Ice nuclei characteristics over Florida; two case studies

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Ice nuclei characteristics over Florida two case
studies

• 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
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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

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HYSPLIT trajectory
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Continuous Flow Diffusion Chamber (CFDC)
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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 gt
  0

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Up close and personal
Koren et al, 2001
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Results dust case of 28 July
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Results no dust case of 18 July
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Results comparison
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Conclusions

• African dust significantly impacts IN
  concentrations in cirrus anvils over Florida
• an increase to gt1 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.

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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

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Counterflow Virtual Impactor (CVI)

• samples particles gt5mm, 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

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Collection method
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Collection method
45 wedge
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Collection method
45 wedge
multiply by 8 for total particle count
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Collection method
45 wedge
multiply by 8 for total particle count
total particles O(100)/Liter
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(No Transcript)
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• 1/3 of particles composed of salts which are
  soluble

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• 1/3 of particles composed of salts which are
  soluble
• But so are sulfates 40 of observed particles
  are soluble highly favors homogeneous freezing!

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• 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)

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• 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)

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• 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

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• 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

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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

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Driving home a point
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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

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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?

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Thanks for listening.