Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U26: Focus Session: Advances in Atmospheric Aerosol Science II |
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Sponsoring Units: DCP Chair: Ruth Signorell, University of British Columbia Room: Morial Convention Center 218 |
Thursday, March 13, 2008 8:00AM - 8:36AM |
U26.00001: Chemistry of Individual Atmospheric Particles Invited Speaker: Aerosols are widely recognized as key elements in atmospheric environment. Chemical and morphological data of individual particles are of crucial importance for understanding of their formation, reactions, atmospheric history and aging. Microprobe analytical techniques have been extensively used in the past to characterize the size, morphology, phase and composition of particles collected in field and laboratory studies. These technique coupled with an appropriate time-resolved aerosol sampling are capable of generating time-resolved single-particle data, which then can be used to follow in detail the time evolution of specific types of aerosols. In this presentation we give a summary of recent research projects carried out in our laboratory that demonstrates how the use of complementary microprobe methods provides new insights into the atmospheric reactions of aerosols, their physical and chemical transformations, and in particular how the obtained data can be utilized to define future directions in laboratory and field studies of aerosols. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U26.00002: Heterogeneous oxidation reactions relevant to tropospheric aerosol chemistry studied by sum frequency generation Grace Stokes, Avram Buchbinder, Julianne Gibbs-Davis, Karl Scheidt, Franz Geiger Unsaturated organic molecules (terpenes) that commonly form molecular films on tropospheric aerosols can be oxidized by ozone, influencing the microphysics of cloud formation and thus the earth's climate. Using a laboratory approach that combines organic synthesis with surface spectroscopy, we track the ozone oxidation reactions of tropospherically relevant terpenes bound to glass surfaces that serve as mimics for mineral dust. Specifically, vibrational broadband sum frequency generation (SFG) is used to study a number of tailor-made terpene-modified glass surfaces and to track their interactions with ozone in real time. Exposure of these surfaces to ppm levels of ozone at 1 atm and 300 K yield initial reaction probabilities that are significantly higher than corresponding gas phase reactions. SFG spectra help elucidate the molecular orientations of the surface-bound terpenes and the accessibility of reactive C=C bonds. Our work shows the successful use of SFG spectroscopy to determine heterogeneous atmospheric reaction probabilities and bridges the gap between atmospheric aerosol science and surface spectroscopy. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U26.00003: Oxidation of the PAH Coronene by Ozone and OH Radical Erin Mysak, Jared D. Smith, John T. Newberg, Kevin R. Wilson, Hendrik Bluhm Reactivity of the polycyclic aromatic hydrocarbon (PAH) coronene to oxidation sources ozone and OH radical is examined. To probe the extent of chemical reaction, product formation, and change in surface morphology as a function of reaction, we examine coronene, adsorbed onto various substrates, from both a surface and bulk perspective, with ambient pressure photoemission spectroscopy (APPES) and aerosol mass spectrometry (AMS), respectively. For bulk on-line analysis, a 20nm thick layer of coronene adsorbed onto NaCl seed particles and reacted with either oxidant in a flow tube showed very little reactant conversion to product in the AMS. However, surface analysis by the APPES of the same reaction where coronene was adsorbed onto model substrates showed up to 50 per cent conversion of the carbon species to oxidized carbon, depending on coronene layer thickness (about 1.5-14A). Data obtained with these complimentary techniques provide evidence for a surface selective reaction. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U26.00004: Morphological effects on coated aerosol kinetics Elias Rosen, Eva Garland, Tomas Baer The fate of organic material in the atmosphere can be strongly dependent on the chemical environment under which oxidation takes place. We have investigated the reaction of gas-phase ozone and oleic acid adsorbed to the surface of polystyrene latex spheres and silica aerosols to better understand how the substrate influences heterogeneous kinetics. Flow tube experiments were performed with an Aerosol Time of Flight Mass Spectrometer using a two-laser vaporization/ionization scheme to minimize analyte fragmentation. Pseudo-first-order rate coefficients suggest that ozonolysis of oleic acid proceeds differently on the hydrophobic latex and hydrophilic silica particles. Mechanistic interpretation of these results has been complicated by the discovery that the morphology of oleic acid vapor deposition onto both particle types is non-uniform, which results in the formation of discrete areas of organic material on the particle surface as evidenced by AFM and SEM/EDS measurements. