Accuracies of field CO<sub>2</sub>−H<sub>2</sub>O data from open-path eddy-covariance flux systems: Assessment based on atmospheric physics and biological environment

Abstract. Ecosystem CO2−H2O data measured vastly from open-path eddy-covariance (OPEC) systems by infrared analyzers have numerous applications in biogeosciences. To assess the applicability, data uncertainties from measurements are needed. The uncertainties are sourced from infrared analyzers in zero drift, gain drift, cross-sensitivity, and precision variability. The sourced uncertainties are individually specified for analyzer performance, but no methodology exists to comprehend these individual uncertainties into a cumulative error for the specification of an overall accuracy, which is ultimately needed. Using the methodology for close-path eddy-covariance systems, this accuracy for OPEC systems is determined from all individual uncertainties via an accuracy model further formulated into CO2 and H2O accuracy equations. Based on atmospheric physics and the biological environment, these equations are used to evaluate CO2 accuracy (±1.21 20 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). Cross-sensitivity and precision variability are minor, although unavoidable, uncertainties. Zero drifts and gain drifts are major uncertainties but are adjustable via corresponding zero and span procedures during field maintenance. The equations provide rationales to assess and guide the procedures. In an atmospheric CO2 background, CO2 zero and span procedures can narrow CO2 accuracy by 40 %, from ±1.21 to ±0.72 mgCO2 m−3. In hot and humid weather, H2O gain drift potentially adds more to H2O measurement uncertainty, which requires more attention. If H2O zero and span procedures can be performed practically from 5 to 35 ºC, the poorest H2O accuracy can be improved by 30 %, from ±0.10 to ±0.07 gH2O m−3. Under freezing conditions, an H2O span is both impractical and unnecessary, but the zero procedure becomes imperative to minimize H2O measurement uncertainty. In cold/dry conditions, the zero procedure for H2O, along with CO2, is an operational and efficient option to ensure and improve H2O accuracy.