Laboratory Studies of Ice Initiation by Atmospheric Aerosol Particles

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

2. Overview • Talk will concern itself onlywith 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. Science according to one “lab” person theory Lab studies Numerical modeling Field Studies

4. Ice nucleation mechanisms

5. Some examples of ice nucleation studies instrumentation Drop freezing devices Aerosol flow tubes (AFTIR)

6. More instrumentation Electrodynamic balance Diffusion chamber (filter processor)

7. Cloud Chamber (AIDA in this case) • 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 Moderate expansion cloud chamber: T Range: 0 to –90 °C P Range: 0.01 to 1000 hPa Ice saturation by ice coated walls

8. 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. 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. Homogeneous freezing of solution drops: Dependence on water activity and freezing point depression - composition irrelevant? Freezing temperatures of solute emulsion drops collapse onto constant water activity difference between solution and ice (Koop et al. 2000) • 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 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. Water activity relation works for many substances, but…ammonium sulfate is a “bugger” Need data as nucleation rate!

13. Another issue: Impacts on homogeneous freezing associated with presence of organics • 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

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

15. Homogeneous and heterogeneous nucleation at low temperatures on ambient tropospheric aerosol particles and suggested impacts on cirrus (“take the lab to the field”) DeMott et al. 2003, PNAS Homogeneous freezing Smaller scale wave forcing and anvil cirrus w Heterogeneous nucleation Synoptic lifting and Subvisual cirrus 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

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

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

18. Laboratory studies of ice formation by mineral dust type particles (Archuleta et al. 2004) Fe2O3 Fe2O3 + H2SO4 Pure H2SO4 homogeneously freezes H2SO4“shell” freezes

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

20. Ice nucleation size effects versus classical theory. Active site theory may do better. Fe2O3coated with H2SO4

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

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

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

25. AIDA Studies Summary (Möhler and colleagues)

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 (<-38C) INSPECT (>-35C)

27. 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?