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Impact of CO2 storage flux sampling uncertainty on net ecosystem exchange measured by eddy covariance

Nicolini, Giacomo; Aubinet, Marc; Feigenwinter, Christian; Heinesch, Bernard; Lindroth, Anders LU ; Mamadou, Ossénatou; Moderow, Uta; Mölder, Meelis LU ; Montagnani, Leonardo and Rebmann, Corinna, et al. (2018) In Agricultural and Forest Meteorology 248. p.228-239
Abstract

Complying with several assumption and simplifications, most of the carbon budget studies based on eddy covariance (EC) measurements quantify the net ecosystem exchange (NEE) by summing the flux obtained by EC (FC) and the storage flux (SC). SC is the rate of change of a scalar, CO2 molar fraction in this case, within the control volume underneath the EC measurement level. It is given by the difference in the quasi-instantaneous profiles of concentration at the beginning and end of the EC averaging period, divided by the averaging period. The approaches used to estimate SC largely vary, from measurements based on a single sampling point usually located at the EC measurement height, to measurements based on profile sampling.... (More)

Complying with several assumption and simplifications, most of the carbon budget studies based on eddy covariance (EC) measurements quantify the net ecosystem exchange (NEE) by summing the flux obtained by EC (FC) and the storage flux (SC). SC is the rate of change of a scalar, CO2 molar fraction in this case, within the control volume underneath the EC measurement level. It is given by the difference in the quasi-instantaneous profiles of concentration at the beginning and end of the EC averaging period, divided by the averaging period. The approaches used to estimate SC largely vary, from measurements based on a single sampling point usually located at the EC measurement height, to measurements based on profile sampling. Generally a single profile is used, although multiple profiles can be positioned within the control volume. Measurement accuracy reasonably increases with the spatial sampling intensity, however limited resources often prevent more elaborated measurement systems. In this study we use the experimental dataset collected during the ADVEX campaign in which turbulent and non-turbulent fluxes were measured in three forest sites by the simultaneous use of five towers/profiles. Our main objectives are to evaluate both the uncertainty of SC that derives from an insufficient sampling of CO2 variability, and its impact on concurrent NEE estimates.Results show that different measurement methods may produce substantially different SC flux estimates which in some cases involve a significant underestimation of the actual SC at a half-hourly time scales. A proper measuring system, that uses a single vertical profile of which the CO2 sampled at 3 points (the two closest to the ground and the one at the lower fringe of the canopy layer) is averaged with CO2 sampled at a certain distance and at the same height, improves the horizontal representativeness and reduces this (proportional) bias to 2–10% in such ecosystems. While the effect of this error is minor on long term NEE estimates, it can produce significant uncertainty on half-hourly NEE fluxes.

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published
subject
keywords
ADVEX, Carbon balance, Forest ecosystems, ICOS, Profile measurement
in
Agricultural and Forest Meteorology
volume
248
pages
12 pages
publisher
Elsevier
external identifiers
  • scopus:85030673114
ISSN
0168-1923
DOI
10.1016/j.agrformet.2017.09.025
language
English
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yes
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1c3eb058-57e3-42a6-9ebb-54a63048af3e
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2017-10-16 07:53:50
date last changed
2018-01-07 12:22:27
@article{1c3eb058-57e3-42a6-9ebb-54a63048af3e,
  abstract     = {<p>Complying with several assumption and simplifications, most of the carbon budget studies based on eddy covariance (EC) measurements quantify the net ecosystem exchange (NEE) by summing the flux obtained by EC (FC) and the storage flux (SC). SC is the rate of change of a scalar, CO<sub>2</sub> molar fraction in this case, within the control volume underneath the EC measurement level. It is given by the difference in the quasi-instantaneous profiles of concentration at the beginning and end of the EC averaging period, divided by the averaging period. The approaches used to estimate SC largely vary, from measurements based on a single sampling point usually located at the EC measurement height, to measurements based on profile sampling. Generally a single profile is used, although multiple profiles can be positioned within the control volume. Measurement accuracy reasonably increases with the spatial sampling intensity, however limited resources often prevent more elaborated measurement systems. In this study we use the experimental dataset collected during the ADVEX campaign in which turbulent and non-turbulent fluxes were measured in three forest sites by the simultaneous use of five towers/profiles. Our main objectives are to evaluate both the uncertainty of SC that derives from an insufficient sampling of CO<sub>2</sub> variability, and its impact on concurrent NEE estimates.Results show that different measurement methods may produce substantially different SC flux estimates which in some cases involve a significant underestimation of the actual SC at a half-hourly time scales. A proper measuring system, that uses a single vertical profile of which the CO<sub>2</sub> sampled at 3 points (the two closest to the ground and the one at the lower fringe of the canopy layer) is averaged with CO<sub>2</sub> sampled at a certain distance and at the same height, improves the horizontal representativeness and reduces this (proportional) bias to 2–10% in such ecosystems. While the effect of this error is minor on long term NEE estimates, it can produce significant uncertainty on half-hourly NEE fluxes.</p>},
  author       = {Nicolini, Giacomo and Aubinet, Marc and Feigenwinter, Christian and Heinesch, Bernard and Lindroth, Anders and Mamadou, Ossénatou and Moderow, Uta and Mölder, Meelis and Montagnani, Leonardo and Rebmann, Corinna and Papale, Dario},
  issn         = {0168-1923},
  keyword      = {ADVEX,Carbon balance,Forest ecosystems,ICOS,Profile measurement},
  language     = {eng},
  month        = {01},
  pages        = {228--239},
  publisher    = {Elsevier},
  series       = {Agricultural and Forest Meteorology},
  title        = {Impact of CO<sub>2</sub> storage flux sampling uncertainty on net ecosystem exchange measured by eddy covariance},
  url          = {http://dx.doi.org/10.1016/j.agrformet.2017.09.025},
  volume       = {248},
  year         = {2018},
}