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Eddy covariance measurements of air–sea CO2 fluxes can beaffected by cross-sensitivities of the CO2 measurement towater vapour, resulting in order-of-magnitude biases. Well-establishedcauses for these biases are (i) cross-sensitivity of thebroadband non-dispersive infrared sensors due to band-broadening andspectral overlap (commercial sensors typically correct for this) and(ii) the effect of air density fluctuations (removed by determiningthe dry air CO2 mixing ratio). Another bias related to watervapour fluctuations has recently been observed with open-path sensors,attributed to sea salt build-up and water films on sensor optics. Twovery different approaches have been used to deal with these watervapour-related biases. Miller et al. (2010) employed a membrane drier tophysically eliminate 97% of the water vapour fluctuations in thesample air before it entered a closed-path gas analyser.Prytherch et al. (2010a) employed the empirical (Peter K. Taylor,PKT) post-processing correction to correct open-path sensor data. Inthis paper, we test these methods side by side using data from theSurface Ocean Aerosol Production (SOAP) experiment in the SouthernOcean. The air–sea CO2 flux was directly measured with fourclosed-path analysers, two of which were positioned down-stream ofa membrane dryer. The CO2 fluxes from the two dried gasanalysers matched each other and were in general agreement with commonparameterisations. The flux estimates from the un-dried sensors agreedwith the dried sensors only during periods with low latent heat flux(≤7 W m−2). When latent heat flux was higher,CO2 flux estimates from the un-dried sensors exhibited largescatter and an order-of-magnitude bias. Applying the PKT correctionto the flux data from the un-dried analysers did not remove the biaswhen compared to the data from the dried gas analyser. The results ofthis study demonstrate the validity of measuring CO2 fluxesusing a pre-dried air stream and show that the PKT correction is notvalid for the correction of CO2 fluxes

Analysis of the PKT correction for direct CO&lt;sub&gt;2&lt;/sub&gt; flux measurements over the ocean

Analysis of the PKT correction for direct CO<sub>2</sub> flux measurements over the ocean