Complex refractive indices of supercooled liquid water at 240, 253, 263, and 273 K, and ice at 200, 210, and 235 K in the mid infrared from 460 to 4000 cm(-1) are reported. The results were obtained from the extinction spectra of small (micron-size) aerosol particles, recorded using the cryogenic flow tube technique. An improved iterative procedure for retrieving complex refractive indices from extinction measurements is described. The refractive indices of ice determined in the present study are in good agreement with data reported earlier. The temperature region and range of states covered in the present work are relevant to the study of upper tropospheric and stratospheric aerosols and clouds.

Dielectric relaxation in liquid water is studied using molecular dynamics (MD) simulations in the temperature range of 240 to 340 K at atmospheric pressure. The main dielectric and fast relaxation mode are identified in the spectra of dipole moment autocorrelation functions. The microscopic origin of the fast dielectric relaxation process, which takes place on a time scale of subpicoseconds at room temperature, is discussed. A new hypothesis for the fast dielectric mode is presented. It is based on the assumption of the intrawell rotational relaxation taking place during the waiting period between thermally activated large angle jumps occurring in the course of changing H-bond partners.

The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE‐FTS) on the SciSat satellite measured nearly 30 spectra of polar mesospheric clouds (PMCs) between 65° and 70°N from July 5 to 14, 2004. The ACE‐FTS measurements are augmented by UV observations made at the same latitude and time period by the Optical Spectrograph and Infrared Imager System (OSIRIS) on the Odin satellite. Our analysis of these measurements shows that PMC particles are composed of nonspherical ice crystals with mean (equivalent spherical) particle radii of 59 ± 5 nm.

Molecular dynamics simulations are performed to study the dielectric response of concentrated NaCl aqueous solutions. The extended simple point charge interaction potential for water molecules and the Higgis-Mayer potential for ion-ion interactions are used. The ion-ion and ion-water distributions are examined for 1 M solution at 298, 373 and 473 K. The solvate-separated ion pairs with the Na ϩ -Cl Ϫ separation of about 5.0 Å are found to form preferentially at ambient temperature. The close contact pairs with the ion-ion separation of 2.9 Å tend to form in high temperature solution. The 3D water-ion arrangements are revealed with the aid of the spatial distribution function. The Na ϩ -O-Cl Ϫ angle in the solvate-separated ion pairs is found to be close to 106°. The correlation times of translations, T , and rotations, R , for water molecules in the solvation shells of the cation and anion are determined. The frequency-dependent dielectric permittivity and absorption coefficient for the concentrated NaCl solution are calculated over wide temperature range. The origin of the complex dielectric spectra for concentrated NaCl solution is analyzed.