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| Home > Publications database > Performance of a Virtual Tall Tower (VTT) approach for estimating CO2 concentrations in the mixed layer from eddy covariance measurements near the surface |
| Conference Presentation (Other) | FZJ-2026-03543 |
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2026
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Please use a persistent id in citations: doi:10.5194/egusphere-egu26-17469 doi:10.34734/FZJ-2026-03543
Abstract: Atmospheric CO₂ measurements provide essential constraints for carbon-budget estimates andatmospheric modelling. Virtual tall tower (VTT) methods are a promising, but yet underexaminedapproach for upscaling ecosystem-level CO₂ concentrations measured at eddy covariance (EC)sites (typically 2–50 m above ground) to atmospheric measurements representative of tall towers(TT; ~100 m and higher). Implementing VTT approaches at existing EC stations could thereforeexpand the currently sparse network of TT observations. In this study, we evaluate the applicabilityof a VTT approach using collocated EC–TT measurements. We use 2024 data from the combinedecosystem-atmosphere station Svartberget site in northern Sweden (SE-Svb, SVB), part of theICOS (Integrated Carbon Observation System) network, with brief examples from one or moreother sites. A key advantage of the ICOS Svartberget station is that ecosystem EC and atmosphericTT measurements are available at the same location, with EC observations at 35 m andTT measurements at 35 m and 150 m. The 35 m TT measurements are an important asset for posthoccalibration correction of the concentrations measured by the EC system, since state-of-the-artEC stations typically do not meet the high calibration requirements of a TT measurement. Weimplemented the VTT method proposed by Haszpra et al. (2015) and tested the gradient functionsof Patton et al. (2003) and Wang et al. (2007) to define a base-run configuration. We thenperformed a sensitivity analysis of key variables in the VTT formulation. Model performance wasevaluated using bias, root mean square error (RMSE), and correlation, by comparing VTTestimatedCO₂ concentrations at the TT top height (150 m) against measured TT concentrations.For 2024, approximately 30% of valid hourly data points met the well-mixed criteria required forVTT application. When treating EC calibration and VTT calculations as separate steps, EC calibrationexerted the largest influence on estimated CO₂ at TT height, highlighting calibration as a criticalprerequisite for reliable mixed-layer concentration estimates. Sensitivity analysis further showedthat, when accounting for both numerical perturbations and measurement uncertainty, theplanetary boundary layer height was the most influential variable, producing the largest changesin performance statistics relative to the target TT concentrations. Taken together, these resultssuggest that VTT approaches could increase the coverage of TT-representative atmospheric CO₂estimates. Improving planetary boundary layer height (PBLH) estimates should further increaseVTT accuracy. Further steps, VTT performance should be tested across additional sites and timeperiods to assess robustness under different conditions.
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