Ice Nucleus Aerosols: Compositional Influences on Activities and Rates of Ice Crystal Formation

Prior cloud chamber studies on ice nucleus aerosols, generated by combustion of acetone solutions of AgI and other ingrediants, have shown that these aerosols display increased activities and rapid rates of ice crystal formation when subjected to transient supersaturations with respect to liquid water. This phenomenon is observed for AgI, Ag(Cl)I, AgI-0.5 NaI, AgI-0.5 KI, and Ag(Cl)I-x NaCl aerosols, where x=0.5 to 5. Studies on Ag(Cl)I- 0.125 XCl aerosols where X was K,Na,Li, or Mg, showed that the choice of cation influenced the activities and rates of ice crystal formation in the absence of transient supersaturations, but had less effect under conditions of transient supersaturation. Effects due to the specific alkali cation present are termed "the lyotropic effect"; are related to the rate of the molecular mechanism of nucleation of ice on the nucleating substrate and appear at alkali chloride concentrations above about 10-3 Molar. Attempts to generate ice nucleus aerosols possessing high activities at temperatures of -5C and very rapid rates of ice crystal formation by pyrotechnic combustion were carried out for application in the current State of Texas weather modification program. Initially, the Nuclei Engineering Inc. formulation, consisting of AgIO3, AL, Mg, Hexachlorobenzene, and epoxy resin binder, was modified to include NaCl, so as to generate Ag(Cl)I-0.5 NaCl aerosol. This pyrotechnic was termed the BF-1 composition. Subsequently, for safety in processing, improved unit costs, and to further improve the rate of function of the aerosol, the composition was modified to become: AgIO3, KClO4, NH4ClO4, Gilsonite, and epoxy resin binder,(termed the RS-3) so as to generate Ag(Cl)I-0.5 KCl aerosol. Cloud chamber studies at Colorado State University (CSU) showed that the latter aerosol functioned to produce ice crystals three times faster than the aerosol containing NaCl, and a much higher activity at -6C. Pyrotechnics generating Ag(Cl)I- x KCl aerosols, where x is 1,5, and 10, are currently in test at CSU to optimize the composition and the nucleus aerosol. Atmospheric in-cloud studies need to be carried out to determine the utility of these pyrotechnically generated ice nucleus aerosols for field use, particularly for application at warm cloud base in cumulus cloud seeding.