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*
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