Thomas Leisner

A Particle-Surface-Area-Based Parameterization of Immersion Freezing on Desert Dust Particles

AbstractIn climate and weather models, the quantitative description of aerosol and cloud processes relies on simplified assumptions. This contributes major uncertainties to the prediction of global and regional climate change. Therefore, models need good parameterizations for heterogeneous ice nucleation by atmospheric aerosols. Here the authors present a new parameterization of immersion freezing on desert dust particles derived from a large number of experiments carried out at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud chamber facility. The parameterization is valid in the temperature range between −12° and −36°C at or above water saturation and can be used in atmospheric models that include information about the dust surface area. The new parameterization was applied to calculate distribution maps of ice nuclei during a Saharan dust event based on model results from the regional-scale model Consortium for Small-Scale Modelling–Aerosols and Reactive Trace Gases (COSMO-ART). The ...

Aging of biogenic secondary organic aerosol via gas-phase OH radical reactions

The Multiple Chamber Aerosol Chemical Aging Study (MUCHACHAS) tested the hypothesis that hydroxyl radical (OH) aging significantly increases the concentration of first-generation biogenic secondary organic aerosol (SOA). OH is the dominant atmospheric oxidant, and MUCHACHAS employed environmental chambers of very different designs, using multiple OH sources to explore a range of chemical conditions and potential sources of systematic error. We isolated the effect of OH aging, confirming our hypothesis while observing corresponding changes in SOA properties. The mass increases are consistent with an existing gap between global SOA sources and those predicted in models, and can be described by a mechanism suitable for implementation in those models.

Resurgence in Ice Nuclei Measurement Research

Understanding cloud and precipitation responses to variations in atmospheric aerosols remains an important research topic for improving the prediction of climate. Knowledge is most uncertain, and the potential impact on climate is largest with regard to how aerosols impact ice formation in clouds. In this paper, we show that research on atmospheric ice nucleation, including the development of new measurement systems, is occurring at a renewed and historically unparalleled level. A historical perspective is provided on the methods and challenges of measuring ice nuclei, and the various factors that led to a lull in research efforts during a nearly 20-yr period centered about 30 yr ago. Workshops played a major role in defining critical needs for improving measurements at that time and helped to guide renewed efforts. Workshops were recently revived for evaluating present research progress. We argue that encouraging progress has been made in the consistency of measurements using the present generation of ic...

Homogeneous nucleation rates of supercooled water measured in single levitated microdroplets

Homogeneous nucleation rates are determined for micrometer sized water droplets levitated inside an electrodynamic Paul-trap. The size of a single droplet is continuously measured by analyzing the angle-resolved light scattering pattern of the droplets with classical Mie theory. The freezing process is detected by a pronounced increase in the depolarization of the scattered light. By statistical analysis of the freezing process of some thousand individual droplets, we obtained the homogeneous nucleation rate of water between 236 and 237 K. The values are in agreement with former expansion cloud chamber measurements but could be determined with considerably higher precision. The measurements are discussed in the light of classical nucleation theory in order to obtain the size and the formation energy of the critical nucleus.

Feedback optimization of shaped femtosecond laser pulses for controlling the wavepacket dynamics and reactivity of mixed alkaline clusters

Abstract In this paper we describe the branching control of different ionization and fragmentation channels of coherently excited Na 2 K by means of feedback optimization of shaped femtosecond laser pulses. For this purpose the system was first excited with one photon into a pair of intersecting electronic states, from where it was ionized in a two-photonic process, so the resulting ions could size selectively be detected. By employing an evolutionary algorithm for optimizing phase and amplitude of the applied laser field, the relative signal intensities of the resulting mother and fragment ions could significantly be influenced. The peak intensities in the obtained pulse shapes correspond very well to the cross sections of the irradiated transitions; their temporal structure reflects perfectly the wavepacket dynamics of the resulting particles. Hence it was possible to extract with the self-learning algorithm – as postulated by Judson and Rabitz [Phys. Rev. Lett. 68 (1992) 1500] – intrinsic properties from the reactive system in real time.

