LECTURE 15
AOSC 434
AIR POLLUTION
RUSSELL R. DICKERSON
2014
STRATOSPHERIC POLLUTION
Without ozone in the atmosphere there could be no life as we know it on
the surface of the Earth. All of the atmospheric ozone, that is the “ozone
column” is only about 0.3 atm cm. In other words, if all the air were
squeezed out of the atmosphere, and the remaining ozone were brought to
STP, it would be only 0.3 cm thick.
– Murphy’s Law is strictly obeyed by NOx pollution in the atmosphere.
– Chemistry of the stratosphere different from troposphere.
Table 15.1 Solar intensity at the Earth’s surface assuming 0.30 atm cm
(300 D.U.) ozone. Note that the maximum flux is about 7x10¹⁵
(photons/(cm²s)/10 nm).
Layers in the atmosphere
Copyright © 2013 R. R. Dickerson
& Z.Q. Li
3
https://www.youtube.com/watch?v=fVsONlc3OUY
Copyright © 2014 R. R. Dickerson
5
λ
(nm)
σ
(atm⁻¹cm⁻¹)
I/Io
250
305
1.0x10⁻⁴⁰
275
162
1.0x10⁻²¹
300
9.5
6.0x10⁻²
325
0.27
9.2x10⁻¹
Where O₃ stops absorbing, sunlight begins to reach the surface of the
Earth. Hartley (1880) measured the ozone spectrum. Fabry and Buisson
(1913) measured the solar spectrum at the Earth’s surface and concluded
that the UV radiation reaching the surface of the Earth must be controlled
by ozone in the upper atmosphere, they even made an accurate estimate of
the amount of ozone!
Today we will examine the various catalytic cycles that control the
level of ozone in the stratosphere. We will calculate the O₃ abundance for
a highly simplified atmosphere containing only O₂ and N₂.
Why do we care about the UVB dosage?
Cholesterol photolysis to Vitamin D
hn
→
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& Z.Q. Li
8
Folic acid (vitamin B-9)
Copyright © 2013 R. R. Dickerson
& Z.Q. Li
9
If you have a weak stomach
Go get a cup of coffee for the next 3 min.
Too little UV
radiation means
rickets;
UV converts
cholesterol to
Vitamin D.
UVC - 100 to 290
nm
UVB - 290 to 320
nm
UVA - 320 to 400
nm
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Dickerson & Z.Q. Li
11
Too much UV
radiation causes
skin cancer and
photodissociates
folate, also called
Vitamin B9.
Deficiency
causes anemia
and birth defects.
Copyright © 2013 R. R.
Dickerson & Z.Q. Li
12
VII. A) OZONE CATALYTIC CYCLES
1)
Chapman Reactions (1931)
O₂ + hn → 2O
O + O₂ + M → O₃ + M†
O₃ + hn → O₂ + O
O + O₃ → 2O₂
(1)
(2)
(3)
(4)
By way of qualitative analysis, Reactions (1) plus (2) produce ozone.
O₂ + hn → 2O
(1)
2 x ( O + O₂ + M → O₃ + M )
(2)
3 O₂ + hn → 2 O₃ NET
While Reactions (3) plus (4) destroy ozone.
O₃ + hn → O₂ + O
(3)
O + O₃ → 2O₂
2O₃ + hn → 3 O₂
(4)
NET
Reactions (3) plus (2) add up to a null cycle, but they are responsible for
converting solar UV radiation into transnational kinetic energy and
thus heat. This cycle causes the temperature in the stratosphere to
increase with altitude. Thus is the stratosphere stratified.
O₃ + hn → O₂ + O
(3)
O + O₂ + M → O₃ + M*
(2)
NULL
NET
By way of quantitative analysis, we want [O₃]ss and [O]ss and [Ox]ss
where “Ox” is defined as odd oxygen or O + O₃. The rate equations
are as follows.
d [O3 ] / dt  R2  R3  R4
d [O] / dt  2 R1  R2  R3  R4
d [O3  O] / dt  d [Ox ] / dt  2 R1  2 R4
(a)
(b)
(a+b)
From the representation for O atom chemistry:
[O]SS 
j (O3 )[O3 ]  2 j (O2 )[O2 ]
k 2 [O2 ][ M ]  k 4 [O3 ]
In the middle of the stratosphere, however, R₃ >>2 R₁ and R₂ >> R₄ thus:
[O]SS 
j (O3 )[O3 ]
k 2 [O2 ][ M ]
(I)
This does not mean that R₄ is unimportant, but it can be ignored in an
approximation of [O]ss at the altitude of the ozone layer.
The ratio of [O] to [O₃] can also be useful:
[O]SS
j (O3 )

[O3 ]SS k 2 [O2 ][ M ]
(II)
Reactions 2 and 3 set the ratio of O to O₃, while Reactions 1 and 4 set the
absolute concentrations. Now we will derive the steady state ozone
concentration fro the stratosphere. From the assumption that Ox is in
ready state we know:
R₁ = R₄
Thus
j(O₂)[O₂] = k₄[O][O₃]
Substituting from (I), the steady state O atom concentration:
k 4 j (O3 )[O3 ]2
j (O2 )[O2 ] 
k 2 [O2 ][ M ]
or
[O3 ]SS 
j (O2 )[O2 ]2 k 2 [ M ]
k 4 j (O3 )
SAMPLE CALCULATION
At 30 km
j (O2 )  6 10 11 s 1
j (O3 )  1 10 3 s 1
k 2  4.5 10 34 cm 6 s 1
k 4  1 10 15 cm 3 s 1
[O ]  30 ppm
3 SS
This is almost a factor of ten above the true concentration! What is
wrong? There must be ozone sinks missing.
