Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D27: Focus Session: Chemical Physics of Clusters: Bridging from Angstrom-scale Clusters to Micron-scale Aerosol Particles I |
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Sponsoring Units: DCP Chair: Bruce Garrett, Pacific Northwest National Laboratory Room: 204B |
Monday, March 2, 2015 2:30PM - 3:06PM |
D27.00001: From Clusters to Atmospheric Aerosol Particles: Nucleation in the CLOUD Experiment at CERN Invited Speaker: Urs Baltensperger Globally, a significant source of cloud condensation nuclei for cloud formation is thought to originate from new particle formation (aerosol nucleation). Despite extensive research, many questions remain about the dominant nucleation mechanisms. Specifically, a quantitative understanding of the dependence of the nucleation rate on the concentration of the nucleating substances such as gaseous sulfuric acid, ammonia, water vapor and others has not been reached. This is of relevance for climate as the atmospheric concentrations of sulfuric acid, ammonia and other nucleating agents are strongly influenced by anthropogenic emissions. By providing extremely well controlled and essentially contaminant free conditions in the CLOUD chamber, we were able to show that indeed sulfuric acid is an important component for such new particle formation, however, for the typical temperatures encountered in the planetary boundary layer the concentrations of sulfuric acid are not high enough to explain the atmospheric observations [1]. Moreover, the effect of ammonia [1], amines [2] and oxidized organic molecules [3] on the nucleation rate of sulfuric acid has been investigated in CLOUD so far. Recent developments in instrument technology such as the Atmospheric Pressure interface-Time Of Flight (APi-TOF) mass spectrometer have allowed us to investigate the chemical composition of charged [4] as well as neutral [5] clusters during such nucleation experiments. References [1] Kirkby, J. et al., Nature, 476, 42-433, 2011 [2] Almeida, J. et al., Nature, 502, 359-363, 2013. [3] Riccobono et al., Science, 344, 717-721, 2014. [4] Schobesberger S. et al., Proc. Nat. Acad. Sci., 110, 17223-17228, 2013. [5] K\"{u}rten, A. et al., Proc. Nat. Acad. Sci., 111, 15019--15024, 2014. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D27.00002: Aerosol Particle Interfacial Thermodynamics and Phase Partitioning Measurements Using Biphasic Microfluidics Cari Dutcher, Andrew Metcalf Secondary organic aerosol particles are nearly ubiquitous in the atmosphere and yet there remain large uncertainties in their formation processes and ambient properties. These particles are complex microenvironments, which can contain multiple interfaces due to internal aqueous--organic phase partitioning and to the external liquid--vapor surface. Interfacial properties affect the ambient aerosol morphology, or internal structure of the particle, which in turn can affect the way a particle interacts with an environment of condensable clusters and organic vapors. To improve our ability to accurately predict ambient aerosol morphology, we must improve our knowledge of aerosol interfaces and their interactions with the ambient environment. Unfortunately, many techniques employed to measure interfacial properties do so in bulk solutions or in the presence of a ternary (e.g. solid) phase. In this talk, a novel method using biphasic microscale flows will be introduced for generating, trapping, and perturbing complex interfaces at atmospherically relevant conditions. These microfluidic experiments utilize high-speed imaging to monitor interfacial phenomena at the microscale and are performed with phase contrast and fluorescence microscopy on a temperature-controlled inverted microscope stage. From these experiments, interfacial thermodynamic properties such as surface or interfacial tension, rheological properties such as interfacial moduli, and kinetic properties such as mass transfer coefficients can be measured or inferred. