Environmental Transport and Fate
Chapter 10
–
Ozone Holes
Benoit Cushman-Roisin
Thayer School of Engineering
Dartmouth College
Recall the vertical structure of the atmosphere
We are now
concerned
about this
l
layer
iin th
the
middle of the
stratosphere
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Temperature increases in this layer of the atmosphere because of absorption
of solar radiation by oxygen and ozone.
Photochemical reactions are:
O2 + (UV radiation < 242 nm) → O● + O●
O● + O2 + M → O3 + M●
(M stands for any molecule nearby)
O3 + (UV-C or UV-B radiation) → O2 + O●
O2 absorption
 < 242 nm
O3 absorption
200 nm <  < 320 nm
2
Cartoon from US EPA
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Example of what is
happening on the
seasonal scale
of ozone
Where was the stratospheric ozone by the end of the winter?
And, it did get worse for a number of years…
1 DU
= 1 Dobson Unit
= thickness of O3
brought to 1 atm @ 0oC
(in meters)
x 105
Historical springtime vertically integrated ozone over Halley Bay, Antarctica (76oS)
(Source: UNEP, 1994)
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http:///www.cpc.noaa.gov/products/strato
osphere/winter_bulletins/sh_07/
Historical data showing
no minimum in spring
Pronounced minimum
in springs from mid1980s to present.
Hole still
occurring in
2010 and 2011
The Antarctic Ozone Hole
The Antarctic Ozone Hole was discovered by the British Antarctic Survey from data
obtained with a ground-based instrument at a measuring station located in Halley Bay,
Antarctica, in the 1981-1983 period.
A first report of October ozone loss was issued in 1985. Satellite measurements then
confirmed
co
ed tthat
at tthe
e sp
springtime
gt e o
ozone
o e loss
oss was
as a co
continent-wide
t e t de feature.
eatu e
Research conducted during the National Ozone Expeditions to the U.S. McMurdo Station
in 1986 and 1987, and NASA stratospheric aircraft flights into the Antarctic region from
Chile in 1987 showed conclusively that the ozone loss was related to chlorine-catalyzed
chemical destruction which takes place following spring sunrise in the Antarctic polar
region.
The chlorine is derived from man-made chloro-fluoro-carbons (CFCs) which have
migrated to the stratosphere and have been broken down by solar ultraviolet light
light,
freeing chlorine atoms.
(Source: http://www.ozonelayer.noaa.gov/science/ozhole.htm)
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Mario J. Molina and F. Sherwood Rowland shared the 1995 Nobel Prize in chemistry for
“explaining how certain man-made chemicals can rise into the atmosphere and harm the
ozone layer that shields us from the ultraviolet radiation of the sun.”
The chemical culprits
These artificial chemicals were thought to be completely inert.
And they indeed are but only under ground-level temperatures,
pressures, and solar radiation.
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Chemical reactions contributing to ozone loss in the stratosphere
1) Chlorine release from the CFCs
ex. CFC-12
CF2Cl2 + UV light → CF2Cl + Cl
CF2Cl + UV light → CF2
+ Cl
2) Catalytic ozone destruction
Cl + O3 → ClO + O2
ClO + O → Cl + O2
One ozone molecule destroyed
as well as one of its precursors,
while the chlorine atom is regenerated
3) Eventual removal of chlorine and of chlorine oxide
Cl + CH4 → HCl + CH3
ClO + NO2 → ClONO2
HCl is water soluble → precipitation
It has been estimated that a chlorine atom can destroy 1000 ozone molecules
before being destroyed in its turn.
Over the poles, there is an extended seasonal period of darkness,
during which no ultraviolet radiation is shed.
The chlorine is then converted into other products, which regenerate the chlorine when the
sun returns in the spring.
4) In the darkness of the polar nigh
(reactions occurring on the surface of icy stratospheric clouds)
ClO + NO2 → ClONO2
ClONO2 + H2O → HOCl + HNO3
HOCl + HCl → Cl2 + H2O
5) Once the sun rises in the spring
Cl2 + sunlight → 2 Cl
HOCl + sunlight → OH + Cl
Cl + O3 → ClO + O2
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These chemical reactions take place in a trapped air zone called the Polar Vortex.
The Polar Vortex is a permanent, though variable, cyclone sitting over the pole.
