Uncertainties in eddy covariance air–sea CO<sub>2</sub> flux measurements and implications for gas transfer velocity parameterisations

Dong, Y; Yang, M; Bakker, DCE; Kitidis, V; Bell, TG. 2021 Uncertainties in eddy covariance air–sea CO<sub>2</sub> flux measurements and implications for gas transfer velocity parameterisations. Atmospheric Chemistry and Physics, 21 (10). 8089-8110. https://doi.org/10.5194/acp-21-8089-2021

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Official URL: http://dx.doi.org/10.5194/acp-21-8089-2021

Abstract/Summary

Air–sea carbon dioxide (CO2) flux is often indi�rectly estimated by the bulk method using the air–sea difference in CO2 fugacity (1f CO2) and a parameterisation of the gas transfer velocity (K). Direct flux measurements by eddy covariance (EC) provide an independent reference for bulk flux estimates and are often used to study processes that drive K. However, inherent uncertainties in EC air–sea CO2 flux measurements from ships have not been well quantified and may confound analyses of K. This paper evaluates the uncertainties in EC CO2 fluxes from four cruises. Fluxes were measured with two state-of-the-art closed-path CO2 analysers on two ships. The mean bias in the EC CO2 flux is low, but the random error is relatively large over short timescales. The uncertainty (1 standard deviation) in hourly averaged EC air–sea CO2 fluxes (cruise mean) ranges from 1.4 to 3.2 mmolm−2 d−1. This corresponds to a relative uncertainty of ∼ 20 % during two Arctic cruises that observed large CO2 flux magnitude. The relative uncertainty was greater (∼ 50 %) when the CO2 flux magnitude was small during two Atlantic cruises. Random uncertainty in the EC CO2 flux is mostly caused by sampling error. Instrument noise is relatively unimportant. Random uncertainty in EC CO2 fluxes can be reduced by averaging for longer. However, averaging for too long will result in the inclusion of more natural variability. Auto-covariance analysis of CO2 fluxes suggests that the optimal timescale for averaging EC CO2 flux measurements ranges from 1 to 3 h, which increases the mean signal-to-noise ratio of the four cruises to higher than 3. Applying an appropriate averaging timescale and suitable 1f CO2 threshold (20 µatm) to EC flux data enables an optimal analysis of K.

Item Type: Publication - Article
Additional Information. Not used in RCUK Gateway to Research.: Correspondence: Yuanxu Dong (yuanxu.dong@uea.ac.uk) and Mingxi Yang (miya@pml.ac.uk)
Divisions: Plymouth Marine Laboratory > National Capability categories > Atlantic Meridional Transect
Plymouth Marine Laboratory > National Capability categories > Long-term Multi-Centre ORCHESTRA
Plymouth Marine Laboratory > Science Areas > Marine Biochemistry and Observations
Depositing User: S Hawkins
Date made live: 27 May 2021 08:25
Last Modified: 27 May 2021 08:25
URI: http://plymsea.ac.uk/id/eprint/9227

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