The relaxation from linear to triangular Ag3 probed by femtosecond resonant two-photon ionization

We present extended NeNePo (negative to neutral to positive) measurements on the ultrafast dynamics in the ground state of neutral, mass-selected Ag3 molecules. A vibrational wave packet in the neutral molecule is created with an ultrashort laser pulse by photodetachment of the excess electron from the corresponding mass-selected anion. The subsequent molecular rearrangement is probed by photoionization after a selected time delay. Complementary to our previous investigations of this process, we now use two-photon photoionization via a resonant state in the probe step. Here, a bound–bound excitation to a well-known state followed by one-photon ionization is used instead of the nonresonant bound/free transition into the ionic continuum. Using radiation with wavelengths near 370 nm for resonant ionization, we observe a fast bending motion of the initially linear Ag3, followed by an ultrafast intramolecular vibrational energy redistribution, interpretable as an intramolecular collision process. The signal sh...

Reaction mechanism for the oxidation of free silver dimers

Abstract The kinetics of the interaction of Ag 2 + with O 2 is studied in the gas phase under multi-collision conditions at various temperatures. A new experimental scheme is employed, which consists of an octopole ion trap of variable temperature inserted into a tandem quadrupole mass spectrometer. From the time evolution of the reactant and product molecule concentrations at different temperatures the corresponding reaction mechanism is extracted. Surprisingly, the product Ag 2 O + is detected, which is formed after molecular adsorption and dissociation of O 2 . We can clearly identify Ag 2 O 2 + as an intermediate in this reaction. In addition, absolute rate coefficients and activation energies for the molecular adsorption of O 2 onto Ag 2 + are presented.

Thermionic emission from free, photoexcited tungsten clusters

We report on delayed electron emission from free tungsten clusters, excited by light from a Q‐switched YAG laser. Using a novel ion extraction lens, electron emission can be analyzed over a time range of 50 ns–5 μs after the laser pulse without interference from prompt ions. All clusters of size 5≤n≤40 exhibit delayed emission on this time scale, while delayed emission from smaller clusters does not occur. We analyze the time dependence and size dependence of the emission rate for different wavelengths and fluences. The yield of delayed ions may exceed the yield of prompt ions for intermediate laser fluences. A statistical model is proposed which is based on the assumption that energy randomization in the electronically excited clusters proceeds much faster than in 50 ns, i.e., that the observed phenomenon is the (cluster) analog of thermionic emission. Good agreement with all our experimental findings is achieved, although the model invokes only one adjustable parameter. We argue that other delayed deexc...

Active sites in heterogeneous ice nucleation—the example of K-rich feldspars

From dust to ice How does ice form on the surfaces of aerosol particles? The process is important for climate and atmospheric properties but poorly understood at the molecular level, in part because the nature of the sites where ice growth begins is unclear. Kiselev et al. used electron microscopy and computer simulations to study the deposition of aligned ice crystals on feldspar, a major component of mineral dust (see the Perspective by Murray). Surface defects of the feldspar were responsible for its high ice-nucleation efficiency. Science, this issue p. 367; see also p. 346 Atmospheric ice nucleation on feldspar dust occurs at surface defects. Ice formation on aerosol particles is a process of crucial importance to Earth’s climate and the environmental sciences, but it is not understood at the molecular level. This is partly because the nature of active sites, local surface features where ice growth commences, is still unclear. Here we report direct electron-microscopic observations of deposition growth of aligned ice crystals on feldspar, an atmospherically important component of mineral dust. Our molecular-scale computer simulations indicate that this alignment arises from the preferential nucleation of prismatic crystal planes of ice on high-energy (100) surface planes of feldspar. The microscopic patches of (100) surface, exposed at surface defects such as steps, cracks, and cavities, are thought to be responsible for the high ice nucleation efficacy of potassium (K)–feldspar particles.

THE CATALYTIC ROLE OF SMALL COINAGE-METAL CLUSTERS IN PHOTOGRAPHY

We model the generation of the photographic latent image in silver-halide systems by the deposition of size-selected silver and gold clusters onto silver-halide microcrystals. Coinage-metal cluster cations were produced in a sputtering arrangement, mass-selected by a quadrupole mass filter, and deposited with a well-controlled kinetic energy onto binderfree silver-bromide crystals. After exposure to the cluster beam, the samples were developed at a defined redox potential. The photographic action of the clusters was investigated as a function of their size, the coverage of the sample with clusters, the kinetic energy of the clusters during the deposition, and the redox potential of the developer. Development was observed after the deposition of or larger silver-cluster cations, and under some development conditions, after deposition of . No development was observed after the deposition of smaller silver clusters, and after the deposition of gold cluster cations (n≤9).