Development of Low-Cost In Situ Gas Sensors for Oceanographic Applications

Abstract

Anthropogenic activity has increased atmospheric carbon dioxide (CO₂) levels, disrupting the global carbon cycle and driving widespread environmental change. The ocean acts as a major sink. Accurate and scalable in situ monitoring of oceanic carbon chemistry is vital for understanding the impacts of climate change and informing marine carbon dioxide removal (mCDR) strategies. Many existing in situ instruments for marine applications are constrained by their size, cost, power requirements, or reliance on consumable reagents. Developing low-cost, compact, low-power, and accurate in situ sensors would significantly enhance the spatiotemporal resolution of oceanographic data and enable widespread monitoring of dissolved gases throughout the ocean. This, in turn, would deepen our understanding of how, where, and when changes are occurring within the marine carbon cycle. Two key variables essential for studying this cycle are the partial pressure of carbon dioxide (pCO₂) and dissolved inorganic carbon (DIC). This thesis presents the development of two sensors, one for in situ pCO₂ measurement and another for novel DIC quantification, both designed to be affordable, reliable, and scalable tools for advancing our understanding of ocean chemistry and the global carbon system. First, the development, calibration, and open-ocean deployment of a miniaturized Dissolved Multi-Gas Sensor (DMGS) that measures pCO₂ and partial pressure of oxygen (pO₂) is presented. The sensor was integrated into a custom-built surface drifter designed to entangle with Sargassum mats and send data autonomously. The drifter utilized commercial off-theshelf (COTS) components and cost roughly $1000 to build. After lab testing, a drifter was deployed in the Great Atlantic Sargassum Belt (GASB) and collected data for 22-days. In addition to gas data, the drifter tracked temperature, light intensity, humidity, pressure, and location sending measurements via an Iridium satellite. The resulting data captured dynamic changes in localized gas concentrations, temperature, and light levels that highlighted photosynthetic and respiratory activity within Sargassum patches. These drifters demonstrate the value of in situ data to investigate marine biogeochemical processes that contribute to the marine carbon cycle, especially in areas with high biologic activity. Next, this thesis presents the iterative development of a novel DIC sensor with potential for future in situ applications. Initial prototypes tested the feasibility of using a COTS CO2 sensor in both static and flow-through configurations, however sensor saturation issues prompted a shift to a pressure-based detection method. Multiple test setups were evaluated for pressure stability and sensor sensitivity, culminating in a bottle-based flow system that demonstrated the potential for reagent-minimized, pressure-based DIC quantification. With the final setup, a COTS pressure sensor that sat behind a gas permeable membrane was found to repeatably and accurately quantify DIC from acidified seawater. This approach of quantifying DIC via pressure change is novel in the field of gas sensing and maintains a low-cost, accessible design. Together, the sensors developed in this thesis expand the toolkit for marine carbon monitoring and provide a foundation for affordable, distributed sensing networks. These technologies enable higher-resolution insights into ocean biogeochemistry and support critical monitoring, reporting, and verification (MRV) frameworks needed to evaluate the effectiveness of mCDR techniques. Continued refinement of these low-cost platforms could play a key role in understanding and mitigating anthropogenic impacts on marine systems.