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U26.00005: Optical and microphysical properties of organic multicomponent aerosol particles Invited Speaker: Atmospheric aerosols affect Earth's climate in direct and indirect manners. The direct effect of aerosols on climate is by scattering and/or absorbing incoming solar and outgoing terrestrial radiation, which strongly modify Earth's radiation budget. In addition, aerosols acting as cloud condensation nuclei (CCN) indirectly affect climate and precipitation by modifying the microphysical properties of clouds and cloud coverage. These climatic effects depend on the chemical composition, size and morphology. We will present laboratory studies aiming at understanding how the organic component of atmospheric affect the climate system Specifically, we present the use of cavity ring down (CRD) spectrometer to derive the extinction and complex refractive index of aerosols containing a significant organic component. By precisely measuring extinction as a function of particle size the real and imaginary refractive indices are obtained, and the single scattering albedo may be calculated.. We will present results on aerosol particles containing humic like substances (HULIS). HULIS are a common component of aerosols in the atmosphere. They contribute to the CCN activity, hygroscopic properties and the density of aerosols. In addition, HULIS absorb throughout the visible range, and hence contribute to the direct climatic effect of aerosols. The absorption by organic aerosols is largely unaccounted for in models. Specifically, we will present how the absorption of aerosols containing HULIS and inorganic salts varies with wavelength, test various optical mixing rules and will present results on the extinction of core-shell particles. In addition, we will discuss how the presence of HULIS affects the surface tension of CCN at activation and of cloud droplets and its implications. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U26.00006: Oxidation of oleic acid monolayers at air/liquid interfaces Laura Voss Field studies of marine and continental aerosols find that fatty acid films form on aqueous tropospheric aerosols. Oxidation of oleic acid monolayers by ozone was studied to understand the fate of fat-coated aerosols from both fresh and salt water sources. Using vibrational sum frequency generation spectroscopy and reflection absorption infrared spectroscopy, we present a molecular-level investigation of fatty acid monolayers at the air-water and air-sodium chloride solution interface and explore reactions with atmospheric oxidants by these model systems. Coupling sum frequency generation spectroscopy with a Langmuir trough, concurrent spectroscopic and thermodynamic data were collected to obtain a molecular picture of the monolayers. No substantial difference was observed between oxidation of monolayers spread on water and on 0.6 molar sodium chloride solutions. Results indicate that depending on the size of the aerosol and the extent of oxidation, the subsequent oxidation products may not remain at the surface of these films, but instead be dissolved in the aqueous sub-phase of the aerosol particle. Results also indicate that oxidation of oleic acid could produce monolayers containing species that have no oxidized acyl chains. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U26.00007: Chemical and Spatial Microscopy of Individual Organic Aerosols Alexei V. Tivanski, Rebecca J. Hopkins, Mary K. Gilles Carbonaceous particles originating from biomass burning can account for a large fraction of organic aerosols in a local environment. Presently, their composition, physical, and chemical properties as well as their environmental effects are largely unknown. A distinct type of biomass burn particles, called ``tar balls'', have been observed in a number of field campaigns, both in fresh and aged smoke. They are characterized by their spherical morphology, high carbon content and ability to efficiently scatter and absorb light. Here, a combination of scanning transmission x-ray microscopy and near edge x-ray absorption fine structure spectroscopy is used to determine the shape, structure and size-dependent chemical composition of 150 individual tar ball particles ranging in size from 0.15 to 1.2 $\mu $m. Oxygen is present primarily as carboxylic carbonyls and oxygen-substituted alkyl functional groups. The observed chemical composition is distinctly different from black carbon and more closely resembles high molecular weight humic-like substances. A detailed examination of the carbonyl intensity as a function of particle size reveals the presence of a thin oxygenated interface layer on the tar balls, indicative of atmospheric processing of biomass burn particles. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U26.00008: OH oxidation of organic aerosols. Jared Smith, Erin Mysak, Musa Ahmed, Christopher Cappa, Stephen Leone, Kevin Wilson Ambient aerosols play a significant role in a variety of atmospheric processes such as direct and indirect effects on radiative forcing. Chemical composition can be an important factor in determining the magnitude of these effects. However, a major fraction of organic aerosols (OA) can not be resolved on a molecular level. Recent identification of high mass oligomeric species as a major component in laboratory and ambient OA has received much attention due to the possibility that these species may account for much of the unknown organic mass in ambient aerosols. Although, a few mechanisms have been proposed, the origin and formation processes of these compounds remain largely unknown. Here we provide strong evidence for a previously unidentified mechanism of extremely rapid oligomer formation, via OH radical initiated oxidation of OA. This process appears capable of converting a sizable fraction of an organic particle to higher mass oligomers within a day of exposure to OH radicals at typical atmospheric concentrations. Furthermore, we have found that rapid volatilization is also important for specific reaction systems, and can lead to the loss of a large fraction of the particle mass. We propose that such a rapid processing is possible due to a radical chain reaction which quickly propagates throughout the entire particle and is only initiated by the surface OH reaction. [Preview Abstract] |
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