HR: 1340h
AN: A53A-0149    [Abstracts]
TI: Effects of Aldehydes and Ketones on Ice Nucleation in Sulfuric Acid Aerosols: Implications for Cirrus Cloud Formation
AU: * Beaver, M R
EM: melinda.beaver@colorado.edu
AF: Department of Chemistry and Biochemistry, University of Colorado Campus Box 216, Boulder, CO 80309 United States
AU: * Beaver, M R
EM: melinda.beaver@colorado.edu
AF: Cooperative Institute for Research in Environmental Sciences, University of Colorado Campus Box 216, Boulder, CO 80309 United States
AU: Garland, R M
EM: rebecca.garland@colorado.edu
AF: Department of Chemistry and Biochemistry, University of Colorado Campus Box 216, Boulder, CO 80309 United States
AU: Garland, R M
EM: rebecca.garland@colorado.edu
AF: Cooperative Institute for Research in Environmental Sciences, University of Colorado Campus Box 216, Boulder, CO 80309 United States
AU: Elrod, M J
EM: matthew.elrod@oberlin.edu
AF: Department of Chemistry and Biochemistry, Oberlin College 119 Woodland Street, Oberlin, OH 44074 United States
AU: Tolbert, M A
EM: margaret.tolbert@colorado.edu
AF: Department of Chemistry and Biochemistry, University of Colorado Campus Box 216, Boulder, CO 80309 United States
AU: Tolbert, M A
EM: margaret.tolbert@colorado.edu
AF: Cooperative Institute for Research in Environmental Sciences, University of Colorado Campus Box 216, Boulder, CO 80309 United States
AB: While it is known that aerosols in the upper troposphere are often internal mixtures of sulfates and organic compounds, little data exists on ice nucleation from such mixtures. Field studies have observed that organic containing particles often act as less efficient ice nuclei than entirely inorganic aerosol particles. Understanding the role of organic compounds in ice nucleation is essential for the predictions of cirrus cloud formation in the upper troposphere. Using an aerosol flow tube apparatus, we have studied the effects of aldehydes (C₃ to C₁₀) and ketones (C₃ and C₉) on ice nucleation in dilute sulfuric acid aerosols. Mixed aerosols were prepared by mixing an organic vapor flow with a flow of sulfuric acid aerosols over a short mixing time (~60s) at room temperature. No acid-catalyzed reactions were observed under these conditions, and physical uptake was responsible for the organic content, as determined by a Mie scattering analysis, of the sulfuric acid aerosols. Overall, very soluble, low-melting organics, C₃, caused an increase in aerosol organic molality, a decrease in aerosol water activity, and a decrease in aerosol ice nucleation temperatures when compared with the ice nucleation temperature of aqueous sulfuric acid aerosol. Organic compounds of eight carbons and greater, of much lower solubility and higher melting temperatures, nucleated ice at temperatures above aqueous sulfuric acid aerosols, presumably by a heterogeneous mechanism initiated by a frozen organic phase on the aerosols. Organic compounds of intermediate carbon chain length, C₄-C₇, (of intermediate solubility and melting temperatures) nucleated ice at the same temperature as aqueous sulfuric acid aerosols. This result is also explained by the formation of a second organic phase that remained unfrozen until the aqueous sulfuric acid core nucleated ice. This study demonstrates that the physical properties of the organics can play an important role in determining the impact on ice nucleation from mixed organic/inorganic aerosols.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0320 Cloud physics and chemistry
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005