Patrick Y' Chuang

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Interactions Between Organic Aerosols and Cloud
Microphysics(the ruthlessly short and biased
version)

• Patrick Y. Chuang
• University of California Santa Cruz

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Overview

• Only the influence of organics on the formation
  of new cloud drops (i.e. activation) will be
  discussed.
• Effects on equilibrium CCN activation
• Effects on kinetics of CCN growth
• After activation, the concentration of all
  soluble species is most likely too small to exert
  much influence of important cloud processes.

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Do Organics Affect Activation?

• Studies from numerous regions worldwide show that
  CCN closure experiments often fail, particularly
  during polluted conditions.
• In general, it is found that
• Predicted CCN conc gtgt Observed CCN conc
• which leads to the question
• Do organics inhibit CCN activation?

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CCN Closure in ARM Aerosol IOP Ponca City, OK May
2003 SS 0.8
Discrepancy between observations and predictions
is a factor of between 2 and 4!
Data courtesy of Tracy Rissman, Caltech
Predicted CCN Concentration (cm3)
Even if we assume that organics are completely
insoluble, CCN concentration is still
overpredicted!
Observed CCN Concentration (cm3)
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• The interaction between
• unactivated and activated particles and
• ambient supersaturation
• is non-linear.

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Organic Effects on Equilibrium Activation
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Generalized Köhler Theory

• The equilibrium vapor pressure of a droplet
  depends on the free energy associated with
  various interactions
• G G(solute-water interactions) (Raoult Effect)
• G(air-solution interface) (Kelvin Effect)
• G(other interactions)
• Are we missing any important interactions?
• (solid-water, solute-solute, organic
  liquid-water, liquid/solid-air)

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Critical Supersaturation

• Using traditional Kohler theory, one can derive
• Therefore, two known major factors in
  determining Sc
• (1) solution surface tension
• (2) number of moles of solute in the aqueous
  drop

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Surface Tension (Fuzzi, Facchini and co-workers)

• Many organic species tend to decrease s.
• At the concentrations appropriate for activation,
  Ds O10.
• Enhances the ability of particles to activate.

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Organic solutes

• Numerous water soluble organics have been
  identified such as
• organic acids
• large humic-like molecules (which generally
  poorly understood!)
• Some are slightly soluble
• can lead to more complex equilibrium behavior

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Is Bulk Solubility Useful?(Pandis, Raymond
co-workers)

• It has been shown that some pure organic
  particles (e.g. glutamic acid, cholesterol)
  activate as if they were highly soluble (even
  though bulk solubility is very low).
• Wettability (contact angle between water and
  organic) appears to correlate with such
  behaviors
• Highly wettable, insoluble substances activate
  like perfectly soluble substances

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Data courtesy Tim Raymond, Bucknell University.
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Image courtesy Tim Raymond, Bucknell University
NaCl at 45 RH
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Image courtesy Tim Raymond, Bucknell University
NaCl at 85 RH
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Image courtesy Tim Raymond, Bucknell University
NaCl reduced to 60 RH
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Image courtesy Tim Raymond, Bucknell University
Leucine at 75 RH
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Image courtesy Tim Raymond, Bucknell University
Leucine at 97 RH
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Image courtesy Tim Raymond, Bucknell University
Glutamic acid at 75 RH
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Image courtesy Tim Raymond, Bucknell University
Glutamic acid at 98 RH, t 170 s
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Image courtesy Tim Raymond, Bucknell University
Glutamic acid at 98 RH, t 300 s
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Image courtesy Tim Raymond, Bucknell University
Glutamic acid at 89 RH, t 60 s
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Organic Effects on Kinetic Activation
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Timescales

• Activation of cloud drops in CCN instruments and
  in real clouds occurs on timescales of O10 s to
  O100 s.
• Processes relevant to activation, such as
• condensation
• dissolution
• that exhibit timescales of this order may lead
  to kinetic inhibition of CCN activationl

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Organic Films

• We know that the presence of organic films at the
  air-water interface can decrease the rates of
  condensation and evaporation of water vapor.
• Do films exist on atmospheric aerosols?
• (probably)
• How much do they inhibit mass transfer?
• (we dont know)

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Evidence of Mass Transfer Effects?

• Cantrell et al. (2001) calculated that mass
  accommodation coefficients a lt 10-4 would be
  sufficient to achieve CCN closure.
• Chuang (2003) measured a for Mexico City
  particles.
• Mass accommodation coefficient a of water is
  defined as the fraction of water vapor molecules
  colliding with the aqueous surface that are
  incorporated into the aqueous phase.

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These particles exhibit a lt 2 x 105!
Particle Diameter (nm)
Chuang, J. Geophys. Res., 2003
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Evidence of dissolution kinetics?
Hegg, et al., Atmos. Res., 2001
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Summary

• Equilibrium effects
• solution interactions
• interfacial interactions
• Kinetic effects
• gas-liquid mass transfer
• solid-liquid mass transfer

Tend to enhance the ability of CCN to activate?
Diminish the ability of CCN to activate (if they
occur).
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Outstanding Questions

• Macromolecules
• Films
• Dissolution

• Surface tension
• WSOC
• Wettability

In what ways is the microscale different from
larger and more familiar scales? Are time scales
for mass transfer relevant to cloud droplet
activation?
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The Good News

• All of these problems appear tractable.
• There are avenues to examine all of these
  questions.
• Whether they lead us to a positive result (i.e.
  CCN closure), however, is unclear.

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Ice Nucleation, Briefly

• How important are cold clouds?
• What are the mechanisms for ice nucleation in
  real clouds? (contact, immersion, deposition,
  and homogeneous freezing)
• What makes a good ice nuclei? Does it depend on
  the mechanism in question?
• Does it even matter? Does ice multiplication
  instead control everything! (Does ice
  multiplication really occur?)
• Organic effects on IN? ? Were quite far from
  asking this question!

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