A laser based sensor for in situ, real time measurement of dissolved gas: application to CO2 in water

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We propose the development of an instrument concept anticipated to enable for the first time the measurement of gases dissolved in water in-situ, in real time, at good resolution and in a compact and deployable package. Current measurement techniques for CO2 dissolved in water typically involve the equilibration of the amount of CO2 dissolved in water with that in a volume of gas, followed by an optical measurement of the CO2 in the gas; or equilibration with an electrolyte behind a semi-permeable membrane and a measurement of the acidity of the resulting liquid. Neither process is especially fast and the former is not suited to immersion in the ocean. Equilibration is a relatively slow process. Consequently it is difficult to measure oceanic CO2 profiles with high spatial or temporal resolution.We propose to develop some innovative laser-based methods to monitor gases dissolved in liquids. The first application targeted is dissolved CO2 in ocean water. With the proposed approach, no equilibration is required and good precision should be achievable in real time (seconds or better). The measurement is in principle tolerant of fouling and should be insensitive to a number of major instrumental and environmental parameters. The main tasks on the project will include initial design and trade-offs of the laser sensors, assembly and initial characterization, development of modulation schemes and quantitative data processing, and final laboratory tests to validate the technology. A fully developed instrument would be compact, have moderate power requirements, be immersible to significant depths and should be compatible with ship inlets, CTD (conductivity, temperature and depth) rosette water samplers and other ocean profilers.The proposed sensor would significantly contribute to the overall study of the Earth carbon cycle by enabling three dimensional measurements of CO2. More specific research areas include carbon chemistry and surface CO2 flux studies, CO2 exchange between marine biosphere and ocean, ocean CO2 circulation, and evaluation of CO2 seeps at ocean ridges and submarine volcanoes to understand deep carbon pathways to the oceans. Good spatial and temporal sampling, including at depth, are also potentially applicable to studies of small-scale or dynamic CO2 distributions, for validating CO2 sequestration experiments and for monitoring oil drilling operations.While we will be demonsrating our technique on CO2, the sensor concepts are widely applicable to the sensing of any molecular species dissolved in liquids. As such other numerous applications would be unlocked such as applications to water quality studies, or industrial characterisation of liquid contamination.