A Re-examination of Density Effects in Eddy Covariance Measurements of CO2 Fluxes

Corrections of density effects resulting from air-parcel expansion/compression areimportant in interpreting eddy covariance fluxes of water vapor and CO2 when open-path systemsare used. To account for these effects, mean vertical velocity and perturbation of the densityof dry air are two critical parameters in treating those physical processes responsible fordensity variations. Based on various underlying assumptions, different studies have obtaineddifferent formulas for the mean vertical velocity and perturbation of the density of dry air,leading to a number of approaches to correct density effects. In this study, we re-examinephysical processes related to different assumptions that are made to formulate the densityeffects. Specifically, we re-examine the assumptions of a zero dry air flux and a zero moistair flux in the surface layer, used for treating density variations, and their implicationsfor correcting density effects. It is found that physical processes in relation to the assumptionof a zero dry air flux account for the influence of dry air expansion/compression on densityvariations. Meanwhile, physical processes in relation to the assumption of a zero moist airflux account for the influence of moist air expansion/compression on density variations. Inthis study, we also re-examine mixing ratio issues. Our results indicate that the assumptionof a zero dry air flux favors the use of the mixing ratio relative to dry air, while theassumption of a zero moist air flux favors the use of the mixing ratio relative to the totalmoist air. Additionally, we compare different formula for the mean vertical velocity, generatedby air-parcel expansion/compression, and for density effect corrections using eddy covariancedata measured over three boreal ecosystems.