Pollen grains are efficient cloud condensation nuclei

Abstract

This letter presents a laboratory study investigating the ability of pollen grains to act ascloud condensation nuclei. The hygroscopicity of pollen is measured under subsaturatedrelative humidities using an electrodynamic balance. It is found, along with other results,that pollen exhibits bulk uptake of water under subsaturated conditions. Through theuse of an environmental scanning electron microscope it was observed that thesurface of pollen is wettable at high subsaturated humidities. The hygroscopicresponse of the pollen to subsaturated relative humidities is parametrized usingκ-Köhler theory and valuesof the parameter κ for pollen are between 0.05 and 0.1. It is found that while pollen grains are only moderatelyhygroscopic, they can activate at critical supersaturations of 0.001% and lower,and thus pollen grains will readily act as cloud condensation nuclei. While thenumber density of pollen grains is too low for them to represent a significantglobal source of cloud condensation nuclei, the large sizes of pollen grains suggestthat they will be an important source of giant cloud condensation nuclei. Lowtemperature work using the environmental scanning electron microscope indicated thatpollen grains do not act as deposition ice nuclei at temperatures warmer than − 15 °C.