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

• Published in: Atmospheric chemistry and physics 2014-04, Vol.14 (7), p.3361
• Main Authors: Landwehr, S, Miller, S. D, Smith, M. J, Saltzman, E. S, Ward, B
• Format: Article
• Language: English
• Subjects: Analysis; Measurement; Measuring instruments; Sensors
• ISSN: 1680-7316; 1680-7324

Eddy covariance measurements of air-sea CO.sub.2 fluxes can be affected by cross-sensitivities of the CO.sub.2 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 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 flux was directly measured with four closed-path analysers, two of which were positioned down-stream of a membrane dryer. The CO.sub.2 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 ( 7 W m.sup.-2). When latent heat flux was higher, CO.sub.2 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 fluxes using a pre-dried air stream and show that the PKT correction is not valid for the correction of CO.sub.2 fluxes.