2) Bates and Nicolet (1950) “HOx”
Odd hydrogen “HOx” is the sum of OH and HO₂ (sometimes H and H₂O₂
are included as well).
HO₂ + O₃ → OH + 2O₂
OH + O₃ → HO₂ + O₂
2O₃ → 3O₂
The following catalytic also destroys ozone.
OH + O₃ → HO₂ + O₂
HO₂ + O → OH + O₂
O + O₃ → 2O₂
(5)
(6)
NET
(6)
(7)
NET
The second catalytic cycle speeds up Reaction 4, that is it effectively
increases k₄. Note that any loss of odd oxygen is the same as loss of
ozone. These catalytic losses are still insufficient to explain the
observed ozone concentration.
3) Crutzen (1970); Johnston (1971) “NOx”
Odd nitrogen or “NOx” is the sum of NO and NO₂. Often “NOx” is used
as “odd nitrogen” which includes NO₃, HNO₃, 2N₂O₅, HONO, PAN
and other species. This total of “odd nitrogen” is better called “NOy” or
“total reactive nitrogen.” N₂ and N₂O are unreactive.
NO + O₃ → NO₂ + O₂
O + NO₂ → NO + O₂
O + O₃ → 2O₂
NET
This is the major means of destruction of stratospheric ozone. The NOx
cycle accounts for about 70% of the ozone loss at 30 km. We will
calculate the implied steady ozone concentration later.
4) Stolarski & Cicerone (1974) “ClOx”
Cl + O₃ → ClO + O₂
ClO + O → Cl + O₂
O + O₃ → 2O₂
NET
This reaction scheme is very fast, but there is not much ClOx in the
stratosphere … yet. Today ClOx accounts for about 8% of the ozone
loss at 30 km. If all these catalytic destruction cycles are added
together, they are still insufficient to explain the present stratosphere
O₃ level.
The general for of a catalytic ozone destruction cycle is:
X + O₃ → XO + O₂
XO + O → X + O₂
O + O₃ → 2O₂
NET
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
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
NET
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Table 15.2 Stratospheric ozone destruction cycles
Cycle
Sources
Sinks
Reservoirs
HOx
H₂O,CH₄,H₂
HNO₃ · nH₂O H₂O,H₂O₂
H₂SO₄ ·
nH₂O
NOx
N₂O + O(¹D)
HNO₃
HO₂NO₂,ClO
NO₂
ClOx
CH₃Cl,CFC
HCl
HCl, HOCl
The sinks involve downward transport to the troposphere and rainout or
other local loss. Note that some sinks are also reservoirs:
HCl + OH → H₂O + Cl
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 current review Is provided by
Solomon Nature, 1990, and “Scientific Assessment of Ozone
Depeation :1991” (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 “denitrify” the
stratosphere (Molina et al., Science, 1987).
ice
HCl  ClONO2 
Cl2 (gas)  HNO3 (aq.) *
Cl2  hν  2Cl
Molecular chlorine is only weakly bound, and can be dissociated by
visible radiation.
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 Ice formed at T ≤ 190K
• They move NOy species from the vapor phase to the condensed phase
as HNO₃.
• The are involved in catalytic cycles with chlorine and bromine
compounds that speed the reaction of ozone with itself to form oxygen.
• They move chlorine from the reservoir species HCl and ClONO₂ to
ClOx.
October 24, 2009
From NASA
http://ozonewatch.gsfc
.nasa.gov/index.html
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
25
Airborne Antarctic Ozone Expedition:
Punta Arenas, Chile,1987
Anderson et al., Science, 1991
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Dickerson & Z.Q. Li
26
Copyright © 2010Clouds
R. R. (PSCs)
Polar Stratospheric
Dickerson & Z.Q. Li
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Copyright © 2010 R. R.
Dickerson & Z.Q. Li
28
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Dickerson & Z.Q. Li
29
World Production of CFCs
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Dickerson & Z.Q. Li
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Copyright © 2010 R. R.
Dickerson & Z.Q. Li
31
From Farman et al., Nature 1985.
Copyright © 2010 R. R.
Dickerson & Z.Q. Li
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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
33
OZONE PROFILES, SOUTH POLE:
UPDATE
35
Ozone Hole Update, II
30
ALTITUDE (km)
25 SEP 29, 1999
20
 90 DU
15
10
5
0
0
OCTOBER
AVERAGE
1967 - 1971
282 DU
D. Hofmann,
NOAA CMDL
5
10
OZONE ABUNDANCE
Copyright
© 2010
R. R. mPa)
(PARTIAL
PRESSURE,
Dickerson & Z.Q. Li
15
34
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
35
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
36
Ozone hole 2013
Copyright © 2013 R. R. Dickerson
http://www.esrl.noaa.gov/gmd/odgi/
37
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
38
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
39