HR: 0830h
AN: A51F-0757    [PDF]
TI: Effect of Morphology and Composition on the Hygroscopicity of Soot Aerosols
AU: * Williams, L
EM: williams@aerodyne.com
AF: Aerodyne Research Inc, 45 Manning Road, Billerica, MA 01821 United States
AU: Slowik, J
EM: slowikja@bc.edu
AF: Boston College, Chemistry Department, Chestnut Hill, MA 02467 United States
AU: Davidovits, P
EM: paul.davidovits@bc.edu
AF: Boston College, Chemistry Department, Chestnut Hill, MA 02467 United States
AU: Jayne, J
EM: jayne@aerodyne.com
AF: Aerodyne Research Inc, 45 Manning Road, Billerica, MA 01821 United States
AU: Kolb, C
EM: kolb@aerodyne.com
AF: Aerodyne Research Inc, 45 Manning Road, Billerica, MA 01821 United States
AU: Worsnop, D
EM: worsnop@aerodyne.com
AF: Aerodyne Research Inc, 45 Manning Road, Billerica, MA 01821 United States
AU: Rudich, Y
EM: Yinon.Rudich@weizmann.ac.il
AF: Weizmann Institute, Department of Environmental Sciences, Rehovot, 76100 Israel
AB: Freshly generated soot aerosols are initially hydrophobic and unlikely to act as cloud condensation nuclei (CCN). However, during combustion many low vapor pressure gas products are formed that may then condense on existing soot aerosols. Additionally, soot particles may acquire coatings as they age, such as acids, salts, and oxygenated organics. An understanding of this aging process and its effect on soot hygroscopicity is necessary to address the potential of soot to act as a CCN. The transformation of soot from hydrophobic to hydrophilic is the focus of this work. An aim here is to determine the minimum coating required for hygroscopic growth. Soot particles produced by combustion of mixtures of fuel and air are size selected by a Differential Mobility Analyzer (DMA) and entrained in a laminar flow passing through a flow tube. The size selected soot particles are mixed with a controlled amount of the gas phase precursors to produce the coatings to be studied. Initial studies are focused on coatings of H2SO4, NH4NO3, and selected organics. The number of particles per unit volume of air is counted by a Condensation Particle Counter (CPC) and the particles are isokinetically sampled into an Aerosol Mass Spectrometer (AMS). Two distinct types of soot aerosols have been observed depending on the type of fuel and air mixture. With soot produced by the combustion of propane and air, the AMS shows a polydisperse particle size distribution with aerodynamic diameters ranging from 100 nm to 400 nm. The aerodynamic diameter is linearly related to the DMA-determined mobility diameter with the product density x shape factor = 1.2. The organic molecules in this soot are mostly PAH compounds. However, when kerosene is added to the propane flame, the soot particle morphology and composition is strikingly altered. While the DMA shows an essentially unchanged mobility diameter distribution, in the range 100 nm to 400, aerodynamic particle diameter is constant at about 100 nm, independent of the mobility diameter. This type of constancy of the aerodynamic diameter has been observed for soot particles in diesel engine exhaust and has been interpreted in terms of a size-dependent effective density. The soot chemical composition is also altered. In this soot the organics are mainly linear hydrocarbons. The differences between these two types of soot with respect to hygroscopicity and effective area are being investigated.
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0345 Pollution--urban and regional (0305)
DE: 0394 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting