Transcript Lecture 12a, Stratospheric Ozone

AOSC 620
Ozone in the Stratosphere
R. Dickerson
Note material on layers in
general will be covered in
AOSC 621.
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Dickerson & Z.Q. Li
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Layers in the atmosphere
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Actual depth of atmospheric layers.
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Why do we care about the UVB dosage?
Cholesterol photolysis to Vitamin D
hn
→
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Folate (vitamin B-9)
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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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Too much UV
radiation causes
skin cancer and
photodissociates
folate, also called
Vitamin B9.
Deficiency
causes anemia
and birth defects.
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The Stratospheric Ozone Layer
Without a protective ozone layer there can be no protein-based life on the
surface of the Earth. Here we examine the catalytic cycles that control the
level of ozone in the stratosphere. We will calculate the O3 abundances
expected for various levels of understanding.
1) Chapman Reactions (1931)
O2 + hv
2O
O + O2 + M
O3 + hv
O + O3
(1)
O3 + M*
O2 + O
(2)
(3)
2O2
(4)
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From The Stratosphere 1981
R. D. Hudson Editor
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O/O3 = j(O3)/(k2 M O2)
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[O3 ] ss =
j(O2 )[O2 ] k2 [M ]
k4 j(O3 )
2
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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, 2 N₂O₅, 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.
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4) Stolarski & Cicerone (1974); Wofsy & McElroy (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. Molina and Rowland (1974) pointed out that
CFC’s release Cl atoms. 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
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The Stratospheric Ozone Layer
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How did Farby and Buisson do?
At 3000 Å (300 nm) 1/100 = I/Io = ln(-ecl)
e300 = 9.5 atm-1cm-1
-ln(0.01)/9.5 = 0.485 atm cm (compare to 0.50)
0.485 atm cm = 485 DU
Correct for zenith angle of 27o for Paris?
(cos 27o = 0.9 so W = 432 DU.)
[O3] = 0.6 cm3 m-3 = 6x10-7 Is that right?
[O3] for uniform mixing ratio = column content/scale height
5 mm/8x106 mm = 6.25 x 10–7 = 625 ppb
As Fabry said this is too much for surface air, and the ozone
layer lies above.
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Slides 27 on will be presented in AOSC621 in 2015.
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