1. June 7, 2004 Laboratory Studies of Ice Initiation by Atmospheric Aerosol Particles Paul J. DeMott With acknowledgment to numerous contributors

2. June 7, 2004 Overview Talk will concern itself only with primary ice initiation. Other laboratory studies of relevance: Secondary ice formation, ice growth, instrumentation testing Ice formation mechanisms Laboratory methodologies of old and new What have we learned about about homogeneous freezing and what remains? What have we learned or not learned about heterogeneous ice nucleation? –Mineral dust revisited: The major source of atmospheric IN? –Soot: Effective or not? –Organics aerosol components and ice nucleation –Real-time assessment of IN composition by mass spectrometry Need synergy with theory, modeling and field studies (will allude to) The future

3. June 7, 2004 Lab studies Field Studies theory Numerical modeling Science according to one “lab” person

4. June 7, 2004 Ice nucleation mechanisms

5. June 7, 2004 Some examples of ice nucleation studies instrumentation Drop freezing devicesAerosol flow tubes (AFTIR)

6. June 7, 2004 More instrumentation Electrodynamic balance Diffusion chamber (filter processor)

7. Cloud Chamber (AIDA in this case) Moderate expansion cloud chamber: T Range: 0 to –90 °C P Range: 0.01 to 1000 hPa Ice saturation by ice coated walls Volume expansion at constant wall temperature: Cooling rates 0.1 to 4 K/min RHi increase up to 100 %/min RHi: > 160 % Duration of expansion  30 min

8. June 7, 2004 Measuring ice formation by aerosols in the laboratory or atmosphere [Continuous flow diffusion chamber (CFDC) – Rogers et al., JAOT, 2001; Now used in US, UK, Japan, Canada, Switzerland (soon)] 1 - 1.5 m, 5 - 30 s or LCVI

9. June 7, 2004 Homogeneous freezing: We believe we quantitatively understand spontaneous freezing of “pure” water, but…issue: Surface vs. volume nucleation Surface crystallization of supercooled water in clouds (A. Tabazadeh, Y. S. Djikaev, and H. Reiss; PNAS, 2002)

10. June 7, 2004 Homogeneous freezing of solution drops: Dependence on water activity and freezing point depression - composition irrelevant? Water activity is defined by freezing point depression experiments (Robinson and Stokes, 1965), so stands to reason that both ideas work as parameterizations of homogeneous freezing nucleation. Both ideas can be formulated to predict nucleation rates (numerous authors). Freezing temperatures of solute emulsion drops collapse onto constant water activity difference between solution and ice (Koop et al. 2000) Freezing conditions of different solute drops is related to melting temperatures by a relatively constant factor (DeMott 2002, via Sassen via Rasmussen)

11. Homogeneous freezing of sulphuric acid droplets (AIDA) Based on Koop et al. 2000

12. June 7, 2004 Water activity relation works for many substances, but…ammonium sulfate is a “bugger” Need data as nucleation rate!

13. June 7, 2004 Organics appear to impact kinetics of homogeneous freezing or are preferentially delayed in freezing compared to sulfates (DeMott et al. 2003, Cziczo et al. 2004) – Talk by D. Cziczo tomorrow Soluble diacids seem not to be the answer (next slide) Organic carbon fraction delays ice formation (Mohler et al. 2004) – see later Another issue: Impacts on homogeneous freezing associated with presence of organics

14. June 7, 2004 CFDC lab studies of ammonium sulfate-dicarboxylic acid mixtures – phase state changes are more important than composition S. Brooks and A. Prenni

15. June 7, 2004 Homogeneous and heterogeneous nucleation at low temperatures on ambient tropospheric aerosol particles and suggested impacts on cirrus (“take the lab to the field”) Gierens (2003): “critical” concentration of heterogeneous IN triggering a switch of predominant mechanism from homogeneous freezing to heterogeneous nucleation, as a function of T and updraft speed Synoptic lifting and Subvisual cirrus Smaller scale wave forcing and anvil cirrus w DeMott et al. 2003, PNAS Homogeneous freezing Heterogeneous nucleation

