HR: 09:10h
AN: A41E-05    [Abstracts]
TI: Ozonolysis of Mixed Oleic-Acid/Stearic-Acid Particles: Reaction Kinetics and Chemical Morphology
AU: * Martin, S T
EM: smartin@deas.harvard.edu
AF: Harvard University, Division of Engineering and Applied Sciences, Cambridge, MA 02138 United States
AU: Katrib, Y
EM: ykatrib@deas.harvard.edu
AF: Harvard University, Division of Engineering and Applied Sciences, Cambridge, MA 02138 United States
AU: Biskos, G
EM: gbiskos@fas.harvard.edu
AF: Harvard University, Division of Engineering and Applied Sciences, Cambridge, MA 02138 United States
AU: Buseck, P R
EM: pbuseck@asu.edu
AF: Arizona State University, Dept of Geol Sci and Dept of Chem, Tempe, AZ 85287 United States
AU: Davidovits, P
EM: paul.davidovits@bc.edu
AF: Boston College, Chemistry Department, Chestnut Hill, MA 02467 United States
AU: Jayne, J T
EM: jayne@aerodyne.com
AF: Aerodyne Research Inc, 45 Manning Drive, Billerica, MA 08121 United States
AU: Mochida, M
EM: mmochida@deas.harvard.edu
AF: Harvard University, Division of Engineering and Applied Sciences, Cambridge, MA 02138 United States
AU: Mochida, M
EM: mmochida@deas.harvard.edu
AF: Hokkaido University, Institute of Low Temperature Science, Sapporo, 060-0819 Japan
AU: Wise, M E
EM: Matthew.Wise@asu.edu
AF: Arizona State University, Dept of Geol Sci and Dept of Chem, Tempe, AZ 85287 United States
AU: Worsnop, D R
EM: worsnop@aerodyne.com
AF: Aerodyne Research Inc, 45 Manning Drive, Billerica, MA 08121 United States
AB: Atmospheric particles directly and indirectly affect global climate and have a primary role in regional issues of air pollution, visibility, and human health. Atmospheric particles have a variety of shapes, dimensions, and chemical compositions, and these physicochemical properties evolve (i.e., "age") during transport of the particles through the atmosphere, in part because of the chemical reactions of particle-phase organic molecules with gas-phase atmospheric oxidants. As a global average, hydroxyl radical (OH) and ozone (O3) are responsible quantitatively for most oxidant aging of atmospheric particles. The reactions of the hydroxyl radical occur in the surface region of a particle because of the nearly diffusion-limited bimolecular rate constant of OH with a variety of organic molecules. Ozone, on the other hand, is a selective agent for the unsaturated bonds of organic molecules and may diffuse a considerable distance into particles prior to reaction. The reaction of oleic acid with ozone has recently emerged as a model system to better understand the atmospheric chemical oxidation processes affecting organic particles. The ozonolysis of mixed oleic-acid/stearic-acid (OL/SA) aerosol particles from 0/100 to 100/0 weight percent composition is studied. The magnitude of the divergence of the particle beam inside an aerosol mass spectrometer shows that, in the concentration range 100/0 to 60/40, the mixed OL/SA particles are liquid prior to reaction. Upon ozonolysis, particles with SA composition greater than 25% change shape, indicating that they have solidified. Transmission electron micrographs show that SA(s) forms needles. For SA compositions greater than 10%, the reaction kinetics exhibit an initial fast decay of OL for low O3 exposure with no further loss of OL at higher O3 exposures. For compositions from 50/50 to 10/90, the residual OL concentration remains at 28ñ2% of its initial value. The initial reactive uptake coefficient for O3, as determined by OL loss, decreases linearly from 1.25(ñ0.2) 10-3 to 0.60(ñ0.15) 10-3 for composition changes of 100/0 to 60/40. At 50/50 composition, the uptake coefficient drops abruptly to 0.15(ñ0.1) 10-3, and there are no further changes with increased SA content. The amount of SA in the particles also decreases during OL ozonolysis. The stabilized Criegee intermediate (SCI) formed by OL ozonolysis attacks the carboxylic acid group of SA to yield an acyloxyalkyl hydroperoxide product. The experimental observations of this study can be explained by the following two postulates: (1) unreacted mixed particles remain as supersatured liquids up to 60/40 composition and (2) SA, as it solidifies, locks in a significant amount of oleic acid. The results of this study point out the important effects of particle phase, composition, and morphology on chemical reactivity. Oleic acid in liquid regions of a particle reacts rapidly with O3 whereas OL trapped inside solid SA is essentially unavailable for reaction with O3. These results contribute to the continuing development of the scientific community's understanding of particle aging process in the atmosphere, for which the ultimate goal is to provide quantitative mechanistic models of physicochemical transformations under atmospheric conditions.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0317 Chemical kinetic and photochemical properties
DE: 0345 Pollution: urban and regional (0305, 0478, 4251)
DE: 0365 Troposphere: composition and chemistry
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005