Quantitative descriptions of ice formation mechanisms of silver iodide-type aerosols

Abstract

Laboratory and numerical cloud model experiments were designed to obtain quantitative data describing ice formation by two specific aerosols used operationally for cloud modification. Experimental methodologies and a quantitative framework for predicting ice formation by these silver iodidebased aerosols were based on four currently accepted nucleation mechanisms (modes) and their inherent dependencies on the varied temperature, pressure, humidity, and cloud conditions that can be encountered in the atmosphere. The processes considered were deposition, condensation freezing, contact freezing and immersion freezing nucleation. Particle size dependencies were also quantified. The Colorado State University dynamic (controlled expansion) cloud chamber was used to reproduce atmospheric cloud conditions experimentally. An adiabatic cloud model was adapted for equivalent simulation of cloud chamber experiments. Hydrophobic AgIAgCl and hygroscopic AgIAgCl4NaCl aerosols were found to function by some combination of all four nucleation mechanisms. AgIAgCl aerosols were found to be potentially most efficient as contact freezing nuclei, but slow collision rates between IN and cloud droplets permit deposition and condensation freezing nucleation to become important for many cloud conditions. Condensation freezing was seen to progressively dominate ice formation when AgIAgCl aerosols were exposed to increasing supersaturation with respect to water in the typical atmospheric range (up to 1%). Immersion freezing nucleation was always less efficient than contact freezing. The hygroscopic AgIAgCl4NaCl aerosols displayed deposition and condensation freezing active site densities about one order of magnitude higher than the AgIAgClaerosols, while contact freezing activity was not discemable. Immersion freezing could be as important or more important than condensation freezing nucleation for these hygroscopic aerosols depending on cloud characteristics and how aerosols are released into cloud. Instantaneous exposure of either ice nucleus aerosol to high water supersaturations caused the dominance of the immersion freezing mechanism. Quantitative results were formulated for use in numerical cloud modeling. Ice crystal formation subsequently predicted in cloud model simulations showed good agreement with ice formation measured in cloud chamber simulations of relevant cloud seeding methodologies.