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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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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CFDC collects all type of aerosol… how are
dust aerosol identified?
 Answer: LIDAR
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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
Koren et al, 2001
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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
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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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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
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
45°
wedge
45°
wedge
multiply by 8 for total
particle count
45°
wedge
multiply by 8 for total
particle count
total particles:
O(100)/Liter
1/3 of particles composed of
salts… which are soluble…
1/3 of particles composed of
salts… which are soluble…
But so are sulfates; ~40% of
observed particles are soluble…
highly favors homogeneous
freezing!
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)
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)
?
?
?
?
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
?
?
?
?
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
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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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
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carbon based aerosol and their role in ice
nucleation processes require further study
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convective processes are important for
distributing various aerosol throughout the
atmosphere
 here, the distributions of salts was of particular
value and importance
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homogeneous freezing processes are much
more important than theory leads us to
believe!
but is this only for coastal areas?