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| Titel |
Immersion freezing of water and aqueous ammonium sulfate droplets initiated by humic-like substances as a function of water activity |
| VerfasserIn |
Y. J. Rigg, P. A. Alpert, D. A. Knopf |
| 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 ; 13, no. 13 ; Nr. 13, no. 13 (2013-07-12), S.6603-6622 |
| Datensatznummer |
250018756
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| Publikation (Nr.) |
 copernicus.org/acp-13-6603-2013.pdf |
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| Zusammenfassung |
| Immersion freezing of water and aqueous (NH4)2SO4 droplets
containing leonardite (LEO) and Pahokee peat (PP) serving as
surrogates for humic-like substances (HULIS) has been
investigated. Organic aerosol containing HULIS are ubiquitous in the
atmosphere; however, their potential for ice cloud formation is
uncertain. Immersion freezing has been studied for temperatures as
low as 215 K and solution water activity, aw,
from 0.85 to 1.0. The freezing temperatures of water and aqueous
solution droplets containing LEO and PP are 5–15 K warmer
than homogeneous ice nucleation temperatures. Heterogeneous freezing
temperatures can be represented by a horizontal shift of the ice
melting curve as a function of solution aw by Δaw = 0.2703 and 0.2466, respectively. Corresponding
hetrogeneous ice nucleation rate coefficients, Jhet,
are (9.6 ± 2.5)×104 and (5.4 ± 1.4)×104 cm−2 s−1 for LEO and PP containing droplets,
respectively, and remain constant along freezing curves
characterized by Δaw. Consequently predictions
of freezing temperatures and kinetics can be made without knowledge
of the solute type when relative humidity and ice nuclei (IN) surface areas are
known. The acquired ice nucleation data are applied to evaluate
different approaches to fit and reproduce experimentally derived
frozen fractions. In addition, we apply a basic formulation of
classical nucleation theory (α(T)-model) to calculate contact
angles and frozen fractions. Contact angles calculated for each ice
nucleus as a function of temperature, α(T)-model, reproduce
exactly experimentally derived frozen fractions without involving
free-fit parameters. However, assigning the IN a single contact
angle for the entire population (single-α model) is not suited to
represent the frozen fractions. Application of α-PDF, active
sites, and deterministic model approaches to measured frozen
fractions yield similar good representations. Furthermore, when using a
single parameterization of α-PDF or active sites distribution to fit
all individual aw immersion freezing data simultaneously,
frozen fraction curves are not reproduced. This implies that these fitting
formulations cannot be applied to immersion freezing of aqueous solutions, and
suggests that derived fit parameters do not represent independent particle
properties. Thus, from fitting
frozen fractions only, the underlying ice nucleation mechanism and
nature of the ice nucleating sites cannot be inferred. In contrast
to using fitted functions obtained to represent experimental
conditions only, we suggest to use experimentally derived
Jhet as a function of temperature and aw
that can be applied to conditions outside of those probed in
laboratory. This is because Jhet(T) is independent of
time and IN surface areas in contrast to the fit parameters obtained
by representation of experimentally derived frozen fractions. |
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