Accuracies of field CO2–H2O data from open-path eddy-covariance flux systems: assessment based on atmospheric physics and biological environment

Abstract

Ecosystem CO₂–H₂O data measured by infrared gas analyzers in open-path eddy-covariance (OPEC) systems have numerous applications, such as estimations of CO₂ and H₂O fluxes in the atmospheric boundary layer. To assess the applicability of the data for these estimations, data uncertainties from analyzer measurements are needed. The uncertainties are sourced from the analyzers in zero drift, gain drift, cross-sensitivity, and precision variability. These four uncertainty sources are individually specified for analyzer performance, but so far no methodology exists yet to combine these individual sources into a composite uncertainty for the specification of an overall accuracy, which is ultimately needed. Using the methodology for closed-path eddy-covariance systems, this overall accuracy for OPEC systems is determined from all individual uncertainties via an accuracy model and further formulated into CO₂ and H₂O accuracy equations. Based on atmospheric physics and the biological environment, for EC150 infrared CO₂–H₂O analyzers, these equations are used to evaluate CO₂ accuracy (±1.22 mgCO₂ m⁻³, relatively ±0.19 %) and H₂O accuracy (±0.10 gH₂O m⁻³, relatively ±0.18 % in saturated air at 35 ^∘C and 101.325 kPa). Both accuracies are applied to conceptual models addressing their roles in uncertainty analyses for CO₂ and H₂O fluxes. For the high-frequency air temperature derived from H₂O density along with sonic temperature and atmospheric pressure, the role of H₂O accuracy in its uncertainty is similarly addressed. Among the four uncertainty sources, cross-sensitivity and precision variability are minor, although unavoidable, uncertainties, whereas zero drift and gain drift are major uncertainties but are minimizable via corresponding zero and span procedures during field maintenance. The accuracy equations provide rationales to assess and guide the procedures. For the atmospheric background CO₂ concentration, CO₂ zero and CO₂ span procedures can narrow the CO₂ accuracy range by 40 %, from ±1.22 to ±0.72 mgCO₂ m⁻³. In hot and humid weather, H₂O gain drift potentially adds more to the H₂O measurement uncertainty, which requires more attention. If H₂O zero and H₂O span procedures can be performed practically from 5 to 35 ^∘C, the H₂O accuracy can be improved by at least 30 %: from ±0.10 to ±0.07 gH₂O m⁻³. Under freezing conditions, the H₂O span procedure is impractical but can be neglected because of its trivial contributions to the overall uncertainty. However, the zero procedure for H₂O, along with CO₂, is imperative as an operational and efficient option under these conditions to minimize H₂O measurement uncertainty.