Uncertainties in Turbulent Statistics and Fluxes of CO2 Associated With Density Effect Corrections

Density effect corrections (DECs) are applied to adjust raw CO2 fluxes measured by eddy covariance (EC) systems with open-path gas analyzers. DEC is also required for adjusting the measured CO2 concentration fluctuations to obtain the adjusted CO2 concentrations for analyzing turbulent statistics or quantifying fluxes. However, our data show that the power spectra of the DEC-adjusted CO2 concentrations are distorted in the high-frequency range, as compared with the corresponding spectra of temperature and water vapor density. This contradicts the similarity behavior of scalars, as suggested by Monin-Obukhov similarity theory. It is demonstrated that such a distortion is caused by the DEC-induced spikes in the DEC-adjusted CO2, altering turbulent statistics of CO2 and scalar similarity between CO2 and other scalars. Our results suggest that CO2 fluxes are overestimated by applying DEC especially under high Bowen ratio conditions, potentially leading to substantial uncertainties in long-term ecosystem carbon exchange in dry regions. Plain Language Summary Eddy covariance (EC) systems are widely used to measure land-surface fluxes. However, the measured water vapor densities and CO2 concentrations by open-path CO2/H2O gas analyzers in EC systems are subjected to the density effects caused by heat and water transfer, the so-called density effects. Density effect corrections (DECs) are required to adjust the measured raw CO2 concentrations to obtain the DEC-adjusted CO2 concentrations. By comparing the power spectra of the DEC-adjusted CO2 concentrations with those of temperature and water vapor density, it is found that the power spectra of the DEC-adjusted CO2 are distorted in the high-frequency range, contradicting the Monin-Obukhov similarity theory that requires similar behaviors of different scalars. This distortion also causes an increase in the variances of CO2. CO2 fluxes are overestimated by about 5-35% under higher Bowen ratio conditions, potentially leading to substantial uncertainties in long-term ecosystem carbon exchange in dry regions.