Laboratory studies of ice formation pathways from ammonium sulfate particles M. E. Wise, K. J. Baustian, and M. A. Tolbert Department of Chemistry and Biochemistry and the Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO 80309, USA
Abstract. Cirrus clouds are composed of ice particles and their formation pathways
have been studied extensively in the laboratory. The ability of ammonium
sulfate particles to act as nuclei for cirrus clouds has been of particular
importance because of their ubiquitous presence in the upper troposphere.
The results of past laboratory experiments of homogeneous ice nucleation
from ammonium sulfate particles show a wide range of freezing conditions. In
the present study, a flow tube apparatus equipped with Fourier transform
infrared spectroscopy was used to reexamine these discrepancies. It was
found that when ammonium sulfate particles were preconditioned at 100%
relative humidity (RH) prior to experimentation, the particles began to
freeze at conditions predicted by the homogeneous ice nucleation model
developed by Koop et al. (2000). If the particles were not
preconditioned at 100% RH, some froze at warmer temperatures and lower
ice saturation ratios than predicted by Koop et al. (2000). It is
hypothesized that a population of effloresced particles affected freezing
conditions for particles that were not preconditioned at 100% RH.
Citation: Wise, M. E., Baustian, K. J., and Tolbert, M. A.: Laboratory studies of ice formation pathways from ammonium sulfate particles, Atmos. Chem. Phys., 9, 1639-1646, doi:10.5194/acp-9-1639-2009, 2009.