Synchrotron-based high-pressure research in materials science Page: 4 of 22
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slowing effect (13, 14). In our previous work, we addressed the problem of accurately
modeling the heat and mass transfer processes through our measurements of the free
evaporation of pure H20 and pure D20 (4, 5, 10). We also studied relative evaporation
rates of isotopomers in mixtures (10, 15). In these studies of pure solutions, condensation
was negligible, allowing the evaporation process to be modeled accurately and then
directly related to the cooling rate of the droplets. We found that evaporation for both
pure H20 and pure D20 occurred at ~ 60% of the maximum rate determined by gas
kinetic theory, too fast to result in a kinetic limit to cloud droplet growth. In the present
study, we take the first step towards accounting for the effects of impurities on the
evaporation rate by performing similar experiments on ammonium sulfate solutions.
Ammonium sulfate was selected as a realistic model system for atmospheric
inorganic aerosol due to its well-documented prevalence in the troposphere. Field studies
using aerosol mass spectrometers (AMS) have revealed significant fractions of the
ambient aerosol to comprise aqueous ammonium and sulfate at the surface in both urban
and rural areas (16). Additionally, single-particle studies have shown that the majority of
atmospheric aerosol particles are internally well-mixed and consist of approximately 50%
ammonium sulfate and 50% carbonaceous components, with little altitude variation (17).
Many thermodynamic studies of ammonium sulfate aerosol have shown a hysteresis in
the deliquescence properties. Solid particles deliquesce at relative humidities greater than
~80%, but can remain in the aqueous phase as the relative humidity drops as low as
~35%, resulting in supersaturated solution up to approximately twice the saturation
concentration before efflorescence occurs (18-20). This suggests that in the atmosphere
much of the ammonium sulfate aerosol will be in the form of concentrated aqueous
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Synchrotron-based high-pressure research in materials science, article, Date Unknown; (https://digital.library.unt.edu/ark:/67531/metadc932478/m1/4/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.