Developing a Miniaturized In-situ Sensor Technology for Simultaneous Measurements of Seawater Dissolved Inorganic Carbon and pCO2

PI: Zhaohui Aleck Wang, WHOI

Project Number:2017-R/RCM-51

Start Date:2017-02-01End Date:2019-01-31

Proposal Summary

Objectives: This proposed project is to develop a miniaturized in-situ sensor, Channelized Optical System II (CHANOS II) for simultaneous, spectrophotometric measurements of seawater dissolved inorganic carbon (DIC) and partial pressure of CO2 (pCO2) with high frequency (~1 Hz) for up to 1000 m of water depth. DIC and pCO2 are a desired pair of CO2 parameters to measure in order to accurately characterize the marine CO2 system. This development will build and improve on the original CHANOS, which makes simultaneous measurements of seawater DIC and pH. The specific objectives of this project are to develop a new pCO2 channel to expand CHANOS’s capability, to design and fabricate a miniaturized fluidic CO2 equilibrating manifold for both DIC and pCO2 channels, and to apply miniature pumps and switches in order to dramatically reduce sensor size, power consumption and cost, and improve the CO2 exchange mechanism and overall robustness of the sensor.

The original CHANOS can be deployed on stationary platforms (e.g., buoys) or used in underway measurements for up to two months before maintenance (i.e., reagent refill). Upon the completion of this project, CHANOS II will be capable of making climatology-quality measurements on both stationary and mobile platforms, such as buoys, profilers, and autonomous underwater vehicles (AUVs), for time-series deployments of >4 months or high-frequency measurements (profiling) on mobile platforms for a few weeks. CHANOS II will be the first in-situ sensor capable of fully resolving the CO2 system in a single package by measuring DIC and pCO2, a desirable pair of parameters. This project will substantially elevate the Technology Readiness Level (TRL) of CHANOS so that it is better positioned for commercialization and thus more readily available to the public.

Methodology: Simultaneous, continuous measurements of seawater DIC and pCO2 by the new in-situ sensor (CHANOS II) are based on similar spectrophotometric principles. We will use a modular design to make two independent but similar channels for the two parameters. The DIC channel adapts a recently-developed spectrophotometric method for continuous detection, which is based on flow-through CO2 equilibration between an acidified sample and a standard pH indicator solution with known alkalinity across a gas-permeable membrane, followed by spectrophotometric pH measurements of that solution. CO2 concentration in the equilibrated indicator (thus sample total CO2 or DIC) can be calculated from indicator pH and alkalinity. The flow-through CO2 equilibration (wherein both indicator and sample are flowing) allows near-continuous (~1 Hz) detection of DIC, thus maximizing the method’s capability to make high-resolution profiling on mobile platforms. A very similar method can be used to measure seawater pCO2, the differences being that, to measure pCO2, the water sample will not be acidified before CO2 equilibration and a different indicator will be used to cover the higher pH range of the CO2-equilibrated indicator.

The design and fabrication of CHANOS II will focus on making significant improvements on the robustness of fluidic control, measuring frequency, size, and power consumption compared to the original CHANOS. The results will be reduced fabrication cost and improved user friendliness, such that non-carbonate chemistry specialists can readily use it. We will develop a miniaturized fluidic CO2 equilibrating manifold with a sample-indicator fluidic channel. It will replace the previous tube-in-tube design of the original CHANOS and serve as the path where CO2 in acidified seawater (for DIC) or natural seawater (for pCO2) samples equilibrates with the indicator across a CO2-permeable Teflon film. It will allow for continuous flow-through CO2 equilibration with only a fraction of the previous footprint. The manifold will be integrated with commercially available miniature pumps and switches into a compact package with minimal connections. Other improvements include LED sources, optical detectors, and antifouling mechanisms. The new sensor will also have built-in in-situ calibration capability to ensure data quality.

The new sensor will undergo rigorous testing and calibration to establish measurement characteristics. Thereafter, the sensor will be test-deployed on a CTD rosette package to make high-resolution profiles of DIC and pCO2 in water column up to 1000 m depth during a research cruise near the shelf break off southern New England. We will collaborate with Dr. A. E. Giblin’s group at Marine Biological Laboratory to test and deploy the new sensor at the Plum Island Ecosystems (PIE) Long-Term Ecological Research (LTER) site. The objective is to use high-resolution DIC and pCO2 measurements to derive lateral export of DIC and alkalinity from intertidal salt marshes. As part of education and outreach activities, we will entrain middle and high students to participate in tests of CHANOS II via Cohasset Center for Student Coastal Research (CSCR, Cohasset, MA).

Rationale: The marine CO2 system plays a critical role in the marine carbon cycle. Currently, the ocean absorbs about one quarter of the anthropogenic CO2 released to the atmosphere, curbing global warming but causing ocean acidification (OA). OA results in complicated responses and feedbacks in the ocean including changes in biogeochemistry of carbon and other chemical species, as well as changes in marine biology and ecology. Development of sensors to enhance our capability to monitor and study OA is one of the focus areas in the current MIT Sea Grant RFP. It has been widely recognized as a research priority in the carbon and OA research communities in order to more effectively study OA and the carbon cycle. The proposed project aligns well with several goals in the MIT Sea Grant Strategic Plan, namely HCE Goal 1, RCE Goals 2-4, and ELWD Goals 1-2.

With continuous efforts invested in developing sensors and instruments of CO2 parameters in recent decades, significant advancements have been made in this field. The recent Wendy Schmidt Ocean Health XPRIZE has acted as a global catalyst for developing new pH sensor technologies to improve seawater pH measurements. Yet a few challenges or limitations remain. One well-known challenge in measurements of seawater carbonate chemistry is that measuring only pH (or any one CO2 parameter) is not sufficient for OA and carbon cycle research, due to the inherent complexity of the marine CO2 system. The marine CO2 system can be described by four measurable, primary parameters: DIC, pCO2 (or CO2 fugacity, fCO2), pH, and total alkalinity (TA). Measurements of any two of the four parameters are required to fully resolve the carbonate system using seawater acid-base equilibria. Simultaneous measurements of two CO2 parameters are thus attractive.

However, different measurement pairs generate a range of calculation errors. Large errors result when pCO2 and pH are used due to their strong co-variation. The errors are often minimized when the DIC-pH or DIC-pCO2 pair is used. Even when TA contains a significant amount of non-carbonate alkalinity, these pairs can still accurately calculate carbonate alkalinity and CaCO3 saturation states, which are of particular interests in OA studies.
However, only in-situ pCO2 and pH sensors are commonly available. In contrast, in-situ DIC sensor technologies are much less mature but are highly desirable. Currently, no commercial DIC sensors are available. A few prototype DIC sensors exist but are designed primarily for time-series measurements at fixed locations. So far there are no DIC sensors that are yet suitable for profiling applications such as deployment on mobile platforms (e.g., AUVs and profilers). Such deployment requires the sensor to have near-continuous measurement capacity, compactness and low power demands. Development of in-situ sensors that can simultaneously measure two desirable CO2 parameters (DIC-pH or DIC-pCO2 pairs) in a single system is in its earliest stage. The technology gap for sensing of CO2 parameters provides the rationale for our proposed sensor development: in-situ, simultaneous measurements of two desirable CO2 parameters in a single system that is versatile and suitable for high-frequency measurements on stationary and mobile platforms. Because of the societal relevance of climate change and OA, this sensor will benefit a wide range of stakeholders, including academic researchers who study OA and the ocean carbon cycle, federal and state agencies who are responsible for monitoring and mitigating OA and climate change, commercial fisheries, and environmental organizations.

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