1. Nocturnal Carbon Efflux can estimates of eddy covariance and chamber measurements be reconciled?
Eva van Gorsel1, Ray Leuning1, Helen A. Cleugh1, Heather Keith2 and Tanja Suni1
1Marine and Atmospheric Research 2Forestry and Forest Products
September 06

2. BACKGROUND // MOTIVATION
Comparisons between micrometeorological and chamber measurements almost consistently show:
Respiration is underestimated
NEE overestimated
The problem occurs due to advection (not usually measured)
What is // can be done?
Nocturnal Carbon Efflux - September 06

3. MEASUREMENTS (CHAMBER)
Leaf Respiration
open-flow-through (IRGA, Licor6400)
Wood Respiration (Stems and Root)
transient (IRGA, Licor6200)
Soil Respiration
non-flow-through steady state (Soda lime chambers)
Soil Respiration: 30 plots
Wood Respiration: transient measurement with IRGA of incrasing CO2 conc over time
Leaf respiration: leaf cuvettes (with temperature control) 8 trees
Nocturnal Carbon Efflux - September 06
Keith H. et al. (2006): Measurement of soil CO2 efflux using soda lime absorption […] Soil Biol. and Biochem.
Keith H. et al. (in preparation): Net carbon exchange in a eucalypt forest […]

4. MEASUREMENTS (EDDY COVARIANCE)
EC-System (70 m)
Ultrasonic Anemometer (Gill HS) IRGA (open path Licor7500)
Profile Measurement (9 levels)
IRGA (closed path Licor6262)
Advection Experiment (50 x 50 x 6 m control volume)
40 m tall Eucalypt forest (evergreen, wet sclerophyll)
Lai~2.4
Forest cover and species homogenous within radius > 5 km
Nocturnal Carbon Efflux - September 06
Leuning et al. (2005) Agric. For. Meteorol. 129,

5. MEASUREMENTS (EDDY COVARIANCE)
40 m tall Eucalypt forest (evergreen, wet sclerophyll)
Lai~2.4
Forest cover and species homogenous within radius > 5 km
40 m tall Eucalypt forest (evergreen, wet sclerophyll)
Lai~2.4
Forest cover and species homogenous within radius > 5 km
Nocturnal Carbon Efflux - September 06

6. MEASUREMENTS ( SITE // TOPOGRAPHY)↑ N
Main mast CV
Streamlines tilted within a range of [-6,2] degrees, depending an wind direction.
1km X 1km topographic map tumba
Shallow basin sloping towards N
Nocturnal Carbon Efflux - September 06

7. MEASUREMENTS ( SITE // TOPOGRAPHY)
Shallow basin sloping towards N
40 m tall Eucalypt forest (evergreen, wet sclerophyll)
Lai~2.4
Forest cover and species homogenous within radius > 5 km
Streamlines tilted within a range of [-6,2] degrees, depending an wind direction.
Nocturnal Carbon Efflux - September 06
Photographs : Dale Hughes

8. METHOD (EDDY COVARIANCE)
Nocturnal Carbon Efflux - September 06
Graphics (Schematic tower): Andreas Christen

9. METHOD (EDDY COVARIANCE)
Advection terms not measured? measurements in homogeneous flat terrain
good vertical mixing strong turbulence
Nocturnal Carbon Efflux - September 06
Graphics (Schematic tower): Andreas Christen

10. METHOD (EDDY COVARIANCE)
Advection terms not measured? measurements in homogeneous flat terrain
good vertical mixing strong turbulence
Nocturnal Carbon Efflux - September 06
Graphics (Schematic tower): Andreas Christen

11. METHOD (EDDY COVARIANCE)
Nocturnal Carbon Efflux - September 06
Graphics (Schematic tower): Andreas Christen

12. RESULTS Nocturnal Carbon Efflux // Participation Eddy Flux and Storage Term
not measured fraction of NCE
cumulative frequency (u*)
Nocturnal Carbon Efflux - September 06

13. METHOD (EDDY COVARIANCE)
CALM
INTERMITTENT WINDY
1) even rather shallow topography can lead to cold air drainage, as the slope length rather than the slope angle is a key controlling parameter.
Nocturnal Carbon Efflux - September 06

14. RESULTS Nocturnal Carbon Efflux // Storage term
Nocturnal Carbon Efflux - September 06

15. RESULTS Nocturnal Carbon Efflux // Storage term
Nocturnal Carbon Efflux - September 06
graph: Aubinet et al.,(2005) Boundary Layer Meteorol., 116: 63-94

16. RESULTS Nocturnal Carbon Efflux // Storage term // Advection
Total NCE
Chamber (Soil + Leaf + Wood)
5.18 ± 0.18 mmol m-2 s-1
4.97 mmol m-2 s-1
5.23 mmol m-2 s-1
1.70 mmol m-2 s-1
0.46 mmol m-2 s-1
Nocturnal Carbon Efflux - September 06

17. RESULTS Nocturnal Carbon Efflux // Storage term // Advection
Nocturnal Carbon Efflux - September 06

18. SUMMARY // CONCLUSIONS
Maximum in storage term in early evening
regularly observed
observed at several stations
Nighttime decrease in storage term is not counter-balanced by eddy flux
Advection leads to (usually) unmeasured loss of CO2 from control volume.
Hypothesis:
the peak occurs because there is a time delay between the onset of radiative cooling and the development of temperature gradients that are strong enough to initiate thermally-driven horizontal and vertical flows that remove the stored CO2.
It has been shown that…
Nocturnal Carbon Efflux - September 06

19. CONCLUSIONS We propose taking advantage of this time delay to develop
relationships between Rmax and Tsoil.
Comparison of chamber measurements and results from the developed temperature response function lead to excellent agreement.
The new parameterization leads to realistic values of nighttime respiration,
and therefore improves estimates of net ecosystem exchange.
It has been shown that…
Nocturnal Carbon Efflux - September 06

20. Thank You CSIRO Marine and Atmospheric Research OzFlux
Dr. Eva van Gorsel
Research Scientist – Land and Atmosphere
+61 (0)
OzFlux
Thank You
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