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.