Polar Stratospheric Clouds -Properties and Climate Impacts

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Polar Stratospheric Clouds
-Properties and Climate Impacts
Haibin Li
Outline


Why these clouds?
Properties of PSCs
Definition; Occurrence; Type; Characteristics

Measurements of PSCs
Methods, example

Climate Impacts
Ozone depletion with PSCs; mechanism
Why??
 Relationship
 Understand
with ozone depletion
the past climate (Sloan et al.,
1998)
 They
are beautiful… “mother of pearl”
http://www.sundog.clara.co.uk/atoptics/nacr1.htm
http://alphais.inep.ksc.ru/knet_in/sky.htm
http://www.nilu.no/projects/theseo2000/images/general/psc8.jpg
Properties of PSCs
 Definition:
High altitude clouds that form in the stratosphere above Antarctica
during the Southern Hemisphere winter. Their presence seems to
initiate the ozone loss experienced during the ensuing Southern
Hemisphere spring. (http://earthobservatory.nasa.gov/Library/glossary.php3)
Large, diffuse, ice-particle clouds that form in the stratosphere usually
over polar regions.(http://www.grida.no/climate/ipcc/aviation/158.htm)

Formation:
During the polar night (i.e. winter) a strong circumpolar wind develops
in the middle to lower stratosphere. This creates a vortex, which
isolates the air above the poles. Due to a lack of sunlight the air inside
the vortex cools and once the temperature falls below about 195K the
probability of PSC formation increases very rapidly.
Temperature decided Type
PSCs Type and physical properties
Type
Temperature
Particle Size
Type Ia
< 195 K
~ 1 μm
Type Ib
Type II
Composition
characterized by NAT (HNO3.3H2O) crystals
(depolarizing solid)
nitric acid/water/sulphuric acid Supercooled
ternary solution (STS)(nondepolarizing
liquid)
~ 188 K
~ 10 μm
Small Ice Crystals
( Crutzen, 1986; Hofmann, 1989;Voigt et al., 2000;Toon et al., 1995)
Measurements of PSCs
Measuring Methods:



ground-based and aircraft LIDAR systems (Stein et al., 1999)
in-situ balloons-borne and aircraft instruments measuring particle
sizes and densities through sampling and backscatter laser devices
( Voigt et al., 2000; Kivi et al., 2001)
Satellite observations of absorption, emission and scattering of light
by PSC particles (Foschi et al., 2002)
Interested in:
Particle size; surface area; volume; frequency of PSC events;
different PSC types; the relation between the PSC occurrence and
the environmental atmospheric conditions.
Example of PSC Measurements
X-axis :the backscatter ratio R
(red line), the
depolarization*20 (thin line),
and the color ratio from two
wavelength measurements.
A: Early stage: few highly
aspherical particles;
B: mid winter: strong PSC
layer of spherical particles;
C: late winter: decaying PSC
signals;
http://www.awi-potsdam.de/www-pot/atmo/psc/psc.html#Lidar_Measurements_of_PSCs
Climate Impacts:Ozone depletion and PSCs
 Phenomenon
 Mechanisms


PSCs act as catalysts converting chlorine from the inert "reservoir"
species to active species by heterogeneous chemical reactions on
their surface.
Details…
HNO3·3H2O
HCl ClONO2
HCl ClONO2
Cl-Cl
Vaporize
Cl-Cl HNO3
HNO3
ClONO2+HCl
ICE
→ Cl2+HNO3
Hv ≤450 nm
Cl-Cl
O3 + Cl → ClO + O2
O3+hv(≤ 1140nm)→O2+O
ClO + O → Cl + O2
Cl* *Cl
Net: 2O3→3O2
ClONO2
ClH
Future Stuff ???
Chlorine activation chemistry is sensitive to the chemical position and the sizes of
PSC particles but PSC properties have never been adequately quantified, either
observationally or theoretically.
So far, size distributions are among the best known PSC properties(light scattering
or parameterized lidar measurements).
PSC chemical compositions are still very poor known --- the best guesses are
based on equilibrium thermodynamic models ( Carslaw, 1995).
How to understand them more realistically?
More advanced&consistent satellite observation + improved models simulation.
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