Analysis of the PKT correction for direct CO 2 flux measurements over the ocean

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Abstract

<p><strong>Abstract.</strong> Eddy covariance measurements of air–sea CO<sub>2</sub> fluxes can be affected by cross-sensitivities of the CO<sub>2</sub> measurement to water vapour, resulting in order-of-magnitude biases. Well-established causes for these biases are (i) cross-sensitivity of the broadband non-dispersive infrared sensors due to band-broadening and spectral overlap (commercial sensors typically correct for this) and (ii) the effect of air density fluctuations (removed by determining the dry air CO<sub>2</sub> mixing ratio). Another bias related to water vapour fluctuations has recently been observed with open-path sensors, attributed to sea salt build-up and water films on sensor optics. Two very different approaches have been used to deal with these water vapour-related biases. Miller et al. (2010) employed a membrane drier to physically eliminate 97% of the water vapour fluctuations in the sample 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. In this paper, we test these methods side by side using data from the Surface Ocean Aerosol Production (SOAP) experiment in the Southern Ocean. The air–sea CO<sub>2</sub> flux was directly measured with four closed-path analysers, two of which were positioned down-stream of a membrane dryer. The CO<sub>2</sub> fluxes from the two dried gas analysers matched each other and were in general agreement with common parameterisations. The flux estimates from the un-dried sensors agreed with the dried sensors only during periods with low latent heat flux (&amp;leq;7 W m<sup>−2</sup>). When latent heat flux was higher, CO<sub>2</sub> flux estimates from the un-dried sensors exhibited large scatter and an order-of-magnitude bias. Applying the PKT correction to the flux data from the un-dried analysers did not remove the bias when compared to the data from the dried gas analyser. The results of this study demonstrate the validity of measuring CO<sub>2</sub> fluxes using a pre-dried air stream and show that the PKT correction is not valid for the correction of CO<sub>2</sub> fluxes.</p>

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Landwehr, S., Miller, S. D., Smith, M. J., Saltzman, E. S., & Ward, B. (2014). Analysis of the PKT correction for direct CO 2 flux measurements over the ocean. Atmospheric Chemistry and Physics, 14(7), 3361–3372. https://doi.org/10.5194/acp-14-3361-2014

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