Abstract
Abstract. Eddy covariance measurements of air–sea CO2 fluxes can be affected by cross-sensitivities of the CO2 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 CO2 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 CO2 flux was directly measured with four closed-path analysers, two of which were positioned down-stream of a membrane dryer. The CO2 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−2). When latent heat flux was higher, CO2 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 CO2 fluxes using a pre-dried air stream and show that the PKT correction is not valid for the correction of CO2 fluxes.
Highlights
Direct measurements of air–sea CO2 flux contribute to the understanding of the earth climate system and can be used to study the fundamental physics of air–sea gas exchange
Measurements of the air–sea CO2 flux over the open ocean were conducted with four infrared gas analysers (IRGAs), two of which had the water vapour fluctuations removed with a membrane dryer (Miller et al, 2010)
The PKT correction reduced the scatter in the flux measurements from the un-dried gas analysers from 1000 to 100 % of the flux signal
Summary
Direct measurements of air–sea CO2 flux contribute to the understanding of the earth climate system and can be used to study the fundamental physics of air–sea gas exchange. S · pCO2 where S is the solubility of CO2 in sea water and Fc is the vertical CO2 flux. In the eddy covariance (EC) method, the turbulent flux is directly calculated from the covariance of the fluctuations in the vertical wind speed (w ) and fluctuations in the CO2 mixing ratio in dry air (xc): Fc = nd w xc , (2). The EC method allows the study of gas transfer with much higher time resolution than both dual tracer experiments (e.g Nightingale et al, 2000; Ho et al, 2006) and measurements of the 14C concentration in sea water (e.g Wanninkhof, 1992; Sweeney et al, 2007)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
