Water ice nucleation on Mars

1
Water ice nucleation on Mars

• A. Määttänen
• University of Helsinki, Finland, presently at
  Service daéronomie, France
• H. Vehkamäki, A. Lauri, I. Napari, M. Kulmala
• Department of Physics, University of Helsinki,
  Finland
• Mars Water Cycle Workshop
• 22.4.2008
• Paris, France

2
Nucleation particle formation

• The first step in cloud formation (for Mars
  mostly heterogeneous on a substrate)
• Combination of two processes
• Overcoming the barrier of energy needed for the
  formation of critical clusters (thermodynamics)
• Collisions of monomers to the cluster and their
  adsorption/desorption (kinetics)
• Nucleation rate the kinetic prefactor and the
  thermodynamic exponential part

3
Some definitions

• Nucleation rate
• Units cm-2 s-1 or s-1
• Not very useful in itself for heterogeneous
  nucleation

• Nucleation probability
• Fraction of CN that have activated (form
  crystals)
• Threshold P1
• All of your CCN are activated and form a crystal
• Threshold P0.5
• Used often

4
Nucleation theory
? Contact angle
? Critical size!
Critical Growth or break-up?

• Heterogeneous cluster geometry

??
FORMATION ENERGY
DROPLET GROWS
5
Why do we need this?

• Its not always easy to grow critical clusters
• It is often assumed that nucleation happens as
  the vapor saturates
• ? equally often this is not the
  case!
• Nucleation theory predicts the correct threshold

6
Martian water ice nucleation

• 300 ppm of water, near-surface conditions, dust
  radius 1µm, contact parameter (cos(contact
  angle)) constant

Määttänen et al., JGR E, 2005
7
Martian water ice nucleation

• Browsing through the parameter space (patm, cH2O)
  for Martian conditions
• Contact parameter constant
• ? Scrit should be calculated for different
  conditions!
• Also RCN affects!

1.55
1.45
1.35
1.25
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How about the parameters?

• Nucleation models need information on ice
  properties (measured)
• The contact parameter m
• Depends on the material of the substrate and the
  condensing species
• Colaprete, Iraci, Phebus, 2008

Spherical geometry
Planar geometry
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The new m(T)
With courtesy from T. Colaprete, L.T. Iraci B.
Phebus
10
Theory, again

• What does the m do?
• Not much in the planar geometry
• Quite a bit more in the spherical geometry

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New results

• Scritical(T)
• New contact parameter
• RCN1 micron

?m(T), spherical
?m(T), planar
Data fit by Colaprete et al ?
m constant ?
12
Sensitivity of J (nucl. rate)

• The nucleation rates are affected by
• Geometry
• RCN
• m
• Nucleation probability gives a better limit for
  cloud formation

J1s-1
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Sensitivity of P (nucl. probability)

• Gives the fraction of CN that have activated for
  cloud formation
• P is affected by m(T) as well

P0.5
14
Conclusions

• Data and models allow us to study the Martian
  water cycle in great detail aerosol dynamics is
  required!
• Scrit gt 1.0 clearly!
• Important for any cloud parameterization
• Nucleation models are needed to predict the
  threshold of cloud formation
• Still more work to be done
• Uncertainties in parameters related to nucleation
  cause large variance in J and P
• Need of more measurements
• Martian pressure and mixing ratio
• Larger T range

15
Having problems with clouds in your model?

• Too many clouds?
• Too small particles?
• Too wet water cycle?
• Just not nice in any way?
• Dont panic! Get a new nucleation model today!

16
Thank you!

• Määttänen et al, Journal of Geophysical Research
  E, 2005
• Vehkamäki et al, Atmospheric Chemistry and
  Physics, 2007
• Määttänen et al, Journal of Chemical Physics,
  2007
• More will follow