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| Titel |
A time dependence study of droplet freezing with immersed mineral dust particles |
| VerfasserIn |
Tina Clauss, Susan Hartmann, Dennis Niedermeier, Alexei Kiselev, Heike Wex, Frank Stratmann |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250057633
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| Zusammenfassung |
| Due to the importance of the ice phase for the radiative and microphysical properties of
clouds, the freezing behaviour of aerosol particles has received an increased attention in the
past years. Especially heterogeneous freezing plays a major role in atmospheric ice formation
processes. There are two contrary approaches concerning the description of heterogeneous
ice nucleation processes which are controversially discussed. In the first approach, the
stochastic hypothesis, it is stated that the presence of an ice nucleus (IN) within a supercooled
droplet increases the likelihood of the random ice nucleation but does not disturb the
stochastic nature of critical cluster formation. Therefore with increasing time the likelihood
of a supercooled droplet to freeze at a given temperature increases too. In contrast, the
singular hypothesis describes that critical cluster formation occurs on specific sites
on the IN surface at a characteristic temperature, i.e., freezing is assumed to be
time-independent. To verify one of those contrary approaches immersion freezing
experiments were performed in the laminar diffusion cloud chamber LACIS (Leipzig
Aerosol Cloud Interaction Simulator, Hartmann et al., 2010; Stratmann et al., 2004).
Size-selected monodisperse Arizona Test Dust (ATD) particles were activated to droplets
which then were supercooled. The freezing of a certain fraction of these droplets
was then observed for one adjusted temperature. Additionally, the residence time
inside LACIS was varied between 2 s and 9 s resulting in different ice nucleation
times in order to validate time dependence or independence. This experiment was
performed for different freezing temperatures. For the determination of frozen and
unfrozen particles downstream LACIS, a novel optical ice detecting instrument LISA
(LACIS Ice Scattering Apparatus) was used. The instrument belongs to the series
of Small Ice Detector 3 (SID3) instruments (Kaye, 2008), capable of capturing
high-resolution two dimensional light-scattering patterns from single particles in
near-forward direction with a high-speed CCD camera. Investigating the immersion
freezing behavior of ATD particles for different ice nucleation times, it was found
that ATD particles nucleate ice over a broad temperature range and the increase
of ice nucleation time from 2 s to 9 s does not significantly change the freezing
temperature distributions. These findings agree with the singular description of the ice
nucleation process. But studies with the ‘soccer ball’ model (see Niedermeier et
al., this session) showed that the variability of the surface properties (e.g., surface
free energy) across the population of the very heterogeneous Arizona Test Dust
particles is most plausibly responsible for the broad temperature range over which
droplets freeze and for the apparent missing time dependence of heterogeneous ice
nucleation.
References:
Hartmann et al. (2010): The Leipzig Cloud Interaction Simulator (LACIS): operating
principle and theoretical studies concerning homogeneous and heterogeneous ice nucleation.
Atmos. Chem. Phys. Discuss., 10, 25577-25617.
Stratmann et al. (2004): Laboratory studies and numerical simulations of cloud droplet
formation under realistic super-saturation conditions, J. Atmos. Oceanic Technol., 21,
876-887.
Kaye et al. (2008): Classifying atmospheric ice crystals by spatial light scattering, Optics
Letters, 33, 1545-1547. |
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