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D27.00003: Formation of Cluster Complexes by Cluster-Cluster-Collisions Masahiko Ichihashi, Hideho Odaka Multi-element clusters are interested in their chemical and physical properties, and it is expected that they are utilized as catalysts, for example. Their properties critically depend on the size, composition and atomic ordering, and it should be important to adjust the above parameters for their functionality. One of the ways to form a multi-element cluster is to employ a low-energy collision between clusters. Here, we show characteristic results obtained in the collision between a neutral Ar cluster and a size-selected Co cluster ion. Low-energy collision experiment was accomplished by using a newly developed merging-beam apparatus. Cobalt cluster ions were produced by laser ablation, and mass-selected. On the other hand, argon clusters were prepared by the supersonic expansion of Ar gas. Both cluster beams were merged together in an ion guide, and ionic cluster complexes were mass-analyzed. In the collision of Co$_2^+$ and Ar$_N$, Co$_2$Ar$_n^+$ ($n=1-30$) were observed, and the total intensity of Co$_2$Ar$_n^+$ ($n \ge 1$) is inversely proportional to the relative velocity between Co$_2^+$ and Ar$_N$. This suggests that the charge-induced dipole interaction between Co$_2^+$ and a neutral Ar cluster is dominant in the formation of the cluster complex, Co$_2^+$Ar$_n$. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 4:06PM |
D27.00004: Investigations of ice nanoparticles and aerosols in molecular beams Invited Speaker: Michal Farnik We have recently set up a versatile experiment which allows for different experiments with molecular clusters and nanoparticles in molecular beams. Here we concentrate on the experiments with ice nanoparticles (large water clusters (H$_2$O)$_N$, $\bar{N}\approx$10$^2$-10$^3$) doped with atmospherically relevant molecules, e.g., hydrogen halides, CFCs, nitric acid, N$_x$O$_y$, etc. Such species are relevant to ozone depletion and other atmospheric processes. We investigate (1) the UV-photochemistry using velocity map imaging techniques, and (2) the uptake cross section for the molecules on the ice nanoparticles from velocity measurements. In addition, we record (3) mass spectra of the particles implementing different ionization methods: electron ionization (EI) at variable electron energies, photoionization, and special method of electron photodetachment after Na-doping (NaPI). The unique combination of all these different methods performed with the same nanoparticles provides detailed molecular level information about the studied species and their (photo)physics and chemistry. In particular, an investigation of mixed water-nitric acid particles by means of EI and NaPI revealed the prominent role of the HNO$_3$ molecule as the condensation nuclei.\footnote{J. Lengyel et al. \textbf{J. Phys. Chem. Lett.} 3, 2012, 3096} The uptake of atmospheric molecules by ice nanoparticles has been studied, and the pickup cross sections for some molecules exceed significantly the geometrical sizes of the ice nanoparticles. It has been argued that the large particles composed of several hundred water molecules which grow in the supersonic expansions tend to have highly irregular shapes -\textit{nanosnowflakes}.\footnote{J. Lengyel et al. \textbf{J. Chem. Phys.} 137, 2012, 034304, \textbf{Phys. Rev. Lett.} 112, 2014, 113401} Photodissociation of hydrogen halides on ice nanoparticles has been investigated, and shown to proceed via excitation of acidically dissociated ion pair and subsequent biradical generation and H$_3$O dissociation.\footnote{V. Poterya et al. \textbf{J. Chem. Phys.} 126, 2007, 071101, \textbf{J. Chem. Phys.} 141, 2014, 074309} The photodissociation of CF$_2$Cl$_2$ molecules in clusters leads to efficient Cl-fragment caging caused by formation of halogen bond.\footnote{V. Poterya et al. \textbf{J. Phys. Chem. A} 118, 2014, 4740} [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D27.00005: What is the structure of aqueous-alkane nanodroplets? Barbara Wyslouzil, Harshad Pathak, Abdalla Obeidat, Gerald Wilemski \textit{In situ} small angle X-ray scattering (SAXS) experiments were conducted on D$_{\mathrm{2}}$O-nonane nanodroplets produced in a supersonic nozzle. Fits to the scattering spectra, using standard models for multicomponent droplets such as well-mixed spheres and core-shell structures, were often poor and furthermore the amount of one of the condensed species often violated mass balance. Better fits were obtained using `lens-on-sphere' models suggested by molecular dynamics simulations. Here the amount of nonane condensed, based on the SAXS fitting parameters, was quite close to that measured by infrared absorption spectroscopy although the amount D$_{\mathrm{2}}$O condensed was only half of that measured spectroscopically. