Eddy-covariance flux errors due to biases in gas concentration measurements: origins, quantification and correction

Errors in gas concentration measurements by infrared gas analysers can occurduring eddy-covariance campaigns, associated with actual or apparentinstrumental drifts or biases due to thermal expansion, dirtcontamination, aging of components or errors in field operations. Ifoccurring on long timescales (hours to days), these errors are normallyignored during flux computation, under the assumption that errors in meangas concentrations do not affect the estimation of turbulent fluctuationsand, hence, of covariances. By analysing instrument theory of operation, andusing numerical simulations and field data, we show that this is not thecase for instruments with curvilinear calibrations; we further show that ifnot appropriately accounted for, concentration biases can lead to roughlyproportional systematic flux errors, where the fractional errors in fluxesare about 30–40% the fractional errors in concentrations. We quantifythese errors and characterize their dependency on main determinants. We thenpropose a correction procedure that largely – potentially completely –eliminates these errors. The correction, to be applied during fluxcomputation, is based on knowledge of instrument calibration curves and onfield or laboratory calibration data. Finally, we demonstrate the occurrenceof such errors and validate the correction procedure by means of a fieldexperiment, and accordingly provide recommendations for in situ operations