This document summarizes efforts to optimize methodologies for measuring surface water CO2 and carbonate chemistry parameters in the North Atlantic Ocean. Researchers from NOC and VLIZ compared measurements of dissolved inorganic carbon, total alkalinity, and partial pressure of CO2 (pCO2) from two research cruises. While most measurements were within the desired accuracy goals, some differences between labs were higher than expected. A larger intercomparison exercise involving multiple labs could help identify sources of error and support further improvement. The researchers concluded methodologies performed reliably but that sampling, storage, and inter-lab comparisons deserve additional attention to optimize surface water carbonate measurement techniques.

1. Surface water CO₂ measurements in the North Atlantic Ocean:
optimize methodologies and analytical procedures
Hannelore Theetaert1, Emy MacGarry 2, Sue Hartman2,
Thanos Gkritzalis1
, Peter Brown2
1: Flanders Marine Institute – VLIZ, Ostend, BE
2: National Oceanography Centre – NOC, Southampton, UK

2. Marine Inorganic Carbonate Chemistry
Climate Goal
pH : 0.003
Ct/At : 2 mmol/kg
pCO2: 0.5%
Weather Goal
pH : 0.02
Ct/At: 10 mmol/kg
pCO2: 2.5%
A pair is enough to characterize the system.
BUT….
(Steinhoff, 2020; doi: 10.18160/vb7c-z758)
ICOS Stations
pCO2 (climate goal)
Carbonate
System
pCO2
Ct
At
pH
Note:
At ≡ TA (Total Alkalinity)
Ct ≡ DIC (Total/Dissolved Inorganic Carbon)

3. History
1988 (14
groups)
1990 (similar)
1990 -1996:
Dickson lab
CRMs for Ct/At
2012: pH of
CRMs
2015 (59
groups)
DCt & Dat
20-30 mmol/kg
Note: Based on measurements of the same Certified samples
1-2 mmol/kg
Desired: 1mmol/kg
Bockmon, Dickson 2015

4. Scope / Rationale
NOC & VLIZ have ICOS Ocean Stations
- NOC: UK-FOS-PAP
- VLIZ: BE-FOS-Thornton Buoy (+ BE-SOOP-Simon Stevin)
- Both have analytical capacity for Marine Inorganic Carbonate parameters (VINDTA, AIRICA)
- Both use Dickson CRMs to report e of Ct and At of ± 2 mmol/kg
- Knowledge exchange
- Training, Improvement, Optimization
- To an extent identify what else might be contributing to errors/uncertainties

5. DY 103
UK-FOS-PAP

6. HydroFIA-TA
HydroC-CO2 FT
UW pCO2 (G2201-i + Marble equilibrator* + VLIZ custom made DAS controls board)
Samples
Niskin bottles from CTD carrousel following SOPs
(Dickson et al., 2007)
DY103
• Equilibrator is based on Frankignoule et al., 2001;
• UW system design follows Pierrot et al., 2009

7. DY103 transect data
UW pCO2 eq. data reductions is based on Pierrot et al., 2009 & Pfeil et al., 2011

8. DY103 UW systems
HydroC-CO2 FT data reduction was based on Fietzek et al. 2014

9. DY103 comparison labs
Weather goal ???
DAt (mmol/kg)
DCt (mmol/kg)
DpCO2 (matm)

10. JC191 Setup
GO-SHIP campaign following the 25 oN (RAPID array)
Setup is very similar to DY103 one

11. JC191 transect data
HydroFIA-TA
UW pCO2 – (Eq+G2201-i)
UW pCO2 eq. data reductions is based on Pierrot et al., 2009 & Pfeil et al., 2011
y = 57.306x + 300.59
R² = 0.9254

12. JC191 UW systems
HydroC-CO2 FT data reduction was based on Fietzek et al. 2014

13. DTA: ± 5 µatm
JC191 comparison labs At

14. DpCO2 ± 10 µatm
DpCO2 ± 20 µatm
JC191 comparison labs pCO2
DAt (mmol/kg)
DCt (mmol/kg)
DpCO2 (matm)
Weather goal!

15. Conclusion
 D between labs is higher than expected
 D is lower on 2nd campaign
- Errors are difficult to identify, but sampling and storing can be responsible for the high Ds
- Equilibrator/CRDS UW pCO2 system performed well, BUT…?
- HydroFIA – TA performed reliably
- HydroC-CO2 performed well beyond it’s specs
- A larger scale exercise (i.e. 5 labs) can be very useful
- ICOS OTC is organizing an UW pCO2 systems intercomparison activity in 2021 (check https://otc.icos-cp.eu/node/107)

16. Thanks