FOREST CANOPY-TROPOSPHERE CO2 AND TRACE GAS EXCHANGE RATES IN THE FLONA TAPAJOS, PARA, BRAZIL,
DETERMINED BY RN-222 CANOPY AND SOIL FLUX MEASUREMENTS
C.S. Martens1, H.P. Mendlovitz11, T.J. Shay1, J.M.S. Moura2, O.L.L. Moraes3, P.M. Crill4, and R.L. Lima2
1University of North Carolina at Chapel Hill, 2Universidade Federal de Para, 3Universidade Federal de Santa Maria, 4University of New Hampshire
CO 2 data and NEE calculations by Stephen Wofsy, Scott Saleska and Dan Matross, of Harvard University
ABSTRACT
Arrays of flow-through radon detectors (Figure 1) have been deployed since April, 2000, at 65meter
tower sites(Figure 2) at primary forest (67km) and selectively logged forest (83km) in the Tapajos National
Forest. A solar powered array has been utilized at pasture site at 77km. Radon activities measured on the
towers range from over 1.0 pCi/l (0.1m) to less than 0.05 pCi/l at heights ranging from 0.1 to 64m (Figure 3).
Radon detector arrays can resolve 0.01 pCi/l/m radon activity gradients throughout the forest canopy using
15 minute counting intervals.
Figure 1
Troposphere
Troposphere
Canopy
Canopy
h
Figure 2
Canopy
Soil
Soil
Determine gas exchange rate constant k using Radon-222 soil flux S and canopy
inventory (dC/dt) measurements
Figure 3. June-July 2001 tower radon data. Note
west wind event late June at all three sites.
h (dC/dt) - S
k=
CCanopy - CTroposphere
Changes in canopy radon inventories and gas
exchange at the tower sites are also closely linked to
meteorological events and conditions including wind
speed and direction plus thickness of the convective
boundary layer (CBL). During late June 2001, a west
wind event lasting approximately 5 days significantly
damped canopy-atmosphere gas exchange as seen in
increased radon canopy inventories (Figures 3 and 4)
and lower calculated gas exchange rate coefficients (k,
Figure 5).
The calculated divergence in radon flux versus
height starting with the measured soil radon flux, S,
(see Mauro et al. poster), can then be utilized to
calculate eddy diffusivities (Kz) as a function of height
within the canopy (Figure 6).
These Kz values are particularly useful for
calculating gas fluxes in the lower 10m of the canopy
where large gas concentration gradients occur. Diel
radon activity variations at both tower sites are
characterized by a nocturnal concentration maxima
(Figure 5) peaking near approximately 0900 and a
secondary maximum at around 1730, the “early
evening transition (“EET”), that produces elevated
inventories largely below 3m height within the canopy.
Figure 4. Radon activity versus height,
radon inventory and calculated radon
fluxes versus canopy height
NEE INFERRED FROM RADON
Figure 5. Calculated gas exchange rate
constants (k). Note the lowered values
during the west wind event
Figure 7. Occurrence of low u* conditions
More than 50% of eddy covariance CO2 flux measurements are lost during calm
nighttime (9:00pm to 4:00am local time) conditions when the friction velocity (u*)
at eddy covariance measurement height is generally less than 0.2 m/sec (Figure 7).
The continuous canopy air plus soil-air flux measurements of radon-222 can be
combined with CO2 concentration and storage data to calculate total CO2 transport.
The total CO2 transport (T) can be calculated directly from radon data based on the
excellent observed correlation between CO2 and radon concentrations using the
equation:
T = {CO2canopy-CO2top} / {Rncanopy-Rntop} * {(S) - [(h) dRn/dt]}
Net Ecosystem Exchange (NEE in µmol/m2sec) can then be calculated by adding
in CO2 storage. Total radon-inferred NEE (CO2 transport plus storage) versus u* is
illustrated in Figure 8. NEE values derived from radon versus directly measured
eddy covariance measurements at low u* are compared in Figure 9. The “lost
carbon” unaccounted for because of the technical problems with eddy covariance
measurements at low u* values is calculated in Figure 10 as the difference between
radon inferred and eddy covariance methods.
Figure 9. Radon inferred
versus eddy covariance NEE
vs u*
Figure 10. “Lost Carbon”
calculated as Radon - eddy
covariance NEE.
Figure 6. Eddy diffusivities versus canopy
height calculated from soil radon flux and flux
divergence.
Figure 8. NEE calculated from radon and CO2
storage data versus u*