region. For testing the hybrid sensors and generating field data, we will focus on profilers in the Arctic to take advantage of on-going scientific projects such as IAOOS Equipex (http://iaoos.ipev.fr) and MOSAIC (www.mosaicobservatory.org).

The IAOOS Equipex platforms are installed on the ice to drift with it. They are equipped with a moving depth profiler and autonomous instruments allowing simultaneous observations of key variables in the ocean, atmosphere and ice. Observations are transmitted in real time via a satellite link. For measurements at depth, the IAOOS platform is equipped with a vertical cable guiding the moving profiler. Current profilers are typically equipped with sensors for depth, temperature, conductivity or salinity, and dissolved oxygen. We will add accurate pH measurement capability to the profilers, to study the dynamics of carbon parameters and acidification in the Arctic Ocean.

The team at LOCEAN involves biogeochemists working on the ocean carbon cycle and physical oceanographers working on the physics of the Arctic Ocean. Fluidion (http://fluidion.com) is a high-technology company specializing in cutting-edge products based on patented MEMS and microfluidic technology. Fluidion technology addresses markets as diverse as water quality/environmental monitoring, industrial process water, and oceanography/subsea applications.

The work is structured in four work packages (WP). WP1 focuses on the development of the pH sensors, WP2 is related to the tests and field measurements and the raw data collected during the project will be analysed and validated in WP3. A dedicated WP4 will focus on communication, website and public outreach as well as valorisation of the results and data obtained during the project. Fluidion expects to generate IP (patents & know-how) from the work related to this project. The economic and scientific benefits are interrelated: the ocean pH sensor market is dominated by academic institutions and research groups, and therefore external academic validation of sensor performance is key for market penetration.

The requested funding for LOCEAN corresponds to small equipment, NKE subcontracting, laboratory measurements, travel expenses, publications, a Ph.-D fellowship and engineer contract. For fluidion, the costs are related to personnel, equipment, IP protection, and outsourcing for mechanical design and manufacturing, two surface hybrid and two profiling hybrid sensors.