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| Titel |
Amorphous and crystalline aerosol particles interacting with water vapor: conceptual framework and experimental evidence for restructuring, phase transitions and kinetic limitations |
| VerfasserIn |
E. Mikhailov, S. Vlasenko, S. T. Martin, T. Koop, U. Pöschl |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 9, no. 24 ; Nr. 9, no. 24 (2009-12-16), S.9491-9522 |
| Datensatznummer |
250007804
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| Publikation (Nr.) |
 copernicus.org/acp-9-9491-2009.pdf |
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| Zusammenfassung |
Interactions with water are crucial for the properties, transformation and
climate effects of atmospheric aerosols. Here we present a
conceptual framework for the interaction of amorphous aerosol particles with
water vapor, outlining characteristic features and differences in
comparison to crystalline particles. We used a
hygroscopicity tandem differential mobility analyzer (H-TDMA) to
characterize the hydration and dehydration of crystalline
ammonium sulfate, amorphous oxalic acid and amorphous
levoglucosan particles (diameter ~100 nm, relative
humidity 5–95% at 298 K). The experimental data and
accompanying Köhler model calculations provide new insights into
particle microstructure, surface adsorption, bulk absorption, phase
transitions and hygroscopic growth. The results of these and related
investigations lead to the following conclusions:
(1) Many organic substances, including carboxylic acids,
carbohydrates and proteins, tend to form amorphous rather than
crystalline phases upon drying of aqueous solution droplets. Depending on
viscosity and microstructure, the amorphous phases can be classified as
glasses, rubbers, gels or viscous liquids.
(2) Amorphous organic substances tend to absorb water vapor and undergo
gradual deliquescence and hygroscopic growth at lower relative humidity
than their crystalline counterparts.
(3) In the course of hydration and dehydration, certain organic substances
can form rubber- or gel-like structures (supramolecular networks) and
undergo transitions between swollen and collapsed network
structures.
(4) Organic gels or (semi-)solid amorphous shells (glassy, rubbery,
ultra-viscous) with low molecular diffusivity can kinetically limit the
uptake and release of water and may influence the hygroscopic growth and activation of
aerosol particles as cloud condensation nuclei (CCN) and ice nuclei (IN).
Moreover, (semi-)solid amorphous phases may influence the uptake of
gaseous photo-oxidants and the chemical transformation and aging of
atmospheric aerosols.
(5) The shape and porosity of amorphous and crystalline particles formed upon
dehydration of aqueous solution droplets depend on chemical composition and
drying conditions. The apparent volume void fractions of particles with
highly porous structures can range up to ~50% or more (xerogels,
aerogels).
(6) For efficient description of water uptake and phase transitions
of aerosol particles, we propose not to limit the terms
deliquescence and efflorescence to equilibrium phase transitions of
crystalline substances. Instead we propose generalized definitions
according to which amorphous and crystalline components can undergo gradual
or prompt, partial or full deliquescence or efflorescence.
We suggest that (semi-)solid amorphous phases may be important not
only in the upper atmosphere as suggested in recent studies of glass
formation at low temperatures. Depending on relative humidity, (semi-)solid
phases and moisture-induced glass transitions may also play a role in
gas-particle interactions at ambient temperatures in the lower atmosphere. |
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