Production of ice in tropospheric clouds by will cantrell and andrew heymsfield
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Production of Ice in Tropospheric Clouds by Will Cantrell and Andrew Heymsfield. Presentation by: Emily Riley 27 February 2007. Outline:. Homogeneous Nucleation Theory Assumptions Heterogeneous Nucleation Different Types of Particles Preactivation Contact Nucleation

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Production of Ice in Tropospheric Cloudsby Will Cantrell and Andrew Heymsfield

Presentation by:

Emily Riley

27 February 2007


Outline:

  • Homogeneous Nucleation

    • Theory

    • Assumptions

  • Heterogeneous Nucleation

    • Different Types of Particles

    • Preactivation

    • Contact Nucleation

  • Secondary Production

    • Hallett-Mossop process

    • Other Mechanisms


Homogeneous Nucleation Theory

Nucleation Occurs:

energy contribution from bulk > energy contribution surface

  • Jo - long explanation

  • E - energy barrier

    • f(, , c, LH, )

      • f(T)

  • k - Boltzmann’s constant

Nucleation Rate:

J = Joexp(-E/kT)


Homogeneous Nucleation Theory

  • Assumes:

    (1) Water molecules randomly arrange into an ice structure

    (2) Liquid transitions directly to hexagonal ice crystal

    (3) occurs in pure water

    (4)The initial ice fragment forms in the bulk


Problem: Nucleation rate disagreed with observations

So, Are Assumptions Good?


Assumption (1):

  • As T decreases, J increases more than the theory predicts! Why?

  • As T decreases, the nature of Hydrogen bonding results in water clustering

  • The clusters grow with decreasing T, while the bond links become more linear

Pruppacher (1995)


Assumption (2)

  • There can be an intermediate step from liquid to hexagonal crystals

  • Cubic crystals

    • occur T < -70C

    • Can cause clouds to dehydrate more effectively in the upper trop.

Riikonen et al. (2000)


Murphy (2003)


Assumption (3)

  • Homogeneous nucleation can occur in solution drops

    • Solute becomes dissolved in water such that the drop is like pure water

  • Use a modified temperature - T* = T + T

    • T - melting pt. depression

    •  - Fudge factor that relates fz pt. depression to T


Assumption (3)

  • Koop et al. (2000)

    • J in an aqueous solution is independent of the nature of the solute, only depends on water activity

    • Good or bad…Don’t know??

  • Seifert et al. (2003a) & Cziczo et al. (2004)

    • While independent of solute nature, may depend on solute size


Assumption (4)

  • Djikaev et al. (2002)

    • Nucleation could be favored at the surface as opposed to the “bulk”

    • Only important for D < 1m


Heterogeneous Nucleation:

  • Basics

  • Particle and Surface Characteristic Studies

  • Preactivation

    • Two Studies

  • Contact Nucleation

    • One Study


Heterogeneous Nucleation:

  • May significantly impact radiative properties of clouds

  • J equation still holds, but nowE is decreaseddue to substrate

  • Common substrates - dust, fly ash, soot

  • Interesting substrates - long-chain alcohols, testosterone


Let’s look at some studies...

The importance of dust:

  • Zuberi et al (2002)

    • Found ammonium sulfate drops with dust (kalonite & montmorillonite) immersed in them had freezing temperatures 10C higher than drops without dust

  • Hung et al. (2003)

    • Found ammonium sulfate drops with aerosols (corundum & hematite) raised the freezing temperature 6C


Let’s look at some studies...

The importance of the surface character:

  • DeMott et al. (1999)

    • Coated soot particles with sulfuric acid

    • For T > -53C untreated, monolayered, and multi-layered soot particles initiated ice formation about the same

    • For T < -53C multi-layered soot particles formed ice better


Let’s look at some studies...

The importance of the surface character:

  • Gorbunov et al. (2001)

    • If the surface contains chemical groups capable of forming hydrogen bonds with water molecules, the soot’s ice-forming potential could be increased.

    • Such surfaces were 3x more efficient as ice nuclei at -20C than surfaces without such capabilities


Preactivation : “memory effect”

  • Increase in the effective freezing T after ice nucleator has catalyzed the phase transition once or cooled below -40C

  • Effect lost if T (of the system) exceeds some threshold

  • Occurs due to an ordered, ice like layer of water molecules on the substrate

  • Basically, aged nucleators more efficient


Yet more studies...

Preactivation : “memory effect”

  • Seeley and Seidler. (2001)

    • Long chain alcohols exhibit preactivation

    • Alcohols could act as effective ice nucleators above the melting point


Yet more studies...

Preactivation : “memory effect” continuted:

  • Rosinski (1991) & Rosinski and Morgan (1991)

    • Conditioned Particles

      1. Particles exposed to SS H2O(v) at T < 0C

      2. Resulting drops evaporated

      3. Particles exposed to H2O(v) SS w.r.t. ice but not H2O(l)

    • Result: conditioned particles formed ice crystals by deposition, while non-conditioned particles did not


Contact Nucleation:

  • Freezing of a drop initiated by contact with an aerosol particle

  • Substance have a different freezing threshold than when they act as deopostion, condensation, or immersion nuclei

    • Indicates freezing mechanism different for different modes


Secondary Production:

  • Important - Ice production frequently exceeds IN concentration

  • How:

    • Hallet-Mossop process (most common)

    • Mechanical Fracture

    • Ice multiplication


Paquita Explains the H-M process….


Paquita Explains the H-M process….


Paquita Explains the H-M process….


Hallett and Mossop (1974), Mossop and Hallett (1974),Mossop (1976)


Studies confirming the Hallett-Mossop process...

  • Hogen et al. (2002)

    • Observed ice crystal concentrations up to 1000 l-1 in UK convection at T = -6C

    • Typical heterogeneous IN concentrations were 3-5 orders of magnitude lower than the measured IN concentrations

  • Ovtchinnikov et al. (2000)

    • Cumulus clouds in NM

  • Modeling studies - confirmed these observations


So, is it always the H-M process?

  • According to Hobbs and Rangno (many papers), Nope. Why not?

    • Clouds glacieated much faster than H-M process could explain

    • Crystal habits observed were often not compatible with the T range in which H-M operates

    • High concentrations of small ice particles appeared concurrently with frozen drizzle drops

  • Conclusion - Origin of ice is a mystery…sounds like a job for more research!


Other Mechanisms:

  • Mechanical Fracture

    • Occurs particularly in dendritic crystals

  • Ice multiplication

    • Occurs when large drops shatter upon freezing


Conclusion:

  • Homogeneous Nucleation - Best Understood

  • Secondary Production - Somewhat Understood

  • Heterogeneous Production - “Progress is desperately needed”


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