Understanding Ice Nuclei Dynamics Over Florida: Two Case Studies

1. Guy Cascella, in association with MPO531, presents: Ice nuclei characteristics over Florida; two case studies Featuring: African dust aerosols as atmospheric nuclei, DeMott et al, 2003 Chemical characteristics of ice residual nuclei in anvil cirrus clouds: evidence for homogeneous and heterogeneous ice formation, Twohy and Poellet, 2005

2. African dust aerosols as atmospheric ice nuclei • experiment designed to test the importance of African dust in cloud properties over Florida – July 2002 • part of the NASA CRYSTAL-FACE program • three main motivations: • understanding cold cloud dynamics: ice phase transitions • dust is a fairly common atmospheric substance • anthropogenic activity: land use change

3. HYSPLIT trajectory

4. Continuous Flow Diffusion Chamber (CFDC)

5. But not all aerosol is due to dust • CFDC collects all type of aerosol… how are dust aerosol identified? • Answer: LIDAR • LIDAR identifies the geometry of the aerosol • calculates the linear depolarization ratio, d, which is the ratio between the alignment of the planes both parallel and perpendicular to the laser • thus, for spherical aerosol, d = 0 • dust aerosol are non-spherical, so we expect d > 0

6. Up close and personal Koren et al, 2001

7. Results – “dust case” of 28 July

8. Results – “no dust case” of 18 July

9. Results – comparison

10. Conclusions • African dust significantly impacts IN concentrations in cirrus anvils over Florida • an increase to >1 cm-3 from a mean of .01 cm-3 • favors heterogeneous freezing • Effects in cloud microphysical and radiative properties, latent heating, and precipitation • Atmospheric dust levels should be monitored • desert dust, anthropogenic factors, etc.

11. Chemical characteristics of ice residual nuclei in anvil cirrus clouds: evidence for homogeneous and heterogeneous ice formation • Previous experiment emphasized geometry, but could not discern chemical composition • Also part of the NASA CRYSTAL-FACE program • Main motivations: • analyze the chemical composition of varying sizes of ice nuclei both in anvils and in immediate surrounding (ambient) air • determine how homogeneous and heterogeneous ice formation contribute

12. Counterflow Virtual Impactor (CVI) • samples particles >5mm, smaller particles deflected by counterflow • particles removed of water • two impacting stages (“large” and “small”) • ~.56mm and ~.38mm • samples sized and analyzed for chemical composition with electron microscope

13. Collection method

14. Collection method 45° wedge

15. Collection method 45° wedge multiply by 8 for total particle count

16. Collection method 45° wedge multiply by 8 for total particle count total particles: O(100)/Liter

18. 1/3 of particles composed of salts… which are soluble…

19. 1/3 of particles composed of salts… which are soluble… • But so are sulfates; ~40% of observed particles are soluble… highly favors homogeneous freezing!

20. 1/3 of particles composed of salts… which are soluble… • But so are sulfates; ~40% of observed particles are soluble… highly favors homogeneous freezing! • Industrial substances; ~15% of total particles (anthropogenic forcing)

21. 1/3 of particles composed of salts… which are soluble… • But so are sulfates; ~40% of observed particles are soluble… highly favors homogeneous freezing! • Industrial substances; ~15% of total particles (anthropogenic forcing) • ~15% of particles have crustal origin (e.g. African dust as we saw earlier)

22. 1/3 of particles composed of salts… which are soluble… • But so are sulfates; ~40% of observed particles are soluble… highly favors homogeneous freezing! • Industrial substances; ~15% of total particles (anthropogenic forcing) • ~15% of particles have crustal origin (e.g. African dust as we saw earlier) • ~25% carbon based particles, function is not clear and is in need of further research ? ? ? ?

23. 1/3 of particles composed of salts… which are soluble… • But so are sulfates; ~40% of observed particles are soluble… highly favors homogeneous freezing! • Industrial substances; ~15% of total particles (anthropogenic forcing) • ~15% of particles have crustal origin (e.g. African dust as we saw earlier) • ~25% carbon based particles, function is not clear and is in need of further research • ~5% other/unknown/mixed particles, most likely Carbon based ? ? ? ?

24. Temperature dependence matters • high variability at different altitudes despite convincing average results shown in previous slides • homogeneous processes dominate at the coldest temperatures • we expect this with soluble particles • as temperatures rise, heterogeneous processes take over; also expected • at the warmest temperatures, virtually no ice processes occur, so this can be excluded from the perspective of this study

25. Driving home a point…

26. Results and Conclusions • salts and sulfates (soluble) dominated the findings • anthropogenic activity again plays a large role • industrial based aerosol • land use leading to increases in atmospheric dust • carbon based aerosol and their role in ice nucleation processes require further study

27. Results and Conclusions • convective processes are important for distributing various aerosol throughout the atmosphere • here, the distributions of salts was of particular value and importance • homogeneous freezing processes are much more important than theory leads us to believe! • but is this only for coastal areas?

28. Thanks for listening.