Volatility of Amorphous Solid Water.

Amorphous solid water is probably the most abundant form of solid water in the universe. Its saturation vapor pressure and thermodynamic properties, however, are not well known. We have investigated the saturation vapor pressure over vapor-deposited amorphous ice at temperatures between 133 and 147 K using a novel experimental method. The new method determines the absolute vapor pressures and the sublimation rates by measuring the mass growth rates of ice-covered nanoparticles under supersaturated water vapor conditions. We find that the vapor pressure of amorphous solid water is up to a factor of 3 higher than that predicted by current parameterizations, which are based in part on calorimetric measurements. We demonstrate that the calorimetric measurements can be reconciled with our data by acknowledging the formation of nanocrystalline ice as an intermediate ice phase during the crystallization of amorphous ice. As a result, we propose a new value for the enthalpy of crystallization of amorphous solid water of Δ H = 2312 ± 227 J/mol, which is about 1000 J/mol higher than the current consensus. Our results shine a new light on the abundance of water ice clouds on Mars and mesospheric clouds on Earth and may alter our understanding of ice formation in the stratosphere.

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.

Temperature-dependent formation of NaCl dihydrate in levitated NaCl and sea salt aerosol particles.

Recent laboratory studies indicate that the hydrated form of crystalline NaCl is potentially important for atmospheric processes involving depositional ice nucleation on NaCl dihydrate particles under cirrus cloud conditions. However, recent experimental studies reported a strong discrepancy between the temperature intervals where the efflorescence of NaCl dihydrate has been observed. Here we report the measurements of the volume specific nucleation rate of crystalline NaCl in the aqueous solution droplets of pure NaCl suspended in an electrodynamic balance at constant temperature and humidity in the range from 250 K to 241 K. Based on these measurements, we derive the interfacial energy of crystalline NaCl dihydrate in a supersaturated NaCl solution and determined its temperature dependence. Taking into account both temperature and concentration dependence of nucleation rate coefficients, we explain the difference in the observed fractions of NaCl dihydrate reported in the previous studies. Applying the heterogeneous classical nucleation theory model, we have been able to reproduce the 5 K shift of the NaCl dihydrate efflorescence curve observed for the sea salt aerosol particles, assuming the presence of super-micron solid inclusions (hypothetically gypsum or hemihydrate of CaSO4). These results support the notion that the phase transitions in microscopic droplets of supersaturated solution should be interpreted by accounting for the stochastic nature of homogeneous and heterogeneous nucleation and cannot be understood on the ground of bulk phase diagrams alone.

Laser-induced plasma cloud interaction and ice multiplication under cirrus cloud conditions.

Potential impacts of lightning-induced plasma on cloud ice formation and precipitation have been a subject of debate for decades. Here, we report on the interaction of laser-generated plasma channels with water and ice clouds observed in a large cloud simulation chamber. Under the conditions of a typical storm cloud, in which ice and supercooled water coexist, no direct influence of the plasma channels on ice formation or precipitation processes could be detected. Under conditions typical for thin cirrus ice clouds, however, the plasma channels induced a surprisingly strong effect of ice multiplication. Within a few minutes, the laser action led to a strong enhancement of the total ice particle number density in the chamber by up to a factor of 100, even though only a 10(-9) fraction of the chamber volume was exposed to the plasma channels. The newly formed ice particles quickly reduced the water vapor pressure to ice saturation, thereby increasing the cloud optical thickness by up to three orders of magnitude. A model relying on the complete vaporization of ice particles in the laser filament and the condensation of the resulting water vapor on plasma ions reproduces our experimental findings. This surprising effect might open new perspectives for remote sensing of water vapor and ice in the upper troposphere.

Contact freezing efficiency of mineral dust aerosols studied in an electrodynamic balance: quantitative size and temperature dependence for illite particles.

Contact freezing has long been discussed as a candidate for cloud ice formation at temperatures warmer than about -25 degrees C, but until now the molecular mechanism underlying this process has remained obscure and little quantitative information about the size and temperature dependent contact freezing properties of the various aerosol species is available. In this contribution, we present the first quantitative measurements of the freezing probability of a supercooled droplet upon a single contact with a size selected illite mineral particle. It is found that this probability is a strong function of temperature and aerosol particle size. For the particles investigated and on the minute time scale of the experiment, contact freezing indeed dominates immersion freezing for all temperatures.

On the role of surface charges for homogeneous freezing of supercooled water microdroplets.

