New developments in biogeochemistry sensors: an autonomous optical sensor for high accuracy pH measurement

Ocean acidification is known to affect the growth of entire groups of species that are basic to the trophic chain, such as some zooplankton and shellfish larvae, and as such is an increasing threat for the entire ecosystem [1]. Ocean acidity is also directly related to the absorption of carbon dioxide (CO2), a major greenhouse gas and one of the key variables of the carbon cycle equation in the ocean [2]. As the concentration of CO2 in the atmosphere has increased with the advent of the industrial age, the ocean continues to play an important role in absorbing it and reducing the impact on global warming, yet at the cost of increasing ocean acidity. There are, therefore, several reasons why monitoring acidity in the ocean is essential, and several initiatives are now in place to ensure pH measurements are made on a continuous basis. For example, the Global Carbon Observing System (GCOS) provides guidance for the coordination of regional carbon-observing systems (e.g., Integrated Carbon Observation System [ICOS]), whether terrestrial, atmospheric, or oceanic, acidity being an essential variable. Tracking the decrease of ocean pH requires high-precision measurements, with sensitivity of the order of a thousandth of a pH unit (mpH) [3]. Reaching such precision is highly challenging, in particular because the stability and accuracy of most sensing techniques depend on the accurate measurement of other variables (e.g., temperature, as illustrated later). When it comes to the development of autonomous systems, additional constraints are added, like compactness, low energy consumption, and low maintenance. This chapter is engineering focused, and the authors recommend consulting the extensive scientific literature for more background on ocean carbon system science, e.g., Refs. [1–3]. The objective of the chapter is to focus on a practical solution based on optics that has been deployed successfully on moorings in the Atlantic and the Mediterranean. The system presented in this chapter has demonstrated its reliability as a high-accuracy solution for open-ocean monitoring. A detailed description of concepts of operation is provided, followed by results and analysis of future work, such as the emerging need for multifunctional capabilities.

Precheur H. & Delory E., 2018. New developments in biogeochemistry sensors: an autonomous optical sensor for high accuracy pH measurement. In: Delory J. & Pearlman J. (Eds.), Challenges and innovations in ocean in situ sensors, pp 27-38. Elsevier. Chapter (restricted access).

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