Nucleation of Clusters Bridging the Scale from Molecules to Nanoparticles
PAUL E. WAGNER, Paul M. Winkler, Fakultaet fuer Physik, Universitaet Wien, Vienna, Austria
Gas to liquid phase transitions are important processes in materials science, aerosol physics and atmospheric science. The recent decade of atmospheric observations has demonstrated particle formation by nucleation to be a frequent phenomenon in the global atmosphere. The underlying activation mechanisms of small molecular clusters are thus of vital importance. However, homogeneous as well as heterogeneous nucleation are still among the least understood phenomena in aerosol science.
New particle formation by homogeneous or heterogeneous nucleation generally proceeds via critical molecular clusters, whose sizes can be directly determined from experimental observables using the nucleation theorem. Homogeneous nucleation rate data provide information on the sizes of critical clusters down to diameters of 2 nm in satisfactory agreement with the Kelvin relation . Experiments on heterogeneous nucleation in n-propanol vapour allowed for the first time to bridge the scale from molecular clusters to nanoparticles . For charged seed particles an enhancement of heterogeneous nucleation and a significant charge sign preference were observed.
Recently we have activated single seed ion molecules at sizes far below the Kelvin-Thomson prediction. This unexpected behaviour has now been explained by quantitative determination of the molecular content of critical clusters . We found these clusters to be significantly larger than the seed particles and in fact fairly well predicted by the Kelvin-Thomson relation. Consequently the fundamental detection limit of Condensation Particle Counters is now considerably extended down to particle diameters of about 1 nm. We have designed a new expansion type measurement system (vSANC), which will be used in joint nucleation experiments at CERN, Geneva .
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 P.M. Winkler et al., Science 319, 1374 (2008).
 P.M. Winkler et al., Phys. Rev. Lett. 108, 085701 (2012).
 J. Kirkby et al., Nature 476, 429 (2011).
Dr. Paul Wagner is currently Ao. Professor at the Fakultät für Physik, Universität Wien, Austria. Previously, he held academic positions as Max Kade Scholar at Clarkson University, USA (1975-76), visiting scientist at Max Planck Institut für Biophysikalische Chemie, Göttingen, Germany (1979), guest scholar at Kyoto University, Japan (1989), visiting professor at the University of Helsinki, Finland (1991, 2004), and visiting fellow at Doshisha University, Japan (2010). Dr. Wagner has received several awards for his work in the field of nucleation and condensation phenomena, including the Smoluchowski Award for Aerosol Research (1986), Fellow of the Japan Society for the Promotion of Science (1989), Honorary Member of the Committee on Nucleation and Atmospheric Aerosols (1996), Honorary Degree of the University of Helsinki (2007), and Honorary Member of the Finnish Association for Aerosol Research (2008). He has served as editorial board member of two scientific journals, vice president of the Gesellschaft für Aerosolforschung (1995-96), chairman of the Committee on Nucleation and Atmospheric Aerosols (1988-96), and co-chairman of five International Conferences. Dr. Wagner has authored ten books and about 180 publications in scientific journals.
Embracing Complexity: Deciphering Origins and Transformations of Atmospheric Organics through Speciated Measurements
ALLEN H. GOLDSTEIN, University of California, Berkeley, CA
Organic material accounts for a large fraction of atmospheric aerosol, with the majority being secondary organic aerosol (SOA) formed through oxidation processes. Primary emissions leading to SOA include thousands of chemicals from a variety of natural and anthropogenic sources ranging over approximately 15 orders of magnitude of volatility. As organics are oxidized they fragment to form smaller volatiles or add functionality leading to SOA formation, dramatically increasing the complexity of compounds present. A continuing challenge in aerosol research is to elucidate the sources, structure, chemistry, fate, climate and health impacts of these organic atmospheric constituents.
