1. Guy Cascella, in association with MPO531, presents: 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.  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

5.  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, , which is the ratio between the alignment of the planes both parallel and perpendicular to the laser  thus, for spherical aerosol,  = 0  dust aerosol are non-spherical, so we expect  > 0

6. Koren et al, 2001

10.  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.  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.  samples particles >5  m, smaller particles deflected by counterflow  particles removed of water  two impacting stages (“large” and “small”)  ~.56  m and ~.38  m  samples sized and analyzed for chemical composition with electron microscope

14. 45° wedge

15. multiply by 8 for total particle count

16. 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.  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.  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

26.  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.  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?