Compressibility of air effects on eddy accumulation flux measurements

Eddy accumulation is a direct flux measurement technique for trace gas exchange between the land surface and the atmosphere. Eddy accumulation complements the now common eddy covariance method in its ability to measure even small fluxes accurately with slow response gas analyzers and being power efficient. However, the physically most direct way of eddy accumulation, also known as true eddy accumulation (TEA), requires the sampling of air at a rate proportional to the vertical wind velocity at a fast rate of typically 10 Hz or more. Lack of suitable methods for high-speed air sampling has been a primary limitation for the practical application of eddy accumulation in the past. The compressibility of air causes a variation of pressure inside the sampling system, which affects the ability to control the sample flow rate accurately, potentially compromising the derived flux measurements. It is therefore essential to quantify the effect of compressibility on fluxes and understand the parameters which allow for mitigating the effect at the design stage.
In this study, we present successful true eddy accumulation measurements over the old-growth forest at the Fluxnet site Hainich (DE-Hai) and quantify the compressibility effects on fluxes. Performing simulations on high-frequency data of CO₂ and vertical wind velocity for a range of system configurations, we are able to quantify the impact of compressibility on fluxes and explain why our measurements were successful. We find that different system configurations lead to flux changes over a representative range of 1 to 25 percent of the flux. Key controlling parameters are the size and arrangement of internal buffer volumes and the appropriate control of the inlet flow rate sampling device as a function of internal and external pressure states. This knowledge allows to mitigate compressibility effects and design accurate true eddy accumulation flux measurements for a range of atmospheric constituents.