Laser depolarization studies of simulated crystallized H2O/H2SO4 clouds

Laboratory experiments were carried out to investigate the linear depolariz ation ratios (LDRs) of crystallized H2O/H2SO4 acid clouds at angles of 45 d egrees, 135 degrees and 157 degrees to the forward direction. LDR is the ra tio of the returned energies in the planes of polarization perpendicular an d parallel to that of the source (Sassen, 1974). Experiments were carried o ut for different concentrations of aqueous solution of sulfuric acid. A clo ud 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 t he ice crystal formation is initiated by momentarily introducing a rod dipp ed 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 scattere d intensity. It is found that the scattered energy is higher in the directi on of 157 degrees and 135 degrees than that at 45 degrees. Microscopic exam ination of the crystals formed from acid concentrations above 50% shows dar k 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 o bserved for pure water clouds and this is found to influence the LDR. (C) 2 000 Elsevier Science B.V. All rights reserved.