Play movie from
http://www.esrl.noaa.gov/gmd/dv/spo_oz/movies/index.html
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The polar vortex – always in a meandering state
except that polar high at ground level
means polar low above
polar vortex over
South Pole, too
As long as the polar vortex retains its integrity, chemicals are trapped inside the vortex,
but, when the meanders of the vortex break off, some of the chemicals are discharged
to lower latitudes where they are exposed to sunlight within hours.
http://www
w.esrl.noaa.gov/gmd/dv/traj/plots/bnd.html
Example of trajectory originating
from the polar vortex (rotating
counterclockwise) and proceeding
to lower latitudes.
BND = Observation station at Bondville, Illinois
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http://www.cpc.noaa.gov//products/stratosphere/winter_bulle
etins/nh_05-06/
We note that not all years have been equally bad. The reason lies in the timing of the
breakup of the polar vortex.
The crucial period is very early spring. At that time, sunlight emerges, and the
polar vortex becomes very unstable. It very much depends on what happens first.
● If the polar vortex breaks early, then the precursor chemicals are dispersed to lower
latitudes before they have a chance to release many chlorine atoms.
There is less damage to the ozone layer, and the hole is not very deep.
● On the contrary, should the polar vortex retain its integrity a while longer, the
precursor chemicals find time to generate many chlorine atoms, and the damage to
the ozone layer can be severe (in terms of decreased O3 concentrations, or in terms
of the area over which ozone concentration falls below a certain threshold).
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Effect at lower latitudes than the polar vortex:
Noticeable but not catastrophic
http://www.cpcc.noaa.gov/products/stratosphere/w
winter_bulletins/nh_05-06/
http://ozonewatch.g
gsfc.nasa.gov/
Largest recorded ozone hole
according to NASA:
24 September 2006
False-color view of total
ozone over the Antarctic
pole. Purple and blue colors
are where there is the least
ozone, whereas yellows and
reds are where there is more
ozone.
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The 2010 ozone hole had two remarkable features: It was one of the latest forming ozone
holes observed and it was one of the longest lasting ozone holes observed.
Ozone depletion
p
typically
yp
y begins
g
in late July
y and early
y August
g
with an observed ozone hole
size of 10 million square kilometers by mid-August. The “ozone hole” is defined as the
area in the polar latitudes where the total column ozone amounts are less than 220 Dobson
Units (DU). In 2010 the ozone hole was not observed via satellite measurements until the
last week in August. It grew to a maximum size of 20.6 million km2 on September 26,
2010.
From this point on, the South Hemispheric (SH) polar vortex and the ozone hole decreased
in size at a much slower rate than previous years. The SH polar circulation was minimally
affected by poleward wave propagation during the austral spring time, remaining zonally
symmetric.
i The
Th absence
b
off substantial
b
i l poleward
l
d heat
h
fl
flux extended
d d the
h transition
i i ffrom
winter to summer circulation patterns over the South Pole in the lower stratosphere. The
2010 ozone hole and polar vortex remained almost intact well into December.
(Source: NOAA – Climate Prediction Center)
http://www.cpc.ncep.noaa.gov/products/stratosphere/winter_bulletins/sh_10/
http://www.cpc.ncep.noaa.go
ov/products/stratosphere/winter_bu
ulletins/sh_10/
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http://www.cpc.ncep.noaa.gov/products/stratosphere/winter_
_bulletins/sh_10/
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It appears that the situation is no longer getting worse.
Could we have rounded the corner?
On the way
back up?
ODS =
Ozone Depleting
Susbtance
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Latest predictions
Recovery at mid-latitudes
circa 2049
Recovery over Antarctica
circa 2078
2078.
Closing concern
Following the phasing out of CFCs per the Montreal Protocol of 1989, chemists
have been hard at work to design new refrigerants, solvents, blowing agents and
foams for fire extinguishers.
In a first wave, they have developed so-called transitional alternatives called
Hydrochlorofluorocarbons
yd oc o o uo oca bo s ((HCFCs),
C Cs), suc
such as
CHFCl2 (di-chloro-fluoro-methane)
C2HFCl4 (tetra-chloro-fluoro-ethane),
all of which still contain some chlorine…
These HCFCs are more reactive and thus have shorter atmospheric lifetimes
than CFCs and deliver less reactive chlorine to the stratosphere.
It is expected that these chemicals will contribute much less to stratospheric
ozone depletion than the earlier CFCs. Because they still contain chlorine and
have the potential to destroy stratospheric ozone, they are viewed only as
temporary substitutes, with phase-out by 2020.
On the longer horizon, plans are to return to well tried but less physically safe
chemicals such as ammonia (NH3) and carbon dioxide (CO2, in supercritical
state).
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