Early (1968–78) Lesser-Known Data on Biogenic Ice: Ground, Airborne and in Clouds

Highly active and ubiquitous biogenic ice nuclei (IN) initiating ice as warm as -1.3C were discovered in the late 1960s. At that time, a spate of papers was published on the topic, many of them in conference proceedings. Within the past 5 years, there has been a resurgence in the field as genetic techniques allow for identification of the biogenic IN proteins in clouds, on inorganic aerosols, and even in precipitation falling in Antarctica. Some of these recent papers seem unaware of some of the earlier research as it does not appear in electronic databases. To wit, there is low awareness that biogenic ice nuclei active at -2C are found in association with marine plankton, have been cultured from marine bacteria, collected alive in fog, and appear to be the main IN in the atmosphere in the Southern oceans. And, that all three source categorizations of biogenic ice nuclei (leaf derived nuclei (LDN), bacteria derived nuclei (BDN) and ocean derived nuclei (ODN)) are active in all three cloud relevant nucleation modes: freezing, contact and deposition. LDN can be stable over long periods; one sample collected in 1970 and stored at room temperature has retained consistent IN activity (-4C with a billion IN/ g active at -8C) for >20 years. Kaolin particles are often mentioned as being an active atmospheric IN, but forgotten work has shown that the IN activity in Kaolin can be removed through heating, then returned to the through exposure to water containing biogenic IN. Airborne desert dusts exhibit similar characteristics as kaolin that may relate to the fact that the dusts originate in depressions where water also deposits decayed plant materials. Atmospheric IN concentrations in Australia show that highest atmospheric IN concentrations are associated with vegetated areas, not the deserts. Outdoor wind tunnel tests show that eroding plant litter can release 500% more IN per unit volume of air than eroding bare soils under the same wind conditions. During the 1974 Sahel drought samples of plant litter and bare soils from across the Sahel showed that the plant litter contained 2-3 orders of magnitude more IN than the bare soils. The thought here is that massive overgrazing prior to the drought may have removed a source of potential precipitation inducing atmospheric IN, thus exacerbating the drought. This remains a speculation.