Laborexperimente zur Mikrophysik der Wolken

This dissertation represents the written scientific work in the frame of my graduation at the Technical University Ilmenau. It deals mainly with the construction of an experimental setup for investigation of atmospheric particles by means of the Electrodynamic Levitation Technique and subsequent realization of three different experiments. The constructed levitators and vacuum chambers allow variation of temperature, pressure and gas composition in the surrounding of the levitated particle to achieve realistic atmospheric conditions. The low device height permits investigations of metastable systems, for instance droplets of supercooled liquids and supersaturated solutions, to be performed in the focus of conventional microscopes as they are used for Raman- or Infrared spectroscopy. With the presented experimental setup it has been achieved to measure for the first time the refractive index of supercooled liquid water in the temperature regime from 16°C to -36°C. In a second experiment it has been shown that the homogeneous nucleation rate of supercooled liquid water droplets with radii larger than 4µm is volume-dominated. In a third experiment the stability of highly charged liquid droplets has been investigated by analyzing induced shape oscillations of the droplet surface. A modified Rayleigh-Limit for rotation-symmetric deformed charged droplets was proposed and is supported by the experimental data.Die vorliegende Dissertation stellt den schriftlichen Abschluss der wissenschaftlichen Arbeiten im Rahmen meiner Promotion an der Technischen Universität Ilmenau dar. Die Arbeit beschäftigte sich vorrangig mit der Konstruktion und dem Aufbau eines Experimentes zur Untersuchung atmosphärisch relevanter geladener Partikel mittels der Technik der elektrodynamischen Levitation und anschließender Durchführung verschiedener Versuche. Die konstruierten Levitatoren und Vakuumkammern wurde dabei derart gestaltet, dass sich Temperatur und Druck, sowie die Zusammensetzung der Gasatmosphäre in dem die Probe umgebenden Volumen zur Simulation atmosphärischer Bedingungen leicht verändern lassen können. Durch die geringe Höhe des Gerätes werden dabei auch Untersuchungen der Partikel im Fokus konventioneller Mikroskope ermöglicht, wie sie zum Beispiel für die Raman- und Infrarotspektroskopie verwendet werden. Mit dem gewählten Aufbau gelang es erstmals, den Brechungsindex stark unterkühlter Wassertropfen im Temperaturbereich von -16°C bis -36°C zu messen. Desweiteren konnte nachgewiesen werden, dass die homogene Nukleationsrate von unterkühlten Wassertropfen mit einem Radius größer 4µm Volumen-dominiert ist.In einem dritten Experiment wurde gezeigt, dass das Stabilitäts-Limit deformierter flüssiger Tropfen gegenüber dem klassischen Rayleigh-Limit mit der Amplitude der Deformation modifiziert werden muss

The vapor pressure over nano-crystalline ice

The crystallization of amorphous solid water (ASW) is known to form nano-crystalline ice. The influence of the nanoscale crystallite size on physical properties like the vapor pressure is relevant for processes in which the crystallization of amorphous ices occurs, e.g., in interstellar ices or cold ice cloud formation in planetary atmospheres, but up to now is not well understood. Here, we present laboratory measurements on the saturation vapor pressure over ice crystallized from ASW between 135 and 190 K. Below 160 K, where the crystallization of ASW is known to form nano-crystalline ice, we obtain a saturation vapor pressure that is 100 to 200 % higher compared to stable hexagonal ice. This elevated vapor pressure is in striking contrast to the vapor pressure of stacking disordered ice which is expected to be the prevailing ice polymorph at these temperatures with a vapor pressure at most 18 % higher than that of hexagonal ice. This apparent discrepancy can be reconciled by assuming that nanoscale crystallites form in the crystallization process of ASW. The high curvature of the nano-crystallites results in a vapor pressure increase that can be described by the Kelvin equation. Our measurements are consistent with the assumption that ASW is the first solid form of ice deposited from the vapor phase at temperatures up to 160 K. Nano-crystalline ice with a mean diameter between 7 and 19 nm forms thereafter by crystallization within the ASW matrix. The estimated crystal sizes are in agreement with reported crystal size measurements and remain stable for hours below 160 K. Thus, this ice polymorph may be regarded as an independent phase for many atmospheric processes below 160 K and we parameterize its vapor pressure using a constant Gibbs free energy difference of 982  ±  182 J mol−1 relative to hexagonal ice

Preferential adsorption of para and ortho water molecules on charged nanoparticles in planetary ice clouds

In the Earth mesopause, nanometer-size singly charged particles form by condensation of evaporated meteorite material. Tey exhibit an enhanced water adsorption cross section due to the strong charge-dipole-interaction. In this work, we study how the nuclear spin state of water molecules afects this enhancement and whether there are conditions that could lead to the formation of spin-polarized ice. Due to symmetry constraints on the total molecular wave function, ortho (proton spins parallel) and para (spins antiparallel) water occupy diferent rotational states, resulting in a diferent average dipole orientation in electric felds. Terefore, we expect ortho and para water to exhibit distinct ad- sorption enhancement factors onto charged nanoparticles. Based on Stark-shifs of individual rotational states of water, average dipole orientations of a molecu- lar ensemble and the resulting collision cross section was calculated for various temperatures and particle sizes. We found that in the mesosphere of the Earth (T~150K) the adsorption enhancement of ortho- and para- water is approxi- mately equal while at lower temperatures prevailing around ice giant planets and their moons, signifcant spin polarizations up to 15% occur

