CHAPTER 10: STRATOSPHERIC CHEMISTRY

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• CHAPTER 10 STRATOSPHERIC CHEMISTRY

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THE MANY FACES OF ATMOSPHERIC OZONE
In stratosphere UV shield
Stratosphere 90 of total
In middle/upper troposphere greenhouse gas
Troposphere
In lower/middle troposphere precursor of OH,
main atmospheric oxidant
In surface air toxic to humans and vegetation
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STRATOSPHERIC OZONE HAS BEEN MEASURED FROM SPACE
SINCE 1979
Method UV solar backscatter
Last Saturdayss ozone layer Notice the
Antarctic ozone hole
l2
l1
Ozone layer
Scattering by Earth surface and atmosphere
Ozone absorption spectrum
l1
l2
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CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE (1930)
Odd oxygen family Ox O3 O
slow
R2
R1
O2
O O3
fast
R3
R4
slow
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STEADY-STATE ANALYSIS OF CHAPMAN MECHANISM
Lifetime of O atoms
is sufficiently short to assume steady state for
O
so the budget of O3 is controlled by the budget
of Ox.
Lifetime of Ox
?Ox
Steady state for Ox
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SOLAR SPECTRUM AND ABSORPTION X-SECTIONS
O3hv
O2hv
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PHOTOLYSIS RATE CONSTANTS VERTICAL DEPENDENCE
quantum yield
absorption X-section
photon flux
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CHAPMAN MECHANISM vs. OBSERVATION
-3
Chapman mechanism reproduces shape, but is too
high by factor 2-3 e missing sink!
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Chapman got it almost rightCATALYTIC CYCLES
FOR OZONE LOSS General Idea
O3 X ? XO O2 O XO ? X O2 Net O3
O ? 2 O2 X is a catalyst
The catalyst is neither created nor destroyedbut
the rate for the catalytic cycle odd-O removal
in this case depends on catalyst concentrations
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WATER VAPOR IN STRATOSPHERE
Source transport from troposphere, oxidation of
methane (CH4)
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HOx-CATALYZED OZONE LOSS
HOx ? H OH HO2 hydrogen oxide radical family
Initiation
Propagation
Termination
slow
OH HO2
H2O
fast
HOx radical family
slow
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NITROUS OXIDE IN THE STRATOSPHERE
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NOx-CATALYZED OZONE LOSS (NOx ? NO NO2)
Also emitted

• Initiation N2O O(1D) ?2NO

Propagation NO O3 ? NO2 O2
NO O3 ? NO2 O2 NO2 h? ? NO O
NO2 O ? NO O2 O O2 M ? O3 M
Null cycle Net O3 O ? 2O2

O3 loss rate
Termination
Recycling NO2 OH M ? HNO3 M HNO3
h? ? NO2 OH NO2 O3 ? NO3 O2
HNO3 OH ?NO3 H2O NO3 NO2 M ?
N2O5 M NO3 h? ? NO2 O N2O5 H2O ?
2HNO3 N2O5 h? ?NO2
NO3
Day
Night
NOy ? NOx reservoirs (HNO3, N2O5, ..)
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ATMOSPHERIC CYCLING OF NOx AND NOy
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STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES
CONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONS
Approximate closure!
Source of Ox
Gas-phase chemistry only
Paul Crutzen shared 1995 Nobel Prize for his work
on the NOx catalyzed destruction of ozone
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STRATOSPHERIC DISTRIBUTION OF CFC-12
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ClOx-CATALYZED OZONE LOSS (ClOx ? Cl ClO)

• Initiation Cl radical generation from
  non-radical precursors (e.g., CFC-12)
• CF2Cl2 hn ? CF2Cl Cl
• Propagation
• Cl O3 ? ClO O2
• ClO O ? Cl O2
• Net O3 O ? 2O2

O3 loss rate
Termination Recycling Cl CH4 ? HCl
CH3 HCl OH ? Cl H2O ClO NO2 M ? ClNO3
M ClNO3 hv ? Cl NO3
Cly ? ClOx reservoirs (HCl, ClNO3)
http//www.atmos.washington.edu/2004Q4/211/09_Ozon
eDep.swf
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ATMOSPHERIC CYCLING OF ClOx AND Cly
Molina and Rowland shared 1995 Nobel Prize for
their work on the ClOx catalyzed destruction of
ozone