Effects of heat and water vapor transport on eddy covariance measurement of CO2 fluxes |
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| Authors: | R Leuning O T Denmead A R G Lang E Ohtaki |
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| Institution: | (1) CSIRO Division of Environmental Mechanics, P.O. Box 821, 2601 Canberra City, A.C.T., Australia;(2) College of Liberal Arts and Sciences, Okayama University, Okayama, Japan |
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| Abstract: | Flux densities of carbon dioxide were measured over an arid, vegetation-free surface by eddy covariance techniques and by a heat budget-profile method, in which CO2 concentration gradients were specified in terms of mixing ratios. This method showed negligible fluxes of CO2, consistent with the bareness of the experimental site, whereas the eddy covariance measurements indicated large downward fluxes of CO2. These apparently conflicting observations are in quantitative agreement with the results of a recent theory which predicts that whenever there are vertical fluxes of sensible or latent heat, a mean vertical velocity is developed. This velocity causes a mean vertical convective mass flux (= 
cw for CO2, in standard notation). The eddy covariance technique neglects this mean convective flux and measures only the turbulent flux 
c w. Thus, when the net flux of CO2 is zero, the eddy covariance method indicates an apparent flux which is equal and opposite to the mean convective flux, i.e.,
c w = –
c w. Corrections for the mean convective flux are particularly significant for CO2 because
cw and
c w are often of similar magnitude. The correct measurement of the net CO2 flux by eddy covariance techniques requires that the fluxes of sensible and latent heat be measured as well. |
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