Ice nucleating behavior of different tree pollen in the immersion mode

Biological particles, including pollen, are released into the atmosphere in large amounts and carry components which can initiate ice nucleation (IN) under cloud conditions. In this study, we investigate in immersion mode the ice nucleation ability of eleven different tree pollen from deciduous and evergreen trees. Droplet freezing experiments are conducted, where we find the IN ability of filtered tree pollen aqueous suspensions to vary over more than 10 °C and ice nucleation to be triggered by subpollen particles that are smaller than 0.45 μm in size. The presence of full pollen grains in the sample does not significantly change the median freezing temperature. The IN activity of the pollen samples is complemented by total organic carbon (TOC) analysis, which allows calculation of the number of ice nucleating particles per mass of carbon. Infrared analysis of the bulk samples does not clearly indicate whether specific functional groups are correlated to the ability of the subpollen particles to nucleate ice.