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

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Y. Dong, M. Yang, D. C. E. Bakker, V. Kitidis, T. G. Bell
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2021
• Subjects: Physics; QC1-999; Chemistry; QD1-999; Arctic
• Online Access: https://doi.org/10.5194/acp-21-8089-2021; https://doaj.org/article/6baa9b7557a6499280220325ce029dfa

Air–sea carbon dioxide ( CO 2 ) flux is often indirectly estimated by the bulk method using the air–sea difference in CO 2 fugacity ( Δ f CO 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 2 flux measurements from ships have not been well quantified and may confound analyses of K . This paper evaluates the uncertainties in EC CO 2 fluxes from four cruises. Fluxes were measured with two state-of-the-art closed-path CO 2 analysers on two ships. The mean bias in the EC CO 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 2 fluxes (cruise mean) ranges from 1.4 to 3.2 <math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">mmol</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">d</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="a4c66177a451915c5530d12ef0d4aec1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-8089-2021-ie00001.svg" width="67pt" height="13pt" src="acp-21-8089-2021-ie00001.png"/></svg:svg> . This corresponds to a relative uncertainty of ∼ 20 % during two Arctic cruises that observed large CO 2 flux magnitude. The relative uncertainty was greater ( ∼ 50 %) when the ...