Laser depolarization studies of simulated crystallized H2O/H2SO4 clouds

Laboratory experiments were carried out to investigate the linear depolarization ratios (LDRs) of crystallized H2O/H2SO4 acid clouds at angles of 45 degrees, 135 degrees and 157 degrees to the forward direction. LDR is the ratio of the returned energies in the planes of polarization perpendicular and parallel to that of the source (Sassen, 1974). Experiments were carried out for different concentrations of aqueous solution of sulfuric acid. A cloud of supercooled droplets is formed inside an experimental chamber kept in a walk-in cold room by heating an aqueous solution of sulfuric acid, and the ice crystal formation is initiated by momentarily introducing a rod dipped in liquid nitrogen into the cloud. A 2-mW polarized He:Ne laser beam is directed through the cloud and a photomultiplier cube measures the scattered intensity. It is found that the scattered energy is higher in the direction of 157 degrees and 135 degrees than that at 45 degrees. Microscopic examination of the crystals formed from acid concentrations above 50% shows dark fuzzy spots on the surface of the ice crystals suggesting that this could be a film of acid drops. This film also alters the shape of the crystals, and the crystal boundaries are no longer sharp and well defined like that observed for pure water clouds and this is found to influence the LDR. (C) 2000 Elsevier Science B.V. All rights reserved.