1. Woods Hole Oceanographic Institution
Developing a Miniaturized In-situ Sensor Technology for Simultaneous Measurements of Seawater Dissolved Inorganic Carbon and pCO2
Zhaohui Aleck Wang
Woods Hole Oceanographic Institution
MIT Sea Grant
November 2016
Intro about self, marine chemist and geochemist. Most of my research is observational: field/cruise and lab work.

2. CO2 Invasion and Ocean Acidification (OA)
Key Q: What is the signal we want to detect?
Basics about in OA and carbonate chemistry, the OA effects are to….;
pH is not all of the story in OA. There are 4 primary parameters to describe the marine CO2 system, pH is one of them.
Measure two to calculate the rest; Often we use CaCO3 saturation states as an OA indicator because shelf-building organisms need it to form their shells (>1 chemically favor precipitation; <1, starts to dissolve).
OA is a relatively small signal
Large cost associated with bottle sampling
Chu et al. JGR-Ocean 2016
9/18/2018

3. Two Challenges for Sensor Development
Simultaneous measurements of two CO2 parameters
Which pairs?
Analytical Errors (Best Practice)
Calculation Errors
Ideally, simultaneous measurements with one instrument/sensor, our original idea
Problem: only pH and pCO2 sensors are readily available, but DIC and TA sensors is much less mature
Millero 2007

4. Carbon Sensor Development
Challenge 2:
Except pH, all other measurements have long response times (many minutes)  Not ideal for mobile platforms (AUV, ROV, gliders etc.) and highly dynamic environments (e.g. coastal oceans)
DIC, pCO2/fCO2 methods: 5-15 minutes (CO2 equilibrating or extracting processes)
TA method: ~10 minutes (titration)
Ideally, simultaneous measurements with one instrument/sensor, our original idea

5. Development Strategy: spectrophotometric methods
Simultaneous measurements of pH/pCO2 and DIC
High-frequency, flow-through measurements
Advantages:
Fully resolve the CO2 system, with good calculation accuracy
Sensitive and stable
Similar principles  modular
Direct measurements of water, deep deployments

6. Version 1: Channelized Optical System (CHANOS) – DIC + pH
Two independent channels: spectrophotometric pH and DIC
Designed for fixed platforms, e.g. buoys
~15 mins/cycle, with ~8 mins continuous measurements
Sensor vs. bottle (1σ difference):
DIC: 0.8±5.2 µmol/kg
pH: ±0.003

7. Sea Grant Project: Simultaneous measurements of DIC and pCO2
Miniaturization of CHANOS
Spectrophotometric DIC and pCO2 Why pCO2 (not pH) + DIC: ) Coastal waters: low-salinity (particularly 0 < S < 15) and high turbidity  Challenges for pH measurements ) Much less prone to the effects of water clarity and particles 3) Similarity between spectrophotometric pCO2 and DIC methods 4) Similar internal calculation consistency as DIC + pH
High-frequency (~1 Hz) measurements, versatile for various platforms, particularly on mobile platforms (CTDs, AUVs, ROVs)

8. Static Equilibration for DIC/pCO2 Measurements
CO2 exchange / equilibrium (~ minutes)
Water Sample (with acidification, for DIC)
(without acidification, for pCO2)
f(CO2)ex
Indicator, base, or air
f(CO2)i
f(CO2)ex = f(CO2)i
Spectrophotometric Conductimetric
Or Infrared detection
Membrane: e.g. silicone, Teflon AF, PTFE
Or gas-water equilibrator (Infrared)

9. Dynamic equilibration for DIC or pCO2 measurement:
Concurrent or Countercurrent flow spectrophotometric method
Acidified seawater
(Modified from Wang et al. 2013, ES&T)
Acidified seawater
Calibration variables
p – percentage equilibration
B(t) – constant
log(p×fCO2)a
log(fCO2)i
(Modified from Byrne et al. 2002)

10. Advantages of Dynamic Equilibration
Improve CO2 diffusion efficiency: 70s vs. 300s
Dynamic (flow-through) equilibrium  almost continuous measurements
Further improvement by using a concurrent design
Full equilibrium: ~70s
Partial equilibrium: Response time ~22s

11. Miniaturization:
Milli-fluidic CO2 equilibrating manifold (centimeter scale)
Minimize # of connections, improve robustness, significantly reduce size and flushing volume (save reagents)

12. Miniaturization: Miniature pump and valves
Re-engineering: deployable under water in high pressure

13. Design strategies
Low cost commercial components, with necessary reengineering.
Low power: Use of LED and low power components (target < 10W)
Size: suitable for AUV, ROV, and CTD rosette deployment
Reagent consumption: sub-mL/min

14. In-situ calibration capability: DIC calibration on temperature effect
Lab Calibration
In-situ calibration using Certified Reference Materials (CRMs):
Reduce lab calibration that may be different from the field
Ensure measurement quality
Evaluate in-situ accuracy
Make the system complex

16. Education and outreach
Testing
Dock testing at the WHOI Iselin Pier
Profiling with a CTD rosette package: one of the OOI maintenance cruises near Pioneer Array
Deployment at a intertidal salt marsh: Plum Island Ecosystems LTER site (with Dr. A.E. Giblin at MBL)
Education and outreach
Engage high school students via the Cohasset Center for Student Coastal Research
WHOI-MIT joint program PhD student (Mallory Ringham)

17. Deliverable: CHANOS II, near commercialization
Examples of applications
Fast response: particularly useful in highly heterogeneous systems (e.g., coastal systems) to study fine-scale variations
Deployable on mobile platforms for high-resolution spatial mapping
In-situ study of biological, ecological, and biogeochemical effects of OA
OA monitoring

18. Acknowledgement
Funding: NIST, MIT Sea Grant, NSF, WHOI Green Tech Award
Development Team:
Engineers: Steve Lerner, Jason Kapit, Fritz Sonnichsen, Yabin Men Chemists: Mallory Ringham, Kate Morkeski