Advances in the True Eddy Accumulation Method: New theory, implementation, and field results

Abstract. The true eddy accumulation method (TEA) provides direct measurements of ecosystem-level fluxes for a wide range of atmospheric constituents. TEA utilizes conditional sampling to overcome the requirement for a fast sensor response usually demanded by the state-of-the-art eddy covariance method (EC).

However, the assumptions and conditions required for the TEA method are often not met. Here we explore the limitations caused by the assumption of zero mean vertical wind velocity during the averaging interval and by the fixed accumulation interval.

We extend the theory of TEA method to non-zero vertical wind velocity by employing information about the scalar transport. We further derive a new method with adaptive time varying accumulation intervals. The new method, termed short-time eddy accumulation (STEA), was successfully implemented and deployed to measure CO₂ fluxes over an agricultural field in Braunschweig, Germany. The measured fluxes matched very well against a conventional EC system (slope of 1.05, R² of 0.87). We provide a detailed description of the setup and operation of the STEA system in the flow-through mode, devise an empirical correction for the effect of buffer volumes, and describe the important considerations for the successful operation of the STEA method.

The new theory developments reduce the bias and uncertainty in the measured fluxes and create new ways to design eddy accumulation systems with finer control over sampling and accumulation. The results encourage the application of TEA and STEA for measuring fluxes of more challenging atmospheric constituents such as reactive species as well as other constituents where no fast gas analyzers are available.