Long‐term intercomparison of two pCO2 instruments based on ship‐of‐opportunity measurements in a dynamic shelf sea environment

Vlad A Macovei,Wilhelm Petersen,Yoana G Voynova,Meike Becker,Jack Triest

doi:10.1002/lom3.10403

Abstract

AbstractThe partial pressure of carbon dioxide (pCO2) in surface seawater is an important biogeochemical variable because, together with the pCO2 in the atmosphere, it determines the direction of air–sea carbon dioxide exchange. Large‐scale observations of pCO2 are facilitated by Ships‐of‐Opportunity (SOOP‐CO2) equipped with underway measuring instruments. The need for expanding the observation capacity and the challenges involving the sustainability and maintenance of traditional equilibrator systems led the community toward developing simpler and more autonomous systems. Here we performed a comparison between a membrane‐based sensor and a showerhead equilibration sensor installed on two SOOP‐CO2 between 2013 and 2018. We identified time‐ and space‐adequate crossovers in the Skagerrak Strait, where the two ship routes often crossed. We found a mean total difference of 1.5 ± 10.6 μatm and a root mean square error of 11 μatm. The pCO2 values recorded by the two instruments showed a strong linear correlation with a coefficient of 0.91 and a slope of 1.07 (± 0.14), despite the dynamic nature of the environment and the difficulty of comparing measurements from two different vessels. The membrane‐based sensor was integrated with a FerryBox system on a ship with a high sampling frequency in the study area. We showed the strength of having a sensor‐based network with a high spatial coverage that can be validated against conventional SOOP‐CO2 methods. Proving the validity of membrane‐based sensors in coastal and continental shelf seas and using the higher frequency measurements they provide can enable a thorough characterization of pCO2 variability in these dynamic environments.

Highlights

Every year there are more observations of the partial pressure of carbon dioxide in seawater collected into qualitycontrolled databases such as the Surface Ocean Carbon Atlas (SOCAT), which grows with every published version (Bakker et al 2018)
Sampling in the Skagerrak The FerryBox sampling of the North Sea covers a wide range of environments with various degrees of coastal influence and different seasonality characteristics
While the Lysbris FerryBox-MBS pCO2 measurements are available over a large area of the North Sea, in order to check and validate the long-term MBS pCO2 record, we focus on the small restricted box in the Skagerrak where many crossovers with the showerhead-equilibrator system (SHS) system on Nuka Arctica were identified

Summary

Introduction

While the Lysbris FerryBox-MBS pCO2 measurements are available over a large area of the North Sea, in order to check and validate the long-term MBS pCO2 record, we focus on the small restricted box in the Skagerrak where many crossovers with the SHS system on Nuka Arctica were identified. The SHS pCO2 was corrected for the difference between the in situ and equilibration temperature, while this was not done for the MBS data since the seawater temperature was not measured immediately after the intake location.

Results

Conclusion