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Pratt, K.A., P.J. DeMott, J.R. French, Z. Wang, D.L. Westphal, A.J. Heymsfield, C.H. Twohy, A.J. Prenni and K.A. Prather, 2009: In-situ detection of biological particles in cloud ice crystals. Nature Geoscience. 2: 398-401, doi: 10.1038/ngeo521.