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. Air–sea carbon dioxide (CO2) flux is often indirectly estimated by the bulk method using the air–sea difference in CO2fugacity (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 ECair–sea CO2 flux measurements from ships have not been well quantified and may confound analyses of K. This paper evaluates theuncertainties in EC CO2 fluxes from four cruises. Fluxes were measured with two state-of-the-art closed-path CO2 analysers ontwo ships. The mean bias in the EC CO2 flux is low, but the random error is relatively large over short timescales. The uncertainty (1standard deviation) in hourly averaged EC air–sea CO2 fluxes (cruise mean) ranges from 1.4 to 3.2 mmolm-2d-1. Thiscorresponds to a relative uncertainty of ∼ 20 % during two Arctic cruises that observed large CO2 flux magnitude. The relativeuncertainty was greater (∼ 50 %) when the CO2 flux magnitude was small during two Atlantic cruises. Random uncertainty in the ECCO2 flux is mostly caused by sampling error. Instrument noise is relatively unimportant. Random uncertainty in EC CO2 fluxes canbe reduced by averaging for longer. However, averaging for too long will result in the inclusion of more natural variability. Auto-covarianceanalysis of CO2 fluxes suggests that the optimal timescale for averaging EC CO2 flux measurements ranges from 1 to 3 h, whichincreases 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.

atmospheric chemistry and physics

Uncertainties in eddy covariance air–sea CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; flux measurements and implications for gas transfer velocity parameterisations