Inside history on droplets

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Inside history on droplets
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Papers

• 1) Nature,Vol.413,586-587,11th October 2001
• news and views ----- cloud physics
• Inside history on droplets (Marcia Baker)
• 2) J. Phys. Chem. A,Vol.105,6458-6464,2001
• Heterogeneous Freezing of Aqueous
• Particles Induced by Crystallized (NH4)2SO4,
• Ice,and Letovicite (Zuberi et al.)

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Inside history on dropletsby Marcia Baker
(Nature)

• 1) Clouds in the troposphere have profound
  influences on air chemistry, weather and global
  climate.
• 2) This is the background to a laboratory study
  by Zuberi and colleagues, published in Journal of
  Physical Chemistry A.
• 3) The paper highlights the potential
  significance of droplet history on droplet
  freezing.

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Two patterns of freezing

• 1) Water can remain liquid, in a metastable
  ('supercooled') state, down to much lower
  temperatures than 0 C.
• 2) In the absence of external surfaces, and
  freezing is equally likely to occur at any point
  within it this is called homogeneous freezing.
• 3) A solid surface is immersed in or in contact
  with the droplet, liquid freezing can be
  catalysed by that surface at relatively high
  temperatures this is called heterogeneous
  freezing.

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Thermal history of droplets

• 1) By subjecting the droplets to a range of
  temperatures, Zuberi et al. found that the
  morphology and sizes of the salt crystallites
  depended on the thermal histories of the
  droplets, as did the temperatures at which the
  droplets eventually froze.
• 2) Freezing was homogeneous in some cases but was
  clearly heterogeneous in others.

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Doubtful classical idea for high-temperature
freezing

• 1) The effects of droplet history have not been
  understood previously.
• 2) The new findings cast doubt on the idea that
  heterogeneous - high-temperature - freezing of a
  droplet can occur only when it comes into contact
  with pre-existing solid surfaces.

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Are pre-existing nuclei required?

• 1) Several studies which have involved collecting
  particles are based on the idea that
  heterogeneous freezing of droplets requires
  pre-existing ice nuclei.
• 2) But the experiments of Zuberi et al. suggest
  this approach may not always be sufficient to
  predict the eventual temperatures of droplet
  freezing.
• 3) Rather, it seems that the formation of ice
  nuclei within droplets may sometimes be part of
  the freezing process itself.

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The applicability of the results of these new
experiments

• 1) There are many differences between conditions
  in laboratory experiments and those in the
  atmosphere, so the applicability of the results
  of these new experiments is likely to be limited.
• 2) Nonetheless, there are times and places in the
  atmosphere at which analogous transformations of
  droplets occur, which might affect the freezing
  modes. The freezing mode and temperature could
  depend on the droplet's history.

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Significance of droplet freezing

• 1) The effects of upper-tropospheric clouds on
  climate are highly sensitive to the amount of
  frozen cloud water.
• 2) IPCC reports the change in the global average
  flux due to freezing is 17 Wm-2.
• (The change due to CO2 is less than 2 W m-2.)
• 3) Droplet freezing is one of the central
  processes in cloud physics but also one of the
  least understood. These experiments provide fresh
  approaches to the subject.

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Heterogeneous Freezing of Aqueous Particles
Induced by Crystallized (NH4)2SO4, Ice, and
Letovicite

• Journal of Physical Chemistry A
• Bilal Zuberi, Allan K. Bertram, Thomas Koop,
  Luisa T. Molina, and Mario J. Molina
• Departments of Earth, Atmospheric and Planetary
  Sciences and of Chemistry, Massachusetts
  Institute of
• Technology

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1.Introduction

• 1) Upper troposheric (UT) clouds play an
  important role in the Earth climate and the
  chemistry of the upper troposphere.
• 2) Homogeneous freezing of aqueous particles was
  considered to be the dominant formation mechanism
  of UT ice clouds.
• 3) Field data suggest that heterogeneous
  nucleation is also occurring in the upper
  troposphere.

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2. Experimental Technique2.1. Optical Microscope

• 1) An optical microscope (Zeiss Axioskop 20) was
  used to investigate heterogeneous freezing.
• 2) Concentrations of (NH4)2SO4-H2O, and
  NH4HSO4-H2O particles were adjusted by exposing
  them to a fixed relative humidity.
• 3) Phase transitions in the particles due to a
  change in light scattering was easily observed.

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2. Experimental Technique2.2. Differential
Scanning Calorimetry

• 1) A commercial Perkin-Elmer DSC-7 instrument was
  used for the calorimetric experiments.
• 2) The DSC technique involved monitoring the
  differential energy required to keep both a
  sample and a reference at the same temperature.
• 3) The changes in the differential energy as a
  function of temperature were plotted as
  thermograms, and peaks in the thermograms
  indicated phase transitions.

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2. Experimental Technique2.3. Ammonium Sulfate
Experiments

• 1) Thermally cycling (NH4)2SO4-H2O particles with
  concentrations between 41.3 and 44.6 wt .
• 2) Determining the phase transition temperatures
  of the particles during the thermal cycling.

