A re-examination of density effects in eddy covariance measurements of CO2 fluxes

Corrections of the density effects resulting from air-parcel expansion/compression are important in interpreting eddy covariance fluxes of water vapour and CO2 when open-path systems are used. To correct these effects, mean vertical velocity and perturbation of dry air density are two critical parameters in treating those physical processes involved in air-parcel expansion/compression. Based on various underlying assumptions, different formula for mean vertical velocity and perturbation of dry air density in previous studies has led to a number of alternative approaches for this correction. This paper explores the fundamental differences in physical processes related to the assumptions that are made to formulate the density effects. It is suggested in this paper that the assumption of zero dry air flux in previous studies implies ignoring the contribution of water vapour to the expansion/compression of total moist air parcel. Instead, the zero moist air flux holds in the surface layer, which reflects the nature of turbulence transfer of the total moist air. We also re-examine the mixing ratio relative to dry air. It is shown that a correction is still required even though the mixing ratio relative to dry air is obtained, which contradicts previous thought. Additionally, we compare different formula for the mean vertical velocity induced by air-parcel expansion/compression, using eddy covariance data measured over three boreal ecosystems.