ICOS Science Conference 2020, Session 16

1. Vlad Macovei, Yoana Voynova, Meike Becker, Jack Triest, Wilhelm Petersen
LONG-TERM INTERCOMPARISON OF TWO pCO2 INSTRUMENTS BASED ON
SHIP-OF-OPPORTUNITY MEASUREMENTS IN THE SKAGERRAK
4th ICOS Science Conference – 17/09/2020
vlad.macovei@hzg.de @OceanVlad

2. 2
IMPORTANCE OF CARBON CYCLING
Oceans as a carbon sink
A quarter of
anthropogenic carbon
emissions have been
absorbed by the oceans.
Direction of gas
exchange given by
difference in seawater
and air partial pressures
of carbon dioxide.
(Friedlingstein et al., 2019)

3. 3
OBSERVING pCO2 IN THE OCEAN
The pursuit of autonomy
Showerhead equilibrators are the
‘gold standard’.
Membrane-based sensors (MBS) are
being developed to simplify the
measuring process.

4. 4
TESTING AGREEMENT BETWEEN INSTRUMENTS
Past intercomparison exercises
Reference MBS instrument Mean residual (µatm) Comments and study duration
Fietzek et al.
(2013)
Contros HydroC FT −3.1 ± 2.9
1.8 ± 3.4
−0.7 ± 2.8
• 2 deployments (the first one with 2
MBS instruments) on the same ship
• two 1-month cruises
Jiang et al.
(2014)
ProOceanus CO2-Pro CV −0.3 ± 3.9 • 1 crossover on different ships
• 1 day time window
Kitidis et al.
(2019)
Contros HydroC FT* 16.7 • 91 crossovers on different ships
• 1 year
Arruda et al.
(2020)
SubCTech OceanPack2** −4.7 ± 2.9
−12.6 ± 2.0
• 2 deployments on the same ship
• a 1-week and a 1-month cruise
Arruda et al.
(2020)
ProOceanus CO2-Pro CV −5.7 ± 4.0
−8.7 ± 3.9†
−26.0 ± 6.8‡
• 2 deployments on the same ship
• a 1-week and a 1-month cruise
*Comparison done with a previous version of the data; **Calibrated with standard gases during deployment;
†Only using data before a severe storm; ‡Only using data after a severe storm

5. 5
STUDY AREA
Identifying crossover locations
Lysbris Seaways (equipped with FerryBox &
membrane-based pCO2 instrument) tracks
intersect Nuka Arctica (equipped with
showerhead equilibrator pCO2 instrument) tracks
in the Skagerrak Strait.
Lysbris Seaways
Nuka Arctica

6. 6
pCO2 in the Skagerrak
COMPARING THE TIME SERIES
Two different measuring methods
Good agreement between the
instruments.
Higher temporal coverage with the
FerryBox sampling.

7. 7
STUDY AREA
Identifying crossover locations
Restricting spatial (5 sub-
regions) and temporal (1
day) separation.
In line with usual water
mass drift in the region.
Further restriction based
on variability of
measurements.

8. 8
COMPARING THE INSTRUMENTS
Restricted crossovers
Close to 1:1 relationship between the
measurements.
Location of the crossover does not
influence the relationship.

9. 9
COMPARING THE INSTRUMENTS
Restricted crossovers
pCO2 difference <10 µatm in most
cases.
Deployments when larger differences
found also feature good agreement
crossovers.
1°C SST change ≈ 16 µatm pCO2 change.
Temperature correction does not
improve the comparison in this case.

10. 10
Anomalous events in the Skagerrak
FERRYBOX-INTEGRATED MEASUREMENTS
Advantages of high frequency sampling
High pCO2 measurements
recorded in October 2016 in the
eastern Skagerrak by one of the
compared instruments.
Ancillary biogeochemical
measurements confirm the
anomalous event.

11. 11
TESTING AGREEMENT BETWEEN INSTRUMENTS
Updated intercomparison exercises
Reference MBS instrument Mean residual (µatm) Comments and study duration
Fietzek et al.
(2013)
Contros HydroC FT −3.1 ± 2.9
1.8 ± 3.4
−0.7 ± 2.8
• 2 deployments (the first one with 2
MBS instruments) on the same ship
• two 1-month cruises
Jiang et al.
(2014)
ProOceanus CO2-Pro CV −0.3 ± 3.9 • 1 crossover on different ships
• 1 day time window
Kitidis et al.
(2019)
Contros HydroC FT* 16.7 • 91 crossovers on different ships
• 1 year
Arruda et al.
(2020)
SubCTech OceanPack2** −4.7 ± 2.9
−12.6 ± 2.0
• 2 deployments on the same ship
• a 1-week and a 1-month cruise
Arruda et al.
(2020)
ProOceanus CO2-Pro CV −5.7 ± 4.0
−8.7 ± 3.9†
−26.0 ± 6.8‡
• 2 deployments on the same ship
• a 1-week and a 1-month cruise
This study Contros HydroC FT 1.5 ± 10.6 • 14 crossovers on different ships
• 5 years
*Comparison done with a previous version of the data; **Calibrated with standard gases during deployment;
†Only using data before a severe storm; ‡Only using data after a severe storm

12. 12
ACKNOWLEDGEMENTS
Co-authors
FerryBox engineering team
Helmholtz Association

13. Thank you!
FOR PEOPLE AND THEIR
FUTURE ENVIRONMENT
Vlad Macovei
Institute of Coastal Research
Integrated Observing Systems
Tel: +494152872369
Max Planck Straße 1
21502 Geesthacht, Germany
www.hzg.de
vlad.macovei@hzg.de @OceanVlad

14. 14
Typical 24 h water mass drift in the area
Drift App of the CoastMap Geoportal
(www.coastmap.org)

15. 15
Removing high variability passages
pCO2 range outliers

16. 16
Removing high variability passages
Temperature range outliers

17. 17
Using a 48 h time window
Similar results to the
24 h time window
selection
Separated by sub-region Separated by time window
= 24 h; = 48 h.