Alexander Yu. Zasetsky

Characterization of atmospheric aerosols from infrared measurements: simulations, testing, and applications

An inversion method for the characterization of atmospheric condensed phases from infrared (IR) spectra is described. The method is tested with both synthetic IR spectra and the spectra of particles that flow in a cryogenic flow tube. The method is applied to the IR spectra recorded by the Atmospheric Trace Molecule Spectroscopy instrument carried by the Space Shuttle during three missions in 1992, 1993, and 1994. The volume density and particle size distribution for sulfate aerosol are obtained as a function of altitude. The density and size distribution of ice particles in several cirrus clouds are also retrieved. The probable radius of the ice particles in the high-altitude (10-15-km) cirrus clouds is found to be approximately 6-7 microm.

Computer simulation study of rotational diffusion in polar liquids of different types

Rotational diffusion in liquid acetonitrile, dimethylsulphoxide (DMSO), water, and methanol is studied with molecular dynamics simulations. The effects of hydrogen bonding and local dipole-dipole correlations (Kirkwood g-factor) on the relationship between the single molecule and collective relaxation are examined. The first rank single molecule dipole moment autocorrelation functions (ACFs) are constructed in the molecule-fixed coordinate frame and the principal components of rotation diffusion tensor are reported. Higher rank orientational ACFs are computed. These ACFs, as a rule, are strongly nonexponential (at least not single exponential) at longer times and the decomposition of these functions into a series of single exponentials results in broad distributions of relaxation times, with the broadening being particularly prominent in the case of higher rank ACFs. The rank dependence of characteristic times calculated as weighted averages over the relaxation time distributions does not follow the pattern of small angle (Debye) diffusion model for all liquids studied in this work except methanol. In contradiction, the same rank dependence computed by direct integration of ACFs leads to good agreement with the Debye diffusion model in the case of acetonitrile, DMSO, and water (but not methanol). The linear-angular momentum cross correlation functions are also computed and the effect of rototranslational coupling on reorientaional relaxation at longer times (>1.0 ps) is found to be small.

Dissolution of Solid NaCl Nanoparticles Embedded in Supersaturated Water Vapor Probed by Molecular Dynamic Simulations

The dissolution process for small, on the order of 1000 atoms, crystalline NaCl particles with defects embedded in highly supersaturated water vapor was studied by the molecular dynamics (MD) simulation method. We found that a breakdown of the crystal lattice does not occur unless (1) the thickness of water layer covering the surface of salt particles exceeds several molecular layers and (2) there are a considerable number of defects in the crystal. The collapse of the crystal lattice starts when the amount of water taken up by a salt particle reaches about half ( approximately 50%) of the amount of salt in this particle. The number of defects required to initiate subsequent dissolution of the NaCl crystal on the time scale accessible by our simulations ( approximately 40 ns) is in the range of 10 to 12%. We also report the estimates for the time required to form supersaturated aqueous solutions of NaCl from originally crystalline particles as a function of the number of defects in the crystal.

Monte Carlo approach to identification of the composition of stratospheric aerosols from infrared solar occultation measurements.

We describe an inversion method for determining the composition, density, and size of stratospheric clouds and aerosols by satellite remote sensing. The method, which combines linear least-squares minimization and Monte Carlo techniques, is tested with pure synthetic IR spectra. The synthetic spectral data are constructed to mimic mid-IR spectra recorded by the Improved Limb Atmospheric Spectrometer (ILAS-I and ILAS-II) instruments, which operate in the solar occultation mode and record numerous polar stratospheric cloud events. The advantages and limitations of the proposed technique are discussed. In brief we find that stratospheric aerosol in the size range from 0.5 to 4.0 02114 microm can be retrieved to an accuracy of 30%. We also show that the chemical composition of common stratospheric aerosols can be determined, whereas identification of their phases from mid-IR satellite remote-sensing data alone appears to be questionable.

Ion–ion and ion–water aggregations and dielectric response of aqueous solutions of Li2SO4: molecular dynamic simulations study

The dielectric properties of Li2SO4 aqueous solutions are studied using the method of molecular dynamic (MD) simulations. The dielectric response is computed for solutions with concentrations between 0.001 and 0.071 mole fraction of salt in the temperature range of 288 to 348 K. Computed values of the static dielectric constant and effective (weighted average) relaxation time are compared with available experimental data. The distributions of dielectric relaxation times are obtained by the decomposition of dipole moment autocorrelation functions (ACF) into a series of single exponentials in the time domain. The distributions of ion–ion and ion–water aggregations in concentrated solutions are computed and their connections to the low frequency modes observed in experimental dielectric spectra are examined. The nature of anomalous (different from single-exponential) dielectric relaxation in Li2SO4 aqueous solutions is discussed.

Characterization of composition, size, and density of atmospheric aerosols from high-resolution IR satellite measurements

We describe a new method for the quantitative characterization of condensed phases in the atmosphere. It uses broad band IR extinction spectra to obtain the density, size distribution, phase and the approximate composition of aerosols within a single retrieval process. The method is based on a linear least squares fitting procedure with physically-based constraints. In this report, the method is applied to the analysis of spectra measured by the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument. The volume density, size distribution and composition of the stratospheric sulfate aerosols observed in several ATMOS missions are reported. The values of these properties for aerosols observed shortly after the eruption of Mount Pinatubo in 1992 are compared with those of aerosols present at much lower levels in 1993 and 1994.