The ice nucleating ability of pollen - Part I: Laboratory studies in deposition and condensation freezing modes

Laboratory experiments are described where the water uptake by a variety of pollen was studied quantitatively, followed by the investigation of the ice nucleating ability of four kinds of pollen in the deposition and the condensation freezing modes. The diameters of the pollen selected for the freezing experiments were between 25 and 70 mum. The freezing experiments in the deposition mode including also pollen resuspended from decayed leaves, and crushed pollen grains were carried out at different temperatures down to - 33 degreesC combined with various supersaturations with respect to ice up to 35%. The condensation freezing experiments were carried out at temperatures down to -18 degreesC at supersaturation with respect to water above 100%. The results showed that all investigated pollen were able to take up significant amounts of water from a humid environment into their interior by capillary effect. The results of the freezing experiments in the deposition mode showed that none of the investigated pollen acted as deposition ice nuclei within the investigated temperature and ice supersaturation ranges. Pollen was found to act as condensation ice nuclei at relatively warm temperatures. The initiation temperature for freezing activation of all pollen was around -8 degreesC, while a mean condensation freezing efficiency of 50% was reached at different temperatures between - 12 and - 18 degreesC. (C) 2001 Elsevier Science B.V. All rights reserved.