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    Surface Science Monday Sessions
       Session SS1-MoM

Paper SS1-MoM7
Scanning Electron Microscopy Studies of the Hydration of Mixed Alkali Halide Aerosols

Monday, October 29, 2001, 11:40 am, Room 120

Session: Aerosol and Related Chemistry
Presenter: S.A. Joyce, Los Alamos National Laboratory
Authors: S.A. Joyce, Los Alamos National Laboratory
J.P. Cowin, Pacific Northwest National Laboratory
J.T. Atherley, Pacific Northwest National Laboratory
Correspondent: Click to Email
The morphologies of aerosols generated from pure NaCl, NaBr, and MgCl@sub 2@ solutions and from NaCl/NaBr and NaCl/MgCl@sub 2@ mixtures have been studied using an environmental scanning electron microscope (ESEM). For aerosols produced by nebulizing pure NaCl solutions, nearly perfect cubic crystals with widths of a few microns are normally observed. Upon raising the water vapor pressure in the specimen chamber, these pure NaCl aerosols are observed to dissolve at relative humidities consistent with previous studies. For aerosols produced by nebulizing NaCl/NaBr mixtures, ranging from 0.1 to 0.5 mole fraction NaBr, polycrystalline particles are often observed. For these polycrystalline samples, deliquescence of individual crystallites occurs at two different relative humidities. The central or core crystallite typically dissolves at relative humidities similar to that for pure NaCl, whereas the outer crystallites dissolve at humidities lower than that for either pure NaCl or NaBr. The deliquescence pressure for these crystallites is independent of the mole fraction of starting solution. The outer region, which dehydrates last, is composed of the NaCl/NaBr eutonic (0.9 mole fraction NaBr), with the inner region composed of nearly pure NaCl. For the aerosols produced by nebulizing NaCl/MgCl@sub 2@ mixtures, a crystalline core surrounded by a featureless outer region is typically observed. X-ray fluorescence measurements show that this outer region is largely magnesium-based while the inner crystallite in sodium-based. Based on available thermodynamic data, no eutonic of NaCl/MgCl@sub 2@ exists, and so phase separation into the pure components is expected. The implication of the phase-separation to surface enrichment of minority components and its potential role in heterogeneous atmospheric chemistry will be discussed. * Pacific Northwest National Laboratory is operated for the Department of Energy by Battelle under Contract DE-AC06-76RLO 1830.