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2. Experimental TechniquePhase Diagram and
Photographs
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2. Experimental Technique2.3. Ammonium Bisulfate
Experiments

• 1) Temperature-cycling experiments were carried
  out on NH4HSO4-H2O particles with the following
  concentrations 36 wt (experiment i), 57 wt
  (experiment ii), and 68 wt (experiment iii).
• 2) From these thermal-cycling experiments, both
  the homogeneous freezing temperatures and
  heterogeneous freezing temperatures of the
  individual particles were determined.

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2. Experimental TechniquePhase Diagram and
Pictorial Illustration
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3. Results and Discussion3.1. Ammonium Sulfate

• 1) The homogeneous freezing temperatures did not
  vary from one experiment to the next.
• 2) The heterogeneous ice-freezing temperatures
  varied as a function of the conditioning
  temperature used in the thermal-cycling
  experiments.
• 3) The higher the conditioning temperature the
  lower is the heterogeneous freezing temperature.

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3. Results and DiscussionFreezing Temperatures
The particles were cooled at 1 K min-1. The
arrow indicates the expected homogeneous freezing
temperature, 202.2 K, for homogeneous nucleation
of a (NH4)2SO4-H2O solution in equilibrium with
a solid (NH4)2SO4 core.
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3. Results and DiscussionFreezing Temperature vs
C-Temperature
This figure shows the median heterogeneous
freezing temperatures as a function of
the conditioning temperature, determined from
the microscope and DSC experiments. The DSC and
microscope results display a similar trend
heterogeneous freezing occurs at warm
temperatures if the conditioning temperature is
close to, but warmer than, the eutectic
temperature.
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3. Results and DiscussionPhotographs of
Particles
Conditioning temperature 291.2 K (higher) only
one microcrystal
Conditioning temperature 255.2 K
(lower) numerous microcrystals
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3. Results and Discussion The Morphology and the
Thermal History
A room temperature B 183 K, all freezing C1
slightly above the eutectic, leaving behind
numerous microcrystals Ch higher
temperature, leaving behind one or two
microcrystals D1 freezing readily Dh
supercooling to a lower temperature, since
the surface area of the solid was
minimized
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3. Results and Discussion Possibility 1
Surface Area

• 1) The heterogeneous freezing temperature depends
  on the thermal history of the crystals.
• 2) There is a clear trend between the surface
  area of the ammonium sulfate crystals and the
  heterogeneous freezing temperature.
• 3) This is consistent with classical nucleation
  theory, which predicts that the heterogeneous
  freezing rate is proportional to the surface area.

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3. Results and Discussion Possibility 2
Surface Microstructure

• 1) The difference in heterogeneous freezing
  temperatures may also be due to the surface
  microstructure of the crystals.
• 2) Heterogeneous nucleation may occur
  predominately at surface defects such as cracks,
  steps, or dislocations.
• 3) These surface defects may be enhanced on the
  microcrystals that are produced at fast crystal
  growth rates.

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3. Results and DiscussionPossibility 3
Preactivation

• 1) Initial formation of solid ammonium sulfate in
  the presence of ice may modify the crystalline
  structure.
• 2) This modified surface may have sites that
  closely match the ice lattice.
• 3) If the temperature is increased only slightly
  above the eutectic, these activated sites may
  continue to exist.
• 4) As a result, heterogeneous freezing of ice may
  occur at much higher temperatures.

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3. Results and DiscussionApplication to the
Atmosphere 1

• 1) When the ratio of ammonia-to-sulfate in the
  atmospheric aerosol is exactly 21.
• 2) At low relative humidities, the particles will
  be completely dry ammonium sulfate.
• 3) When the temperature decreases and the
  relative humidity increases, the particles will
  deliquesce and temporarily exist as a
  solid-liquid mixture.

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3. Results and DiscussionHypothetical
Atmospheric Trajectory
Deliquescence Relative Humidity of (NH4)2SO4
crystals
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3. Results and DiscussionApplication to the
Atmosphere 2

• 1) The results might also be applicable when the
  ammonia-to-sulfate ratio in atmospheric particles
  is nonstoichiometric. In these cases, the
  particles can exist as partially crystalline
  ammonium sulfate.
• 2) Despite this fact, the freezing temperatures
  of nonstoichiometric particles can be predicted
  on the basis of the ice saturations required for
  heterogeneous freezing determined in the
  experiments.

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3. Results and Discussion3.2. Ammonium Bisulfate

• 1) One or two large crystals of ice or
  letovicite (NH4)3H(SO4)2 were in equilibrium
  with a liquid.
• 2) The heterogeneous freezing temperatures were
  close to the homogeneous freezing temperatures,
  similar to the (NH4)2SO4-H2O experiments.
• 3) Heterogeneous nucleation by ice or letovicite
  with a similar morphology is not an important
  atmospheric process.

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4. Summary and Conclusions

• 1) The heterogeneous freezing of (NH4)2SO4-H2O
  and NH4HSO4-H2O particles by solid inclusions of
  crystallized (NH4)2SO4, ice, and letovicite was
  investigated.
• 2) The temperature at which ice nucleates
  heterogeneously was found to be dependent on the
  surface area and microstructure of the solid, as
  well as the thermal history of the particles.