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Relative roles of biogenic emissions and Saharan dust as ice nuclei in the Amazon basin


Some aerosol particles, known as ice nuclei, can initiate ice formation in clouds, thereby influencing precipitation, cloud dynamics and the amount of incoming and outgoing solar radiation. In the absence of biomass burning, aerosol mass concentrations in the Amazon basin are low1. Tropical forests emit primary biological particles directly into the atmosphere; secondary organic aerosols form from the emission and oxidation of biogenic gases2. In addition, particles derived from biomass burning in central Africa, marine aerosols, and wind-blown dust from North Africa3,4,5 often reach the central part of the Amazon basin during the wet season. The contribution of these aerosol sources to ice nucleation in the region is uncertain. Here we present observations of the concentration and elemental composition of ice nuclei in the Amazon basin during the wet season. Using transmission electron microscopy combined with energy-dispersive X-ray spectroscopy, we show that ice nuclei are primarily composed of carbonaceous material and dust. We show that biological particles dominate the carbonaceous fraction, whereas import of Saharan dust explains the intermittent appearance of dust-containing nuclei. We conclude that ice-nucleus concentration and abundance can be explained almost entirely by local emissions of biological particles supplemented by import of Saharan dust. Using a simple model, we tentatively suggest that the contribution of local biological particles to ice nucleation is increased at higher atmospheric temperatures, whereas the contribution of dust particles is increased at lower temperatures.

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Figure 1: Ice-nucleus number concentrations.
Figure 2: Dust relation to ice-nucleus measurements.
Figure 3: Predicted ice-nucleus number concentrations.


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This work was financially supported by the NASA New Investigator Program, grant NNG04GR44G. S.T.M. acknowledges support received from the US National Science Foundation (ATM-0723582). P.A. acknowledges financial support from the Millennium Institute Program from CNPq. The authors wish to acknowledge R. J. Lee Group for TEM-EDX analysis. The authors also wish to acknowledge the entire AMAZE-08 team, and specifically J. Jimenez, D. Farmer and Q. Chen for their contributions to the AMS data collection and analysis.

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Authors and Affiliations



A.J.P. and M.D.P. carried out ice-nucleus measurements, processed ice-nucleus data and analysed ice-nucleus data in the context of the aerosol measurements. S.M.K. aided in the ice-nucleus analysis and in writing the manuscript. C.L.H. did simulations with GEOS-Chem. S.T.M. served as one of the project organizers of AMAZE-08 and helped with interpretation of AMS data. P.A. served as one of the project organizers of AMAZE-08, made the PIXE measurements and helped with interpretation of PIXE data. R.M.G. carried out the ultraviolet-APS measurements and helped process ultraviolet-APS data. A.G.W. helped process ultraviolet-APS data and helped with interpretation of ultraviolet-APS data. U.P. served as PI for ultraviolet-APS measurements and helped with interpretation of ultraviolet-APS data.

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Correspondence to Anthony J. Prenni.

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Prenni, A., Petters, M., Kreidenweis, S. et al. Relative roles of biogenic emissions and Saharan dust as ice nuclei in the Amazon basin. Nature Geosci 2, 402–405 (2009).

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