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Sökning: WFRF:(Woodward C) > (2000-2004) > (2004)

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  • Martens, C.S., et al. (författare)
  • Radon fluxes in tropical forest ecosystems of Brazilian Amazonia: night-time CO2 net ecosystem exchange derived from radon and eddy covariance methods.
  • 2004
  • Ingår i: Global Change Biology. - 1354-1013 .- 1365-2486. ; 10:5, s. 618-629
  • Tidskriftsartikel (refereegranskat)abstract
    • Radon-222 (Rn-222) is used as a transport tracer of forest canopy–atmosphere CO2 exchange in an old-growth, tropical rain forest site near km 67 of the Tapajós National Forest, Pará, Brazil. Initial results, from month-long periods at the end of the wet season (June–July) and the end of the dry season (November–December) in 2001, demonstrate the potential of new Rn measurement instruments and methods to quantify mass transport processes between forest canopies and the atmosphere. Gas exchange rates yield mean canopy air residence times ranging from minutes during turbulent daytime hours to greater than 12 h during calm nights. Rn is an effective tracer for net ecosystem exchange of CO2 (CO2 NEE) during calm, night-time hours when eddy covariance-based NEE measurements are less certain because of low atmospheric turbulence. Rn-derived night-time CO2 NEE (9.00±0.99 μmol m−2 s−1 in the wet season, 6.39±0.59 in the dry season) was significantly higher than raw uncorrected, eddy covariance-derived CO2 NEE (5.96±0.51 wet season, 5.57±0.53 dry season), but agrees with corrected eddy covariance results (8.65±1.07 wet season, 6.56±0.73 dry season) derived by filtering out lower NEE values obtained during calm periods using independent meteorological criteria. The Rn CO2 results suggest that uncorrected eddy covariance values underestimate night-time CO2 loss at this site. If generalizable to other sites, these observations indicate that previous reports of strong net CO2 uptake in Amazonian terra firme forest may be overestimated.
  • Forsman, Jan, et al. (författare)
  • Evaluating the accuracy of a density functional theory of polymer solutions with additive hard sphere diameters
  • 2004
  • Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606. ; 120:1, s. 506-510
  • Tidskriftsartikel (refereegranskat)abstract
    • We assess the accuracy of a density functional theory for athermal polymer solutions, consisting of solvent particles with a smaller radius than that of the monomers. The monomer and solvent density profiles in a slit bound by hard, flat, and inert surfaces are compared with those obtained by a Metropolis Monte Carlo simulation. At the relatively high density at which the comparison is performed, there are considerable packing effects at the walls. The density functional theory introduces a simple weight function to describe nonlocal correlations in the fluid. A recent study of surface forces in polymer solutions used a different weighting scheme to that proposed in this article, leading to less accurate results. The implications of the conclusions of that study are discussed. (C) 2004 American Institute of Physics.
  • Woodward, C E, et al. (författare)
  • Density functional study of surface forces in solutions containing star-shaped polymers
  • 2004
  • Ingår i: Macromolecules. - : The American Chemical Society (ACS). - 0024-9297. ; 37:18, s. 7034-7041
  • Tidskriftsartikel (refereegranskat)abstract
    • Density functional theory is used to study polymer solutions between surfaces in equilibrium with a bulk solution. We investigate the effect on the surface interaction free energy of having star polymers with different numbers of arms. In addition, we consider the role played by the strength of the adsorption potential and the concentration of the polymer molecules. The interaction free energy is found to scale quite well with the number of arms on the stars, and the separation dependence of the interaction scales approximately with the radius of gyration. When the adsorption potential is weak, the polymer molecules are depleted. A free energy barrier is nevertheless present-a phenomenon often referred to as depletion stabilization. The barrier scales with the number of arms on the star polymers.
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  • Resultat 1-4 av 4

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