Ice nucleation activity of airborne pollen: A short review of results from laboratory experiments

Airborne pollen impacts cloud formation and thus affects radiative forcing and climate. Therefore, there is a growing interest in its ice nucleation activity (INA). In this paper, the latest research progresses in pollen ice nucleating particles (INPs) in the atmosphere are overviewed. Newly developed droplet-freezing assays utilizing sampled particles for subsequent offline processing or post-processing, and different cloud chambers for direct processing of aerosol particles have been widely employed in laboratory experiments to study the INA of pollen. Recent studies focusing on the INA of pollen are gradually shifting from pollen grains to pollen derived mac-romolecules or nanoscale particles. There are differences in the median freezing temperature (T50) among different types of pollen, which is generally in the range of-10 --25 degrees C and slightly lower than those of bacteria and fungal spores. Polysaccharides and/or proteinaceous molecules located in/on the pollen provide a distinct survival advantage by controlling the formation and growth of ice crystals. Atmospheric processing (e.g., oxidants, acidity) during long-range transport, meteorological factors (e.g., high humidity, precipitation), and anthropogenic forcing can alter the pollen INA by affecting the abundance and properties of pollen grains and/or subpollen particles in the atmosphere. The spatial and temporal distribution, as well as the ice nucleation mechanisms and processes of pollen INPs in the atmosphere, require further investigations by applying field observation, advanced measurement techniques, and model simulation in future studies.