Improving the CO2 storage measurements in a tall temperate forest

The storage change term is calculated to account for the CO₂ accumulation in the air column underneath the eddy covariance system when calculating net surface exchange. The rigorous form requires continuous measurements in space and time. Historically, a standard approximation was to measure CO₂ concentrations solely at one point, the eddy covariance system measurement height. Since the storage term can contribute significantly to annual fluxes, e.g., the Net Ecosystem Exchange of CO₂, other methods have been investigated, such as the vertically distributed measurements within a profile system.

This work aims at providing a physically sound storage change calculation method for long-term atmospheric stations such as ICOS. Additionally, the extrapolation to historical time series brings the possibility to correct the storage term when only the one-point measurements were available.

We used one year of eddy covariance and profile system data collected in the Sorø temperate forest in Denmark and compared different methods to calculate the storage change. We compared the top tower measurements with the results from the vertical profile system. We also considered the temporal resolution by contrasting two methods to average the samples: subtracting the end and beginning concentrations as the first method and using the times series linear regression slope as the second. We highlighted the effects on annual budgets, surface fluxes, and the interactions with other turbulence-driven variables, namely the friction velocity and the standard deviation of vertical velocity fluctuations.

These results are thus relevant for high canopies and other landscapes investigated with tall towers, where the storage change is expected to impact the annual budgets considerably.