On the formation of Langevin ions in water superasaturted air jets

Abstract : The expansion of moist air through a supersonic nozzle which incorporates a copious supply of ions has been used to clarify some basic water condensation problems. The experimental arrangement used allows for the introduction of small ions into a supersaturated region in which the effects of surfaces, foreign condensation nuclei and self nucleation products can be completely excluded. The role of the ions is shown to be the stabilization of preformed but thermodynamically unstable water clusters thus supporting heterophase nucleation theories. After nucleation, condensation proceeds further to produce charged particles with a final size corresponding to that of large atmospheric or Langevin ions. The large number densities obtained and the high flow used suggest an attractive new method to be used in actual cloud seeding experiments. The interaction between electrostatic and thermodynamic energy in the nozzle has been used to determine from experimental data an order of magnitude for the minimum size of charged condensation nuclei. (Author)