Charge induced enhancement of water adsorption on nanoparticle ions

Water and other polar molecules experience an attractive force in the inhomogeneous electric field of small molecular ions or charged nanoparticles. This charge induced attractive force increases the collision cross section, and, hence, impacts the adsorption rates compared to neutral particle interactions. While ion-molecule interactions have been studied extensively, experimental data are still lacking regarding the interaction of polar molecules with nanoparticles whose radii exceed the Langevin capture radius. Precise knowledge of this effect is crucial, e.g. for describing the formation and growth of atmospheric nanoparticles and for understanding the role of charged particles in cloud formation. We present experimental results for the charge induced enhancement of the collision cross section between H2O molecules and singly charged nanoparticles with radii between 1.4 nm and 3 nm. The enhancement factor Γ with respect to the geometrical cross section increases with decreasing particle size. We also present a new model for Γ based on Stark effect adiabatic dipole orientations, which is in excellent agreement with the experimental findings

Unravelling the microphysics of polar mesospheric cloud formation

Polar mesospheric clouds are the highest water ice clouds occurring in the terrestrial atmosphere. They form in the polar summer mesopause, the coldest region in the atmosphere. It has long been assumed that these clouds form by heterogeneous nucleation on meteoric smoke particles which are the remnants of material ablated from meteoroids in the upper atmosphere. However, until now little was known about the properties of these nanometre-sized particles and application of the classical theory for heterogeneous ice nucleation was impacted by large uncertainties. In this work, we performed laboratory measurements on the heterogeneous ice formation process at mesopause conditions on small (r=1 to 3 nm) iron silicate nanoparticles serving as meteoric smoke analogues. We observe that ice growth on these particles sets in for saturation ratios with respect to hexagonal ice below Sh=50, a value that is commonly exceeded during the polar mesospheric cloud season, affirming meteoric smoke particles as likely nuclei for heterogeneous ice formation in mesospheric clouds. We present a simple ice-activation model based on the Kelvin–Thomson equation that takes into account the water coverage of iron silicates of various compositions. The activation model reproduces the experimental data very well using bulk properties of compact amorphous solid water. This is in line with the finding from our previous study that ice formation on iron silicate nanoparticles occurs by condensation of amorphous solid water rather than by nucleation of crystalline ice at mesopause conditions. Using the activation model, we also show that for iron silicate particles with dry radius larger than r=0.6 nm the nanoparticle charge has no significant effect on the ice-activation threshold

Composition, Mixing State and Water Affinity of Meteoric Smoke Analogue Nanoparticles Produced in a Non-Thermal Microwave Plasma Source

Abstract The article reports on the composition, mixing state and water affinity of iron silicate particles which were produced in a non-thermal low-pressure microwave plasma reactor. The particles are intended to be used as meteoric smoke particle analogues. We used the organometallic precursors ferrocene (Fe(C5H5)2) and tetraethyl orthosilicate (TEOS, Si(OC2H5)4) in various mixing ratios to produce nanoparticles with radii between 1 nm and 4 nm. The nanoparticles were deposited on sample grids and their stoichiometric composition was analyzed in an electron microscope using energy dispersive X-ray spectroscopy (EDS). We show that the pure silicon oxide and iron oxide particles consist of SiO2 and Fe2O3, respectively. For Fe:(Fe+Si) ratios between 0.2 and 0.8 our reactor produces (in contrast to other particle sources) mixed iron silicates with a stoichiometric composition according to FexSi(1−x)O3 (0≤x≤1). This indicates that the particles are formed by polymerization of FeO3 and SiO3 and that rearrangement to the more stable silicates ferrosilite (FeSiO3) and fayalite (Fe2SiO4) does not occur at these conditions. To investigate the internal mixing state of the particles, the H2O surface desorption energy of the particles was measured. We found that the nanoparticles are internally mixed and that differential coating resulting in a core-shell structure does not occur.</jats:p

Laser vaporization of cirrus-like ice particles with secondary ice multiplication

We investigate the interaction of ultrashort laser filaments with individual 90-μm ice particles, representative of cirrus particles. The ice particles fragment under laser illumination. By monitoring the evolution of the corresponding ice/vapor system at up to 140,000 frames per second over 30 ms, we conclude that a shockwave vaporization supersaturates the neighboring region relative to ice, allowing the nucleation and growth of new ice particles, supported by laser-induced plasma photochemistry. This process constitutes the first direct observation of filament-induced secondary ice multiplication, a process that strongly modifies the particle size distribution and, thus, the albedo of typical cirrus clouds