Accuracies of field CO 2 –H 2 O data from open-path eddy-covariance flux systems: assessment based on atmospheric physics and biological environment

[摘要] Ecosystem CO 2 –H 2 O data measured by infraredgas analyzers in open-path eddy-covariance (OPEC) systems have numerousapplications, such as estimations of CO 2 and H 2 O fluxes in theatmospheric boundary layer. To assess the applicability of the data forthese 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 sourcesare individually specified for analyzer performance, but so far no methodologyexists yet to combine these individual sources into a composite uncertaintyfor the specification of an overall accuracy, which is ultimately needed.Using the methodology for closed-path eddy-covariance systems, this overallaccuracy for OPEC systems is determined from all individual uncertaintiesvia an accuracy model and further formulated into CO 2 and H 2 Oaccuracy equations. Based on atmospheric physics and the biologicalenvironment, for EC150 infrared CO 2 –H 2 O analyzers, theseequations are used to evaluate CO 2 accuracy ( ±1.22  mgCO 2  m −3 , relatively ±0.19  %) and H 2 O accuracy ( ±0.10  gH 2 O m −3 , relatively ±0.18  % in saturated air at 35  ∘ C and 101.325 kPa). Both accuracies are applied to conceptualmodels addressing their roles in uncertainty analyses for CO 2 andH 2 O fluxes. For the high-frequency air temperature derived fromH 2 O density along with sonic temperature and atmospheric pressure, therole of H 2 O accuracy in its uncertainty is similarly addressed. Amongthe four uncertainty sources, cross-sensitivity and precision variabilityare minor, although unavoidable, uncertainties, whereas zero drift and gaindrift are major uncertainties but are minimizable via corresponding zero andspan procedures during field maintenance. The accuracy equations providerationales to assess and guide the procedures. For the atmosphericbackground CO 2 concentration, CO 2 zero and CO 2 spanprocedures can narrow the CO 2 accuracy range by 40 %, from ±1.22 to ±0.72  mgCO 2  m −3 . In hot and humid weather, H 2 Ogain drift potentially adds more to the H 2 O measurement uncertainty,which requires more attention. If H 2 O zero and H 2 O span procedurescan be performed practically from 5 to 35  ∘ C, the H 2 Oaccuracy can be improved by at least 30 %: from ±0.10 to ±0.07  gH 2 O m −3 . Under freezing conditions, the H 2 O spanprocedure is impractical but can be neglected because of its trivialcontributions to the overall uncertainty. However, the zero procedure forH 2 O, along with CO 2 , is imperative as an operational and efficientoption under these conditions to minimize H 2 O measurement uncertainty.

[学科分类] 再生能源与代替技术