AOSC 620
The Ozone Hole
R. Dickerson
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
1
Recap:
The general for of a catalytic ozone destruction cycle is:
X + O₃ → XO + O₂
XO + O → X + O₂
O + O₃ → 2O₂
NET
Where X is OH, NO, Cl, or Br.
But nobody saw the Ozone Hole coming!
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
2
October 24, 2009
From NASA
http://ozonewatch.gsfc
.nasa.gov/index.html
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
3
Antarctic Ozone Hole
In the Antarctic winter there is no sunlight and even in the spring there is
too little UV to generate enough O atoms to destroy ozone. The
annual loss of ozone over Antarctica is driven by heterogeneous
chemistry and visible radiation. A good review is provided by
Solomon Rev. Geophys., 1999, and “Scientific Assessment of Ozone
Depletion: 2006” (WMO). The destruction of ozone is usually
moderated by the production of chlorine nitrate, an important reservoir
species.
NO₂ + ClO + M → ClONO₂ + M†
In the Antarctic winter, heterogeneous reactions liberate chlorine from
long-lived HCl and ClONO2 and sedimentation of polar stratospheric
clouds “denitrifies” the stratosphere (Solomon et al., Nature, 1986;
McElroy et al., Nature, 1986; Toon et al., GRL, 1986).
ice Cl (gas) + HNO (aqueous)
HCl + ClONO2 
2
3
Cl2 + h  2Cl
HNO3(aqueous) sediments (falls) out of stratosphere,
removing a potential source of NOx.
Copyright © 2012 R. R. Dickerson & Z.Q. Li
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Molina and Molina (1987)
2(Cl + O₃ → O₂ + ClO)
ClO + ClO + M → (ClO)₂ + M
(ClO)₂ + hv → Cl + ClOO
ClOO + M → Cl + O₂ + M
2O₃ → 3O₂
NET
Two types of Polar Stratospheric Clouds (PSC’s) exist.
Type I = HNO₃ ● 3H₂O Nitric acid trihydrate, formed at T ≤ 195K
Type II = H₂O Water ice formed at T ≤ 190K
• They move NOy species from the vapor phase to the condensed phase
as HNO₃.
• They move chlorine from the reservoir species HCl and ClONO₂ to
ClOx.
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
5
Molina and Molina (1987)
2(Cl + O₃ → O₂ + ClO)
ClO + ClO + M → (ClO)₂ + M
(ClO)₂ + hv → Cl + ClOO
ClOO + M → Cl + O₂ + M
2O₃ → 3O₂
NET
McElroy, Salawitch, et al. (1986)
Cl + O₃ → ClO + O₂
Br + O₃ → BrO + O₂
ClO + BrO → Cl + Br + O₂
2O₃ → 3O₂
NET
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
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Airborne Antarctic Ozone Expedition:
Punta Arenas, Chile,1987
Anderson et al., Science, 1991
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
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THE ANTARCTIC OZONE HOLE
Southern Hemisphere ozone column seen from TOMS, October
DU
1 Dobson Unit (DU) = 0.01 mm O3 STP = 2.69x1016 molecules cm-2
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
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Copyright © 2010Clouds
R. R. (PSCs)
Polar Stratospheric
Dickerson & Z.Q. Li
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Copyright © 2010 R. R.
Dickerson & Z.Q. Li
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Copyright © 2010 R. R.
Dickerson & Z.Q. Li
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World Production of CFCs
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
12
Multiphase Processes
Gas-Surface Reactions
Key to ozone hole formation are reactions of gaseous species with
PSC’s. Multiphase reactions are important for tropospheric
chemistry as well – the formation of HONO appears to take place
predominantly on organic surfaces.
The probability of a molecule A striking a unit area is
Z = ¼ M[A] A
Where M is the molecular number density and [A] is the mixing ratio
of A, and A is the rms velocity of molecules A (see Kinetics Lecture).
The number of collisions with a single spherical particle of radius r is
proportional to the area of the particle, 4pr2. For an ensemble of
particles with total area Stot (units area of particles per volume of air
or cm2 cm-3) the total collision rate is
R = ¼ Stot M[A] A
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
13
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
14
From Farman et al., Nature 1985.
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
15
Antarctic Ozone Loss: Hole cannot get wider or deeper.
Ground Based
TOMS
OMI
After Farman et al., Nature, 315, 207, 1985
• Models now provide good overall simulation of Antarctic ozone loss.
• Scientific understanding of polar ozone depletion led to international ban of CFC production
Copyright © 2010 University of Maryland.
This material may not be reproduced or redistributed, in whole or in part, without written permission from Ross Salawitch or Tim Canty.
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OZONE PROFILES, SOUTH POLE:
UPDATE
35
Ozone Hole Update, II
ALTITUDE (km)
30
25
SEP 29, 1999
 90 DU
20
15
OCTOBER
AVERAGE
1967 - 1971
282 DU
10
5
0
D. Hofmann,
NOAA CMDL
0
5
10
OZONE ABUNDANCE
(PARTIAL
PRESSURE,
Copyright
© 2010
R. R. mPa)
Dickerson & Z.Q. Li
15
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Accommodation Coefficients
• Condensed phase has lower entropy than
gas phase.
• Accommodation coefficients (reaction
probabilities) should be greater at lower
temperatures.
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
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Heterogeneous Chemistry: Faster at low temperatures
In all cases,  must be measured in the laboratory (thanks, RJS 2010)
Reaction probabilities given for various surface types, with formulations of various
degrees of complexity, in Section 5 of the JPL Data Evaluation.
Atmospheric Chemistry and Physics by Seinfeld and Pandis provides extensive treatment
of aqueous phase chemistry, properties of atmospheric aerosol, organic aerosols, etc.
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
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Ozone hole 2013
Copyright © 2013 R. R. Dickerson
http://www.esrl.noaa.gov/gmd/odgi/
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Summary of Ozone Hole Formation
•
•
•
•
•
•
•
Threat to DNA-based life forms.
Not predicted by any models
First observed by Farman et al., (Nature 1985).
Ozone destruction nearly complete.
Halogen (Cl & Br) reactions are responsible.
Polar stratospheric Clouds play a central role.
Multiphase (heterogeneous) reactions denitrify
stratosphere.
• Reaction rates depend on accommodation
coefficients, f(T).
• Replacement of CFC’s should heal ozone hole.
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
21
Summary of Ozone Hole Formation
•
•
•
•
•
•
•
Threat to DNA-based life forms.
Not predicted by any models
First observed by Farman et al., (Nature 1985).
Ozone destruction nearly complete.
Halogen (Cl & Br) reactions are responsible.
Polar stratospheric Clouds play a central role.
Multiphase (heterogeneous) reactions denitrify
stratosphere.
• Reaction rates depend on accommodation
coefficients, f(T).
• Replacement of CFC’s should heal ozone hole.
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
22