Charge induced changes in homogeneous freezing rates of water have been proposed to constitute a possible link between the global atmospheric electric circuit and cloud microphysics and thus climate. We report here on high precision measurements of the homogeneous nucleation rate of charged, electro-dynamically levitated single water droplets as a function of their surface charge. No evidence has been found that the homogeneous volume specific ice nucleation rate of supercooled microdroplets is influenced by surface charges in the range between +/-200 elementary charges per µm(2). It has also been suggested that filamentation in highly electrified liquids can induce freezing at temperatures well above the homogeneous freezing limit. We report here the observation of Coulomb instabilities of highly charged droplets that are accompanied with the formation and ejection of fine filaments from the liquid supercooled droplets. Down to temperatures of 240 K, which is close to the homogeneous freezing limit of uncharged water, no filamentation induced freezing has been detected. At even lower temperatures, the droplets froze before the instability was reached. These findings rule out that filamentation exerts an important influence on ice formation in supercooled water. Combining these findings, we conclude that the surface charges (even at their maximum possible density) have no significant effect on the homogeneous ice nucleation rate of supercooled cloud droplets.

Fluorescence quenching induced by conformational fluctuations in unsolvated polypeptides.

Time-resolved measurements were conducted to relate the fluorescence lifetimes of dye-derivatized polypeptides to local conformational dynamics in trapped, unsolvated peptide ions. This research was performed to better understand the intramolecular interactions leading to the observed increase of fluorescence quenching with temperature and, in particular, how this quenching is related to conformational fluctuations. Dye-derivatized polyproline ions, Dye-[Pro] n -Arg (+)-Trp, are formed by electrospray ionization and trapped in a variable-temperature quadrupole ion trap where they are exposed to a pulsed laser which excites fluorescence. Lifetime data exhibit fluorescence quenching as a result of an interaction between the dye and tryptophan (Trp) side chain. This result is consistent with solution measurements performed for comparison. The lifetime temperature dependence is closely fit over the range 150-463 K by an Arrhenius model of the ensemble averaged quenching rate, k q. Model fits of the measured lifetimes yield a frequency prefactor of approximately 10 (11) s (-1) for k q characteristic of collective motions of the side chains identified in molecular dynamics (MD) simulations. The data fits also yield activation barriers of approximately 0.3 eV, which are comparable to intramolecular electrostatic interactions calculated between the unshielded charge on the Arg residue and the dye. As a result, the quenching rate appears to be determined by the rate of conformational fluctuations and not by the rate of a specific quenching mechanism. The peptide sequence of Dye-Trp-[Pro] n -Arg (+) was also studied and identified a dependence of the quenching rate on the electrostatic field in the vicinity of the dye, Trp pair. Molecular dynamics simulations were performed over the range of experimental measurements to study trajectories relevant to the quenching interaction. The MD simulations indicate that as the temperature is increased, conformational fluctuations in the presence of strong electrostatic fields of the charged Arg (+) residue can result in both (a) an increased number of dye and Trp separations <8 A and (b) increased exothermicity for electron transfer reactions between the dye and Trp. Consequently, the MD simulations are consistent with increased fluorescence quenching with temperature resulting from the occurrence of conformers having specific positions of the dye, Trp, and Arg (+). As a result, the fluorescence lifetime provides a local probe of conformational fluctuations averaged over the ion ensemble.

Shedding light on biomolecule conformational dynamics using fluorescence measurements of trapped ions.

Biomolecule conformational change has been widely investigated in solution using several methods; however, much less experimental data about structural changes are available for completely isolated, gas-phase biomolecules. Studies of conformational change in unsolvated biomolecules are required to complement the interpretation of mass spectrometry measurements and in addition, can provide a means to directly test theoretical simulations of biomolecule structure and dynamics independent of a simulated solvent. In this Feature Article, we review our recent introduction of a fluorescence-based method for probing local conformational dynamics in unsolvated biomolecules through interactions of an attached dye with tryptophan (Trp) residues and fields originating on charge sites. Dye-derivatized biomolecule ions are formed by electrospray ionization and are trapped in a variable-temperature quadrupole ion trap in which they are irradiated with either continuous or short pulse lasers to excite fluorescence. Fluorescence is measured as a function of temperature for different charge states. Optical measurements of the dye fluorescence include average intensity changes, changes in the emission spectrum, and time-resolved measurements of the fluorescence decay. These measurements have been applied to the miniprotein, Trp-cage, polyproline peptides and to a beta-hairpin-forming peptide, and the results are presented as examples of the broad applicability and utility of these methods. Model fits to Trp-cage fluorescence data measured as a function of temperature provide quantitative information on the thermodynamics of conformational changes, which are reproduced well by molecular dynamics. Time-resolved measurements of the fluorescence decays of Trp-cage and small polyproline peptides definitively demonstrate the occurrence of fluorescence quenching by the amino acid Trp in unsolvated biomolecules.

Time-resolved explosion dynamics of H2O droplets induced by femtosecond laser pulses.

Series of time-resolved still images of the explosion dynamics of micrometer-sized water droplets after femtosecond laser-pulse irradiation were obtained for different laser-pulse intensities. Amplified pulses centered around a wavelength of 805 nm with 1-mJ energy and 60-fs duration were focused onto the droplet to initiate the dynamics. Several effects, such as forward and backward plumes, jets, water films, and shock waves, were investigated. Additionally, the influence of different pulse durations produced by chirping the laser pulses was observed.