be explained by the indirect aerosol cloud effect. The use of a parcelmodel to determine the cloud droplet number concentrationenables us to separate the effects of the cloud LWP and clouddroplet number concentration on the cloud optical depth. Anexamination of the TOA shortwave flux from the radiative transfermodel applied to the two sites does not directly confirm the indirecteffect, because the observed surface albedos at the NSA site for ourcases (0.6 ^ 0.28) are significantly larger than those from the SGPsite (0.2 ^ 0.02). However, the model can be used to estimate theoutgoing flux difference if the clouds from the NSA site had thesame average surface albedo and average zenith angle as those fromthe SGP site (see Fig. 3). This analysis indicates that these sitesprovide important evidence corroborating the effect of aerosols oncloud optical properties and on shortwave fluxes at both the surfaceand the TOA. Moreover, the analysis indicates that a parameteriza-tion of the effects of aerosols on clouds on the basis of an adiabaticparcel model and average aerosol size distributions such as thoseused in current general circulation models18,22,23provides a goodestimate of cloud optical properties determined over a broad rangeof aerosol concentrations. AReceived 28 May; accepted 17 November 2003; doi:10.1038/nature02234.1. Penner, J. E. et al. in Climate Change 2001: The Scientific Basis (eds Houghton, J. T. et al.) 289–348(Cambridge Univ. Press, Cambridge, UK, 2001).2. Rosenfeld, D. & Feingold, G. Explanation of discrepancies among satellite observations of the aerosolindirect effects. Geophys. Res. Lett. 30, doi:10.1029/2003GL017684 (2003).3. Brenguier, J.-L., Pawlowska, H. & Schu¨ller, L. J. Cloud microphysical and radiative properties forparameterization and satellite monitoring of the indirect effect of aerosol on climate. J. Geophys. Res.108, doi:101029/2002JD002682 (2003).4. Sheridan, P. J., Delene, D. J. & Ogren, J. A. Four years of continuous surface aerosol measurementsfrom the Department of Energy’s Atmospheric Radiation measurement Program Southern GreatPlains Cloud and Radiation Testbed site. J. Geophys. Res. 106, 20735–20747 (2001).5. Dong, X., Ackerman, T. P., Clothiaux, E. E., Pilewskie, P. & Han, Y. Microphysical and radiativeproperties of stratiform clouds deduced from ground-based measurements. J. Geophys. Res. 102,23829–23843 (1997).6. Dong, X., Ackerman, T. P. & Clothiaux, E. E. Parameterizations of microphysical and shortwaveradiative properties of boundary layer stratus from ground-based measurements. J. Geophys. Res. 102,31681–31393 (1998).7. Dong, X., Mace, G. G., Minnis, P. & Young, D. F. Arctic stratus cloud properties and their effect on thesurface radiation budget: selected cases from FIRE ACE. J. Geophys. Res. 106, 15297–15312 (2001).8. Dong, X. et al. Comparison of stratus cloud properties deduced from surface, GOES, and aircraft dataduring the March 2000 ARM Cloud IOP. J. Atmos. Sci. 59, 3265–3284 (2002).9. Dong, X. & Mace, G. G. Profiles of low-level stratus cloud microphysics deduced from ground-basedmeasurements. J. Atmos. Ocean. Tech. 20, 42–53 (2003).10. Liljegren, J. C., Clothiaux, E. E., Mace, G. G., Kato, S. & Dong, X. A new retrieval for liquid water pathusing a ground based microwave radiometer and measurements of cloud temperature. J. Geophys. Res.106, 14485–14500 (2001).11. Dong, X. & Mace, G. G. Arctic stratus cloud properties and radiative forcing derived from ground-based data collected at Barrow Alaska. J. Clim. 16, 445–461 (2003).12. Liu, X. & Seidl, W. Modeling study of cloud droplet nucleation and in-cloud sulfate productionduring the Sanitation of the Atmosphere (SANA) 2 campaign. J. Geophys. Res. 103, 16145–16158(1998).13. Delene, D. J. & Deshler, T. Vertical profiles of cloud condensation nuclei above Wyoming. J. Geophys.Res. 106, 12579–12588 (2001).14. Quinn, P. K. et al. A three-year record of simultaneously measured aerosol chemical and opticalproperties at Barrow, Alaska. J. Geophys. Res. D 107, doi:101029/2001JD001248 (2002).15. Climate Modeling and Diagnostics Laboratory data archive khttp://www.cmdl.noaa.gov/info/ftpdata.htmll (2000).16. Twomey, S. The nuclei of natural clouds formation. Part II: The supersaturation in natural clouds andthe variation of cloud droplet concentration. Geofis. Pura Appl. 43, 243–249 (1959).17. Rogers, E., Deaven, D. G. & DiMego, G. J. The regional analysis system for the operational “early” etamodel: original 80-km configuration and recent changes. Weath. Forecast. 10, 810–825 (1995).18. Lohmann, U., Feichter, J., Chuang, C. C. & Penner, J. E. Prediction of the number of cloud droplets inthe ECHAM GCM. J. Geophys. Res. 104, 9169–9198 (1999).19. Brenguier, J.-L. et al. Radiative properties of boundary layer clouds: droplet effective radius versusnumber concentration. J. Atmos. Sci. 57, 803–821 (2000).20. Liu, Y. & Daum, P. H. Indirect warming effect from dispersion forcing. Nature 419, 580–581 (2002).21. Lin, B., Wielicki, B., Minnis, P. & Rossow, W. Estimation of water cloud properties from satellitemicrowave, infrared, and visible measurements in oceanic environments. 1, Microwave brightnesstemperature simulations. J. Geophys. Res. 103, 3873–3886 (1998).22. Ghan, S. J., Easter, R. C., Hudson, J. & Breon, F.-M. Evaluation of aerosol indirect radiative forcing inMIRAGE. J. Geophys. Res. 106, 5317–5334 (2001).23. Chuang, C. C. et al. Cloud susceptibility and the first aerosol indirect forcing: Sensitivity to blackcarbon and aerosol concentrations. J. Geophys. Res. D 107, doi:101029/2000JD000215 (2002).Acknowledgements We thank P. Quinn for providing the composition data at the ARM SGP andNSA sites. During this study, X.D. was also supported by the NASA CERES project. This work wassupported by the DOE ARM programme.Competing interests statement The authors declare that they have no competing financialinterests.Correspondence and requests for materials should be addressed to J.E.P. ([email protected])...............................................................Tungsten isotope evidence thatmantle plumes contain nocontribution from the Earth’s coreAnders Scherste´n1, Tim Elliott1, Chris Hawkesworth1& Marc Norman21Department of Earth Sciences, University of Bristol, Will’s Memorial Building,Queen’s Road, Bristol BS8 1RJ, UK2Research School of Earth Sciences, The Australian National University, Canberra,Australian Capital Territory 0200,