Uncertainties in eddy covariance air-sea CO.sub.2 flux measurements and implications for gas transfer velocity parameterisations.

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From: Atmospheric Chemistry and Physics(Vol. 21, Issue 10)
Publisher: Copernicus GmbH
Document Type: Brief article
Length: 329 words

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Abstract :

Air-sea carbon dioxide (CO.sub.2) flux is often indirectly estimated by the bulk method using the air-sea difference in CO.sub.2 fugacity (ÎfCO.sub.2) 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 CO.sub.2 flux measurements from ships have not been well quantified and may confound analyses of K. This paper evaluates the uncertainties in EC CO.sub.2 fluxes from four cruises. Fluxes were measured with two state-of-the-art closed-path CO.sub.2 analysers on two ships. The mean bias in the EC CO.sub.2 flux is low, but the random error is relatively large over short timescales. The uncertainty (1 standard deviation) in hourly averaged EC air-sea CO.sub.2 fluxes (cruise mean) ranges from 1.4 to 3.2 mmolm-2d-1. This corresponds to a relative uncertainty of â¼ 20 % during two Arctic cruises that observed large CO.sub.2 flux magnitude. The relative uncertainty was greater (â¼ 50 %) when the CO.sub.2 flux magnitude was small during two Atlantic cruises. Random uncertainty in the EC CO.sub.2 flux is mostly caused by sampling error. Instrument noise is relatively unimportant. Random uncertainty in EC CO.sub.2 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 CO.sub.2 fluxes suggests that the optimal timescale for averaging EC CO.sub.2 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 ÎfCO.sub.2 threshold (20 µatm) to EC flux data enables an optimal analysis of K.

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Gale Document Number: GALE|A663146491