1999 Summer Colloquium
ICE FORMATION IN THE ATMOSPHERE
MEASUREMENTS OF ICE NUCLEI
By David C. Rogers, Colorado State University
(a) freeze (exhibit phase transition)
(b) grow to detectable size as crystals
Deposition (sorption), condensation-freezing, immersion-freezing, contact-freezing, (electro-freezing, ..}
Temperature, humidity, time, thermodynamic history, presence of supercooled liquid water, epitaxy and lattice misfit, solubility, hydrophobic/hydrophillic, surface concentration of hydrogen bonds, surface charge density,..?
(compare with variables for activation of CCN)
Membrane filters (Bigg, Huffman, Rosinski, others)
Supercooled drops of pure water (Vali, Deshler)
Static diffusion (Bigg, Huffman, Schaller 2D Ñ temp)
Forced ventilation on filters (Langer)
Slow expansion (DeMott)
Rapid expansion (Popoff, Bigg/Warner)
Mixing (Schaefer, Vonnegut, Langer)
Sedimenting droplets (Ohtake, Cooper)
Continuous flow thermal gradient diffusion (Tomlinson, Hussain, Rogers)
Horizontal flat parallel plates
Vertical flat parallel plates and horizontal flowVertical Concentric cylinders and vertically downward flow
Each technique emphasizes some nucleation mechanism(s) and may bias against others.
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Deposition |
Condensation-Fz |
Immersion-Fz |
Contact-Fz |
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Cold Box |
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Drop Freezing |
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Membrane Filters: |
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- Vapor only |
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- Sedimenting Drops |
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Particle capture by drops in flow |
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Chambers: |
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- 2D Ñ temp |
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- Rapid expansion |
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- Mixing |
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- Sedimenting drops |
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- Continuous flow |
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Examples:
Contact freezing detection is enhanced with high droplet concentration in mixing chambers, whereas detection is reduced by small residence time in flow chambers (~5s).
Vapor competition in static
Practical advantages/disadvantages (size, power, complexity, response time, etc.)
Who is presently doing IN measurements and what methods are being used?
The background response (need for blank or placebo measurement)
Sampling statistics are often weak since IN numbers are small at atmospherically interesting temperatures.Trade-off between sample volume and temporal/spatial resolutionSubstrate effects
Vapor competition
Which nucleation mechanism(s) is important in particular cloud processes?
Drop freezing nucleus spectra
Climatologies of IN
Membrane filter data - Bigg, Rosinski, Berezinski, Huffman, etc.
Vertical profiles
Effect of particle size on IN activity (filters)
AgI sniffing? (Heimbach and Super)
Bag samples (WISP94 DCC, filters, CFD)
IN activity of aircraft exhaust? (Popoff, SUCCESS, lab studies)
Source regions? (Arctic 96,98)
Aggregate data - IN concentration versus temperature, supersaturation
Single particle studies - Kumai, Rosinski, Chen
IN instrument comparison workshops (1970, 1975)
Design field program to study ice formation in different kinds of cloud types, such as wave clouds, stratus or isolated cumulus. Include instrument list, flight plans and outline analysis plans.
Make a flow chart showing conceptual transfer processes involving ice nuclei in the atmosphere. Possible links might include activation, cloud entrainment/detrainment, sources, cloud cycling, CCN, biological feedback, biomass burning, alien invasions, warfare, etc. Identify some possible distinctions between continental and maritime regions, winter and summer. Estimate time scales.
Design a mixing type ice nucleus chamber that could provide carefully controlled limits on temperature and supersaturation, would provide adequate time for the nucleation mechanisms you choose and would provide counting uncertainties of <10% when sampling at –10°C. Provide a reality check – how large is this device? Could it be operated in airborne field studies?
Supersaturation in a static diffusion chamber
Temperature and humidity in a mixing process
Ice nucleation activity for two AgI compounds