The complex chemical composition of organic aerosols presents unique measurement challenges. Dr. Goldstein’s group and close collaborators have developed the Thermal Desorption Aerosol Gas chromatograph (TAG) system for hourly in-situ speciation of a wide range of primary and secondary organic compounds in aerosols. This instrument combines a particle collector with thermal desorption followed by GCMS detection to provide hourly separation, identification, and quantification of organic constituents at the molecular level. We incorporated two-dimensional chromatography (GCxGC), providing dramatically enhanced speciation. We developed a semivolatile collection and analysis system that allows simultaneous measurement of specific organics in the gas and particle phases, enabling analysis of their partitioning. We also developed a combined TAG-AMS (Aerosol Mass Spectrometer) instrument for simultaneous measurements of the total and speciated aerosol composition. We are currently exploring soft ionization with vacuum ultraviolet radiation using a high resolution time of flight mass spectrometer (GCxGC/VUV-HRTOFMS) to more fully separate and identify compounds in complex mixtures such as diesel fuel, motor oil, fire emissions, in controlled oxidation studies, and in ambient samples. This talk will review recent developments (TAG, 2DTAG, SVTAG, TAG-AMS, GCxGC/VUV-HRTOFMS), and present new atmospheric observations, source characterizations, and controlled oxidation studies to more fully characterize atmospheric organic sources and transformation processes.
Allen H. Goldstein is currently a professor in the Department of Civil and Environmental Engineering and in the Department of Environmental Science, Policy, and Management, at the University of California, Berkeley where he served as department chair from 2007-2010. Professor Goldstein received his BA and BS degrees from the University of California at Santa Cruz in politics and chemistry, and his MA and PhD degrees in chemistry from Harvard University. His research program encompasses anthropogenic air pollution, biosphere-atmosphere exchange of radiatively and chemically active trace gases, and development and application of novel instrumentation to investigate the organic chemistry of earth’s atmosphere. He engages in field measurement campaigns, controlled laboratory experiments, and modeling activities covering urban, rural, regional, intercontinental, and global scale studies of ozone, aerosols, and their gas phase precursors. His comprehensive research questions include; What controls atmospheric concentrations of greenhouse gases, photochemical oxidants, and aerosols? How do terrestrial ecosystems interact chemically and physically with earth's atmosphere? Professor Goldstein has published approximately 200 peer-reviewed articles and holds a patent (with Susanne Hering) for On-Line Gas Chromatographic Analysis of Airborne Particles. His honors include being elected a fellow of the American Geophysical Union (2011), selected as a Miller Foundation Researcher Professor (2010-11), and a Fulbright Senior Scholar in Australia (2005).
A Tangled Web: Occupants, Squames, Ozone, SOA and SVOCs in Indoor Environments
CHARLES WESCHLER, UMDNJ-Robert Wood Johnson Medical School, New Brunswick, NJ
By their very presence and independent of their activities, humans influence the environments they occupy. The outer layer of human skin — the stratum corneum — is covered by lipids including squalene and unsaturated fatty acids. These compounds react readily with ozone, significantly reducing its indoor concentration and generating oxidized byproducts. From first principles, one expects that some byproducts of ozone-lipid chemistry partition between the gas phase and airborne particles, contributing to secondary organic aerosols (SOA) in occupied rooms. Conversely, the lower indoor ozone levels as a consequence of titration by the exposed skin, hair and clothing of occupants means less generation of SOA from ozone-initiated reactions with terpenoids and other unsaturated organic compounds that can serve as SOA precursors indoors. Occupants also inadvertently transfer their skin oils to exposed indoor surfaces and continuously shed their skin as small flakes, known as “squames.” A typical adult sloughs 200,000 - 600,000 squames per minute, equivalent to 30 - 90 mg per hour. Consistent with their origin, these squames contain squalene (~1% by weight) and unsaturated fatty acids. By transferring their oils and depositing their skin flakes onto indoor surfaces, occupants alter indoor environments even when they are no longer present. Evidence for what humans leave behind includes measured levels of squalene in airborne particles and settled dust, as well as human skin microbiota found in airborne particles and on indoor surfaces as determined by rDNA gene-sequence analysis. The squalene and unsaturated fatty acids in settled dust and on indoor surfaces further impact the levels of ozone and, indirectly, SOA. In turn, indoor particles, including squames, their fragments and ozone-derived SOA, can alter the concentrations and fates of co-occurring SVOCs. All else being equal, as particle concentrations increase, gas-phase concentrations of SVOCs decrease, and emission rates of SVOCs from indoor surfaces increase. For any given SVOC, the larger the ratio of its particle- to gas-phase concentration, the larger the influence of particles on its overall dynamics. Additionally, it has been argued recently that airborne particles might enhance the net flux of SVOCs to human surfaces by as much as a factor of five for realistic indoor conditions. In summary, dynamic physical and chemical processes involving people and particles can markedly influence pollutant exposures that humans experience in indoor environments.
