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 Secondary Production –Hallett-Mossop process –Other Mechanisms

Nucleation Rate: J = J o exp(-  E/kT) Homogeneous Nucleation Theory J o - long explanation  E - energy barrier –f( ,, c, LH,  ) f(T) k - Boltzmann’s constant Nucleation Occurs: energy contribution from bulk > energy contribution surface

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 decreased due to substrate Common substrates - dust, fly ash, soot Interesting substrates - long-chain alcohols, testosterone

Let’s look at some studies... 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 The importance of dust:

Let’s look at some studies... 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 The importance of the surface character:

Let’s look at some studies... 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 The importance of the surface character:

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... Seeley and Seidler. (2001) –Long chain alcohols exhibit preactivation –Alcohols could act as effective ice nucleators above the melting point Preactivation : “memory effect”

Yet more studies... Rosinski (1991) & Rosinski and Morgan (1991) –Conditioned Particles 1. Particles exposed to SS H 2 O (v) at T < 0C 2. Resulting drops evaporated 3. Particles exposed to H 2 O (v) SS w.r.t. ice but not H 2 O (l) –Result: conditioned particles formed ice crystals by deposition, while non-conditioned particles did not Preactivation : “memory effect” continuted:

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….

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”