To develop theories and numerical models of the formation and microstructure of clouds and precipitation, it is necessary to identify the potential sources of ice nuclei in the atmosphere. However, the subject remains an area of debate. According to the most accepted theory, the great majority of atmospheric ice nuclei constitute soil mineral particles. But some evidence appears to favor the hypothesis of a nonnegligible contribution to the population of effective ice nuclei made by biogenic material, living or dead. Moreover, some specific human activities have been identified as prolific sources of particles on which ice crystals can be generated. In contrast, it has also been suggested that some anthropogenic effluents deactivate nuclei naturally occurring in the atmosphere. This paper summarizes present knowledge about the biogenic and anthropogenic sources of atmospheric ice nuclei. Recent research reveals an increasingly greater variety of sources and activities of ice nuclei. However intriguing and potentially significant these findings are, the overall picture emerging from the review is one of inconclusive, and sometimes contradictory, results. A standarization of measurement techniques and a more coordinated and systematic effort in the search for a general theory of heterogeneous ice nucleation are needed to answer the fundamental questions, what is the origin of atmospheric ice nuclei, and what is their activity spectrum?
*Universite du Quebec a Montreal, Montreal, Quebec, Canada.
+McGill University, Montreal, Quebec, Canada.
