Uncertainties in eddy covariance air–sea CO<sub>2</sub> flux measurements and implications for gas transfer velocity parameterisations
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Published:2021-05-26
Issue:10
Volume:21
Page:8089-8110
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Dong YuanxuORCID, Yang MingxiORCID, Bakker Dorothee C. E.ORCID, Kitidis Vassilis, Bell Thomas G.ORCID
Abstract
Abstract. Air–sea carbon dioxide (CO2) flux is often indirectly estimated by the bulk method using the air–sea difference in CO2
fugacity (ΔfCO2) 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-2d-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 ΔfCO2 threshold (20 µatm) to EC flux data enables an optimal analysis of K.
Funder
Natural Environment Research Council European Space Agency China Scholarship Council
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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