Lagrangian dispersion of 222Rn, H2O and CO2 within the Amazonian rain forest

Simon, Eric; Lehmann, B.E.; Ammann, C.; Ganzeveld, L.; Rummel, U.; Meixner, F.X.; Nobre, A.D.; Araújo, A.; Kesselmeier, J. (2005). Lagrangian dispersion of 222Rn, H2O and CO2 within the Amazonian rain forest. Agricultural and forest meteorology, 132(3-4), pp. 286-304. Elsevier 10.1016/j.agrformet.2005.08.004

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The present study focuses on the description of the vertical dispersion of trace gases within the Amazon rain forest. A Lagrangian approach is parameterised using in-canopy turbulence measurements made at a site in Rondônia (Reserva Jaru). In contrast to common scaling schemes that solely depend on friction parameters measured above the canopy, a combined scaling that also includes night-time free convective mixing in the lower part of dense vegetation canopies is proposed here. 222Rn concentration profiles and soil flux measurements made at a second site near Manaus (Reserva Cuieiras) are used to evaluate the derived parameterisation and the uncertainties of the forward (prediction of concentration profiles) and inverse (prediction of vertical source/sink distributions) solution of the transfer equations. Averaged day- and night-time predictions of the forward solution agree with the observations within their uncertainty range. During night-time, a weak, but effective free convective mixing process in the lower canopy ensures a relatively high flushing rate with residence times of <1 h at half canopy height in contradiction to earlier estimates for Amazon rain forest.

The inverse solution for 222Rn source/sink distributions shows a high sensitivity to small measurement errors, especially for day-time conditions, when there is efficient turbulent mixing in the upper canopy and profile gradients are small. The inverse approach is also applied to CO2 and H2O profiles. The predicted net fluxes show a reasonable agreement with Eddy Covariance (EC) measurements made above the forest canopy, although the scatter is large and the day-time solutions for CO2 are very sensitive to measurement errors. However, this is not the case for typical night-time conditions, where the CO2 profile gradients in the upper canopy are large. The inverse approach predicts a mean CO2 emission flux of 7.5 μmol m−2 s−1 for the investigation period. This value is somewhat larger compared to estimates based on EC measurements, which are quite uncertain at night-time and thus reduces the upper bound of the estimated carbon sink strength for Amazonian rain forest.

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