Biogenic Ice Nuclei: Part I. Terrestrial and Marine Sources

Abstract

Decayed plant leaf litters from North America, Europe and Asia have been found to contain copious numbers of ice nuclei, some active at ?4°C. The abundance of nuclei in a litter was noted to vary according to the climate of the plant's origin; litters from tropical, A-type climates (according to the Köppen classification) contain fewer ice nuclei (103 g?1 active at ?10°C) than litters from mid-latitude, C-type climates (105 g?1 active at ?10°C) which in turn contain fewer nuclei than litters from high-latitude, D-type climates (109 g?1 at ?10°C). The rate of release of freezing nuclei to the atmosphere from in situ litters from D-type climates was determined experimentally: the flux of nuclei active at ?12°C was found to be 101?103 cm?2 day?1 during daylight hours. Active ice nuclei also have been found in seawaters rich in phytoplankton; seawaters devoid of plankton are poor sources of ice nuclei. Some of these nuclei are active at temperatures around ?4°C and concentrations reach up to 107?103 nuclei at ?10°C per gram of plankton. Using numerous measurements from around the globe, atmospheric ice nucleus concentrations, and also freezing nucleus concentrations in rainfall, were shown to exhibit a climatic dependence similar to that of biogenic nuclei sources at the surface. This correlation suggests that large proportions of atmospheric ice nuclei are possibly of biogenic origin.