Uncertainties in eddy covariance air–sea CO2 flux measurements and implications for gas transfer velocity parameterisations

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Dong, Yuanxu, Yang, Mingxi, Bakker, Dorothee C. E., Kitidis, Vassilis, Bell, Thomas G.
• Format: Text
• Language: English
• Published: 2021
• Subjects: Arctic
• Online Access: https://doi.org/10.5194/acp-21-8089-2021; https://acp.copernicus.org/articles/21/8089/2021/

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 CO 2 flux magnitude was small during two Atlantic cruises. Random uncertainty in the EC CO 2 flux is mostly caused by sampling error. Instrument noise is relatively unimportant. Random uncertainty in EC CO 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 2 fluxes suggests that the optimal timescale for averaging EC CO 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 Δ f CO 2 threshold (20 µatm ) to EC flux data enables an optimal analysis of K .