Based on Atmospheric Physics and Ecological Principle to Assess the Accuracies of Field CO2 /H2O Measurements From Infrared Gas Analyzers in Closed-Path Eddy-Covariance Systems

Authors

Xinhua Zhou, Institute of Applied Ecology
Tian Gao, Institute of Applied Ecology
Yunchao Pang, Institute of Applied Ecology
Hayden Mahan, Campbell Scientific Incorporation
Xiufen Li, Institute of Applied Ecology
Ning Zheng, Institute of Applied Ecology
Andrew E. Suyker, University of Nebraska - LincolnFollow
Tala Awada, University of Nebraska - LincolnFollow
Jiaojun Zhu, Institute of Applied Ecology

ORCID IDs

Xinhua Zhou

Tian Gao

Yunchao Pang

Ning Zheng

Jiaojun Zhu

Document Type

Article

Date of this Version

9-21-2021

Citation

Zhou, X., Gao, T., Pang, Y., Mahan, H., Li, X., Zheng, N., et al. (2021). Based on atmospheric physics and ecological principle to assess the accuracies of field CO2 /H2 O measurements from infrared gas analyzers in closed-path eddy-covariance systems. Earth and Space Science, 8, e2021EA001763. https://doi.org/10.1029/2021EA001763

Comments

OPEN ACCESS

Abstract

Field CO₂ /H₂O measurements from infrared gas analyzers in closed-path eddy-covariance systems have wide applications in earth sciences. Knowledge about exactness of these measurements is required to assess measurement applicability. Although the analyzers are specified with uncertainty components (zero drift, gain drift, cross-sensitivities, and precision), exactness for individual measurements is unavailable due to an absence of methodology to comprehend the components as an overall uncertainty. Adopting an advanced definition of accuracy as a range of all measurement uncertainty sources, the specified components are composited into a model formulated for studying analyzers’ CO₂ /H₂O accuracy equations. Based on atmospheric physics and environmental parameters, the analyzers are evaluated using the equations for CO₂ accuracy (±0.78 µmolCO2 mol⁻¹, relatively ±0.18%) and H₂O accuracy (±0.15 mmolH₂ O mol⁻¹). Evaluation shows that precision and cross-sensitivity are minor uncertainties while zero and gain drifts are major uncertainties. Both drifts need adjusting through zero/span procedures during field maintenance. The equations provide rationales to guide and assess the procedures. H₂O span needs more attentions under humid conditions. Under freezing conditions while H₂O span is impractical, this span is fortunately unnecessary. Under the same conditions, H₂O zero drift dominates H₂O measurement uncertainty. Therefore, automatic zero becomes a more applicable and necessary tactic. In general cases of atmospheric CO₂ background, automatic CO₂ zero/ span procedures can narrow CO₂ accuracy by 36% (±0.74 to ± 0.47 µmolCO2 mol⁻¹). Automatic/manual H2 O zero/span procedures can narrow H₂O accuracy by 27% (±0.15 to ±0.11 mmolH₂O mol⁻¹). While ensuring system specifications, the procedures guided by equations improve measurement accuracies.