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

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 mmolm-2d-1. 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 .