16. June 7, 2004 Homogeneous freezing on natural aerosol particles compared to laboratory surrogates Homogeneous freezing of pure sulfates from Chen et al. (2000) or Koop et al. (2000) NASA-SUCCESS RH i inside/outside cirrus, |w|<|1m/s (Jensen et al., JGR, 2001) water saturation Ice saturation

17. June 7, 2004 What is the dominant composition of heterogeneous ice nuclei? Statistics of PALMS cluster analyses of particle types 20% industrial 80% mineral dust (1/4 with any detectable S)

18. June 7, 2004 Laboratory studies of ice formation by mineral dust type particles (Archuleta et al. 2004) Fe 2 O 3 Fe 2 O 3 + H 2 SO 4 H 2 SO 4 “shell” freezes Pure H 2 SO 4 homogeneously freezes

19. June 7, 2004 Can heterogeneous freezing be parameterized using concepts applied to homogeneous freezing? – Seems so. Coating freezing homogeneously

20. June 7, 2004 Ice nucleation size effects versus classical theory. Active site theory may do better. Fe 2 O 3 coated with H 2 SO 4

21. June 7, 2004 Resuspending actual dust samples (Asian dust – Archuleta et al. 2004) 200 nm Ca, Si, S, Mg Si, Al, Fe Homogeneous freezing points of sulfuric acid aerosols Heterogeneous nucleation by dust

22. June 7, 2004 Natural dust samples (nucleation mechanism unknown) CFDC (K. Koehler) and AIDA (Mohler) studies of one test dust agree on sense of size effects Hygroscopic dusts (OL) are less effective in CFDC (insoluble size?) Unusual (?) uniformity of Arizona and Asian sample

23. June 7, 2004 Combustion soot as an ice nucleus (AIDA studies). Contrast with some other studies suggest morphology, surface properties, chemistry are important.

24. June 7, 2004 Two expansions at identical pumping speed and temperature profiles 16% OC content: Many ice particles 40% OC content: Less ice particles

25. June 7, 2004 AIDA Studies Summary (Möhler and colleagues) AerosolMixed CloudCirrus Spark generator sootImmersion freezingDeposition freezing S IN ≈ 1.1 to 1.3 SA coated sootImmersion freezingS IN ≈ 1.4 to 1.6 Flame soot (-60 °C)Increasing OC content suppresses IN Arizona Test Dust (ATD)Deposition freezing, S IN ≈ 1.0 to 1.2. Highest IN temperature: -15°C. Large fraction of activated mineral particles. Saharan Dust (SD2 Asian Dust (AD1) Liquid activation and homogeneous freezing around -35°C. Very few deposition nuclei. Immersion freezing at higher T (up to -5°C). Deposition nucleation starts at S IN ≈ 1.05 to 1.15. Number of particles activated at low RHi increases with decreasing T. Two IN modes at intermediate T (-50°C)

26. Lab studies of processed natural ice nuclei suggest need for parameterizations based on aerosol properties rather than generalization of concentrations Meyers et al. INSPECT (>-35C) INSPECT (<-38C)

27. June 7, 2004 Some thoughts on future studies What are the fundamental ice nucleation mechanisms (e.g., Cantrell, Shaw talks tomorrow)? Investigations of missing primary or secondary mechanisms New and improved instruments needed, especially for examining the role of different ice nucleation mechanisms Need for relatively portable instruments that have utility in both the laboratory or on aircraft To what extent are we missing information with existing instrumentation due to kinetics of nucleation and influence of preactivation processes? Continued studies of IN morphology, chemistry, and attempts to tie such properties explicitly to IN activity (e.g., no overarching parameterizations that ignore aerosol properties) What are the various influences of organic and inorganic carbon compounds on ice nucleation? –Combustion byproducts, surface active types, biomass burning-related Biological ice nuclei: Do they play a significant role?