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D27.00006: Nanodroplets of immiscible fluid pairs adopt nonspherical shapes Gerald Wilemski, Fawaz Hrahsheh To help understand recent experimental results for nonane/water condensation [Pathak, et al. J. Chem. Phys. \textbf{140}, 224318 (2014)], the structure of water/nonane nanodroplets was investigated using classical molecular dynamics (MD) simulations of SPC/E water and a unified atom model of nonane. Because nonane and water are essentially immiscible fluids that only partially wet each other, one might expect unusual nanodroplet structures to arise. Indeed, nonspherical, phase-separated Russian Doll (RD) structures were found to occur for these nanodroplets over the entire temperature range studied in the MD simulations, 220K -- 300K. An idealized, but realistic lens-on-sphere model for the observed RD structures consists of a spherical nonane lens that partially wets a spherical water droplet. This model was used to analyze the experimental small angle x-ray scattering measurements. The simulated contact angle of nonane on water was found to be quite sensitive to the value of the Lennard-Jones energy parameter $\varepsilon _{OC}$ for the cross-interaction between oxygen and carbon atoms. The standard geometric mean approximation for $\varepsilon_{OC\, }$yielded contact angles in the range 70$^{o}$- 80$^{o}$, while a 19{\%} increase in $\varepsilon_{OC\, }$reduced the simulated contact angle close to the experimental value of 33.6$^{o}$ at 295 K. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 5:06PM |
D27.00007: Vapor and Condensed Phase Clusters Invited Speaker: Shawn Kathmann Given the difficulty of directly observing clustering mechanisms underlying nucleation, classical and \textit{ab initio} statistical mechanics provide crucial insight into the thermodynamics and kinetics of these processes. Recent experiments have shown nucleation can emit (e.g., crystalloluminescence) and be induced (e.g., IR lasers) by electromagnetic radiation. This opens up the possibility of using luminescence as an exquisite probe of the nucleation mechanism in addition to doing the reverse process by imposing external electromagnetic fields to activate specific modes of nucleation. The inclusion of electronic degrees of freedom as well as excited electronic states lies beyond classical theory. In addition, the osmotic coefficients of sub- and supersaturated aqueous electrolytes may provide quantitative insights into salt cluster distribution functions and free energies of crystal formation. Here we outline the chemical physics relevant to these findings and their consequences on how we understand and model nucleation to control and exploit the synthesis of matter. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D27.00008: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 5:18PM - 5:30PM |
D27.00009: First-Principles Exploration of the Competing Mechanisms for the Sign Preference in Ion-induced Nucleation Liubin Xu, Wei Chen, Ping Cui, Zhenyu Zhang Ion-induced nucleation plays an important part in aerosol formation, under both atmospheric and experimental conditions. However, the dominant mechanism underlying the so-called sign preference, a phenomenon that ions of the same magnitude of charge but opposite signs exhibit notably different enhancement of nucleation, remains an enigma. Recent experiments revealed a negative charge affinity of 1-propanol molecules condensing on tungsten oxide seeds, while prevailing first-principles calculations indicated a positive preference. In this study, we investigate the adsorption of organic molecules on charged transition metal oxide via density functional theory (DFT) calculations by including new physical factors that were absent in previous theoretical studies. First, we demonstrate the significant role of van der Waals interactions in such systems. Furthermore, we show that the change of charge state effectively varies the spin moment of the seeds, which can be utilized to selectively influence the strengths of the intermolecular bindings. These new factors may prove to be instrumental in gaining an eventual complete understanding of the long-standing sign preference puzzle in ion-induced nucleation. [Preview Abstract] |
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