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

Abstract

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.

Uncertainties in temperature statistics and fluxes determined by sonic anemometer due to wind-induced vibrations of mounting arms

“…The data processing mainly entailed despiking, double rotation for the wind components (Wilczak et al, 2001), sonic temperature conversion (Liu et al, 2001;Schotanus et al, 1983), and application of DEC to the raw fluxes of latent heat and CO2 (Webb et al, 1980). However, corrections for the effects of humidity and density fluctuations in this study are applied to the turbulent fluctuations of sonic temperature, 𝜌 % , and 𝜌 " , respectively (Detto and Katul, 2007;Gao et al, 2020;Sahlée et al, 2008;Schotanus et al, 1983;Webb et al, 1980). For each 30-min interval, the corrected turbulent fluctuations of sonic temperature (𝑇 !…”

“…Accurate air temperature measurements are not only important for determining H but also crucial for estimating other scalar fluxes (e.g., water vapor and CO2) through density effect corrections (DEC hereafter; Detto and Katul, 2007;Gao et al, 2020;Lee and Massman, 2011;Sahlée et al, 2008;Webb et al, 1980). The measured high-frequency time series of densities of water vapor, CO2, and other scalars are subjected to the effects of density fluctuations of dry air and other components in the atmosphere, as well as the fluctuations of air pressure (Lee and Massman, 2011;Webb et al, 1980).…”

“…The measured high-frequency time series of densities of water vapor, CO2, and other scalars are subjected to the effects of density fluctuations of dry air and other components in the atmosphere, as well as the fluctuations of air pressure (Lee and Massman, 2011;Webb et al, 1980). Correcting for these effects involves applying corrections to either the calculated raw fluxes or to the high-frequency time series of the scalar density fluctuations (Webb et al, 1980;Detto and Katul, 2007;Gao et al, 2020;Sahlée et al, 2008). Any errors or uncertainties in sonic-derived temperature are anticipated to propagate, and in some certain cases, be amplified by the correction algorithms applied to the scalar fluxes, leading to heightened uncertainties in these fluxes (Liu et al, 2006).…”

Technical note: Uncertainties in eddy covariance CO<sub>2</sub> fluxes in a semiarid sagebrush ecosystem caused by gap-filling approaches

“…The ANN algorithm has been successfully applied for filling NEE gaps in various ecosystems (Baldocchi and Sturtevant, 2015;Knox et al, 2016;Moffat et al, 2007;Papale and Valentini, 2003;Tramontana et al, 2016). In this study, we employed the "neuralnet" R package (Günther and Fritsch, 2010), the resilient back-propagation algorithm that has proven to be capable of gap-filling flux data (Dengel et al, 2013;Jammet et al, 2015;Kim et al, 2020;Knox et al, 2016). The required parameters for the ANN algorithm include the number of hidden layers and the number of nodes in each layer.…”

Estimation of Oceanic and Land Carbon Sinks Based on the Most Recent Oxygen Budget

“…However, these results are limited by the lack of spatially explicit observations of carbon changes in vegetation and soils (Arneth et al, 2017) and insufficiency of physical, chemical and biological observations in the ocean (DeVries et al, 2017). Studies also reveal that direct measurement of CO 2 flux may underestimate the uptake due to non-closure of the surface energy balance (Gao et al, 2019(Gao et al, , 2020. Under this circumstance, estimates of the global oxygen budget could provide unique insights for quantifying carbon sinks.…”