Charles J. Weschler is an adjunct professor, Department of Environmental and Occupational Medicine, Universityof Medicine and Dentistry of New Jersey (UMDNJ)/Robert Wood Johnson Medical School and the UMDNJ- School of Public Health. In addition he is a visiting professor (ongoing) at the TechnicalUniversity of Denmark and Tsinghua University, Beijing. Professor Weschler received his BS in chemistry from Boston College and his MS (physical sciences) and PhD (chemistry) from the Universityof Chicago. His expertise lies in indoor pollutant exposures; their contributions to total pollutant exposures and consequent health effects; chemical reactions among indoor pollutants and their products, including ozone derived free radicals and secondary organic aerosols; gas/particle and gas/surface partitioning in indoor environments and factors that influence the concentrations, transport and surface accumulations of indoor pollutants. Dr. Weschler has written 108 peer-reviewed journal articles; 13 articles cited more than 100 times; four editorials; 50 articles in conference proceedings; and 12 articles and chapters in books. He is a member of numerous professional societies and associations including the Air & Waste Management Association, the American Association for the Advancement of Science, the American Chemical Society, and the American Association for Aerosol Research. In 1999 he was elected to the International Academy of Indoor Air Sciences.
Multiphase Oxidation Chemistry: Impacts on Both the Gas Phase and Aerosol
JONATHAN ABBATT, University of Toronto, Toronto, Ontario, Canada
Whereas gas phase oxidation mechanisms are comparatively well understood, considerable uncertainties remain with respect to the nature and potential importance of multiphase oxidation processes in which oxidative reactions occur either within a condensed phase or at a gas-particle interface. While such chemistry has long been recognized to be important in the oxidation of sulfur(IV) to sulfur(VI) in cloudwater and for promoting the Ozone Hole, its prevalence with tropospheric aerosol and at the Earth’s surface remains poorly quantified. As well, such reactions are likely to occur indoors, given the relatively slow gas-phase chemistry that prevails in these environments. In part, these uncertainties arise because of the complexity of the chemistry: oxidants may either form in-situ or be delivered from the gas phase, chemistry can occur at interfaces or in the bulk, and mass transport limitations can be important in both the gas and condensed phases. After an introduction to the general issues involved in multiphase oxidation chemistry, this talk will illustrate the complexity of the field by focusing on specific examples from across the realm of aerosol and atmospheric chemistry. Attention will be given to: i) organic aerosol oxidation, whereby the overall oxidation state and hygroscopicity of the particle may change, ii) transformations of trace, toxic species (such as polycyclic aromatic hydrocarbons) within particles, and iii) oxidation processes that occur within cloudwater. In addition, the potential for multiphase chemistry occurring at the Earth’s surface, such as at high latitudes with salty substrates or with the marine surface microlayer, will also be discussed.
Jonathan Abbatt is a professor in the Department of Chemistry at the University of Toronto, where he is also the associate director of the Centre for Global Change Science. He received his B.Sc. in chemistry from the University of Toronto and PhD. in chemistry from Harvard University. Dr. Abbatt’s research is concentrated in atmospheric aerosol chemistry: i.e. multiphase chemical processes, aerosol phase transitions, ice and liquid water cloud formation mechanisms, and field studies of tropospheric aerosol processes. Professor Abbatt has been co-editor of Atmospheric Chemistry and Physics since 2003 and co-editor for Atmospheric Measurement Techniques since 2011. He became a fellow of the American Geophysical Union in 2012 and won the CIC Environment Division Research and Development Award in 2012. Having over 3,700 citations, Dr. Abbatt has been invited to give over 100 seminars internationally. He was co-chair of the 2011 Gordon Conference in Atmospheric Chemistry and is currently a member of the IGAC Scientific Steering Committee.
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