Lecture 7: The Ozone Hole
Ozone layer in the news: Nov 23, 2003
“[The US] administration is insisting on a sharp increase
in spraying of the most dangerous ozone-destroying
chemical still in use, the pesticide methyl bromide,
even though it is due to be phased out under the
Montreal Protocol in little more than a year.
…the United States could withdraw from the treaty's
provisions altogether if its demand is not met…
…
US delegates…even rejected a European Union proposal
that would have allowed farmers to use the same amount
of the pesticide as at present, even though this, itself,
would violate the spirit of the protocol.”
The Independent (London-based UK national newspaper)
Ozone hole - satellite failure to detect
Ozone hole - discovery
Antarctic Ozone Hole
1985: Ozone Hole Discovery
•Total ozone over Halley Bay
(75.5°S, 26.65°W), Antarctica, in
October (Southern Spring)
October mean, Halley Bay, 1957-2001
•Satellite view of ozone hole
•TOMS: Total Ozone Mapping
Spectrometer
•TOMS launched in 1979
• Joe Farman et al., (1985) long-term,
ground-based measurements
•Ozone hole developed over
Antarctica in the early 1980s
Not clear until 1980s that had been a
decrease since mid-1970s
•So, naturally, the TOMS satellite
discovered it, right?
WRONG! Low values were
flagged by the TOMS
computer as instrumental
error.
•later confirmed by NASA’s TOMS
(Total Ozone Mapping Spectrometer)
satellite measurements
Kump, Fig 1-5
Figure source: http://www.al.noaa.gov/WWWHD/pubdocs/Assessment02/Q&As.html
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Science and Policy
Ozone hole - state of knowledge in 1987
1987: State of Knowledge
Science
and policy:
normal
times and
unusual
times
NORMAL TIMES (theory-driven)
1974 Mario Molina and Sherwood Rowland propose catalytic
destruction of stratospheric ozone by CFCs. They predict 7%
loss over the next 50-100 years (Nature, 249, 194-196).
1978
US bans CFC use in aerosol sprays.
1979
NASA launches TOMS satellite to monitor global ozone.
UNUSUAL TIMES (event-driven)
1985 Springtime "ozone hole" discovered over Antarctica
(Farman et al., Nature, 315, 207-210).
1987
Montreal Protocol calling for strict limits on CFC emissions
is signed by 59 nations, including U.S. (Reagan administration).
1987
Cause of ozone hole is still in question. The leading theories are:
- dynamics and natural variability
- nitric oxide (NO) and sunspots
- CFCs and polar stratospheric clouds
Stratospheric "Ozone Hole": Explanation
Ozone hole
- outline of
explanatio
n
Four years (1985-1988) of frantic research
Led to scientific consensus that the CFC/PSC explanation was correct
(as given in Kump textbook, p.353-357):
Polar Stratospheric Clouds
(PSCs) in twilight
1. polar vortex (extremely cold conditions)
2. formation of polar stratospheric clouds (PSCs)
3. heterogeneous reactions (reactions on surfaces)
4. removal of NO2 (and H)
5. liberation of Cl (normally tied up via bonding with NO2 and H)
6. massive, catalytic destruction of O3
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Recall vertical distribution of chlorine species
Heterogeneous Reactions
Key to catastrophic ozone destruction
Vertical distribution of CFCs
Only the reactive forms of chlorine, Cl and ClO, are catalysts and a
danger to stratospheric ozone.
Ozone hole
- outline of
explanatio
n
unreactive chlorine
Almost all Cl, esp. in lower stratosphere, is bound up in unreactive
forms (recall earlier vertical distribution diagram) mainly
HCl (hydrogen chloride) and ClONO2 (chlorine nitrate)
Reactions on crystal surfaces of polar stratospheric clouds (PSCs)
do two things: (1) sequester NO2 and (2) liberate molecular chlorine
The second of these is:
ClONO 2 +
HCl
=
Cl2
+ HNO3
chlorine nitrate + hydrogen chloride = chlorine molecules + nitric acid
catalytic chlorine
source: Turco (1987)
Earth Under Seige
The chlorine gas molecules are photolyzed to make Cl atoms
These heterogeneous reactions were left out of the early models
that calculated modest ozone destruction from CFCs.
"Smoking gun": Correlation with ClO from aircraft
Question:
What is the ratio of
ClO to O 3 within
the ozone hole?
O3 in ppb
ClO in ppt
Prediction: Ozone destruction should correlate with regions
with high concentrations of activated chlorine (Cl and ClO)
Nowadays, we can see the same
correlation from satellite
Kump Figure 1-6
smoking gun: correlation from aircraft
smoking gun: correlation from satellite
1100 ppt ClO
1000 ppb O 3
~1 ppt
1 ppb
1 ppt
1000 ppt
= 1/1000
Not much ClO, but
it has big effect.
Figure source: http://www.al.noaa.gov/WWWHD/pubdocs/Assessment02/Q&As.html
(WMO Scientific Assessment of Ozone Depletion, 2002.)
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1998 Antarctic hole
Polar vortex: An added twist?
Stratospheric winds
from west to east
Cold at pole: air sinks
Like a whirlpool
Sinking air removes NO2
in ice particles
(which otherwise inactives ClO)
Vortex isolates air over Antarctica:
Little new O3 brought in.
Kump Fig 17-10
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International Treaties and Scientific Assessments
Int'l
Treaties
and
Scientific
Assessment
s
Global trends
Natural variability:
1) Stratosphere winds
cycle annually and
every 27 months
-affects seasonal
ozone levels
2) 11-year sunspot cycle
causes changes of 2-3
% (solar max - more
UV and more O3)
Year
1981
Policy Process
1985
Vienna Convention
1987
1988
Montreal Protocol
Scientific Assessment
The Stratosphere 1981. Theory and
Measurements . WMO No. 11.
Atmospheric Ozone 1985. Three
volumes. WMO No. 16.
Farman et al., ozone hole discovery
International Ozone Trends Panel Report
1988 . Two volumes. WMO No. 18.
Scientific Assessment of Stratospheric
Ozone: 1989 . Two volumes. WMO No. 20.
1989
Substract these out to get
global trend
1990
1991
London Adjustments
1992
1994
Copenhagen Adjustments
1995
1997
1998
Vienna Adjustment
Montreal Adjustment
2002
Scientific Assessment of Ozone Depletion:
1991 . WMO No. 25.
Scientific Assessment of Ozone Depletion:
1994 . WMO No. 37.
Scientific Assessment of Ozone Depletion:
1998 . WMO No. 44
Scientific Assessment of Ozone Depletion:
2002 .
Figure source: WMO, 2002 assessment
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