Spectral versus co-spectral correction of eddy-covariance fluxes: a multi-site assessment across the ICOS network

Abstract. Accurate correction of high-frequency attenuation remains a critical challenge in eddy-covariance measurements of turbulent gas fluxes. Experimental correction approaches based on either power spectra or co-spectra are widely used, yet their relative behaviour and implications for flux estimates have not been systematically assessed across sites and atmospheric conditions.

This study presents a multi-site comparison of these two approaches using one year of CO₂ and H₂O flux data from 38 ecosystem stations of the Integrated Carbon Observation System (ICOS) equipped with a standardized enclosed-path gas analyser setup. To support reproducibility and facilitate future methodological intercomparisons, we additionally provide an open-source and configurable software tool implementing both correction approaches.

Overall, the ICOS setup exhibited limited high-frequency attenuation, with correction factors generally below 1.2 for CO₂ and occasionally approaching 2 for H₂O under humid conditions. For CO₂, differences between methods remained small after standard turbulence filtering, with cumulative flux differences typically below 2 %. In contrast, H₂O corrections showed larger discrepancies, frequently reaching 5–10 % on cumulative fluxes. The largest differences occurred under stable atmospheric conditions, at low measurement heights, and under strong attenuation. Results indicate that CO₂ attenuation is dominated by sensor-separation effects, whereas H₂O attenuation is primarily controlled by adsorption–desorption processes within the sampling system.

The comparison highlights methodological limitations of both approaches. Both methods rely on assumptions regarding spectral similarity, but departures from these assumptions were found to affect gas spectra much more strongly than co-spectra, particularly for H₂O under humid conditions. In addition, spectral corrections were frequently affected by high-frequency noise and required dedicated denoising procedures, while sensor-separation effects had to be introduced through analytical formulations. Overall, the co-spectral approach provided more robust and physically consistent results across sites and atmospheric conditions.

A comparison with fluxes produced by the ICOS Ecosystem Thematic Centre revealed systematic differences that were often larger than those observed between the spectral and co-spectral approaches themselves, especially for H₂O. These results demonstrate the effectiveness of setup standardisation across the ICOS network while identifying high-frequency attenuation correction as a remaining source of uncertainty. They also highlight the need for a broader reassessment of this processing step within the ICOS flux-processing pipeline.