Transcript Chapman Mechanism (~1930, Sidney Chapman)

© 2012 W. H. Freeman & Co.
Skin cancer
© 2012 W. H. Freeman & Co.
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O2
Ozone production
O
O2
O
O2
O3
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Ozone destruction
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O2
Ozone production
O
O2
O
O2
O3
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Ozone destruction
14a–7
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O2
Ozone production
O
O2
O
O2
O3
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Ozone destruction
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O2
Ozone production
O
O2
O
O2
O3
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Ozone destruction
O2 + hn  O + O
498 kJ/mol
O3 + hn  O + O2
383 kJ/mol)
O2 + hn  O + O
498 kJ/mol <242.4 nm
O3 + hn  O + O2
383 kJ/mol <280 nm
Chapman Mechanism
(~1930, Sidney Chapman)
O2 + hn  O + O
498 kJ/mol <242.4 nm slow
O + O2 + M  O 3 + M
(-106 kJ/mol fast)
O3 + hn  O + O2
383 kJ/mol <280 nm (fast)
O + O3  2O2
291 kJ/mol slow
O + O + M  O2 + M
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O2
λ ≤ 240 nm
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O2
Ozone production
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O2
λ ≤ 280 nm
O3
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Ozone destruction
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• Intensity
Transmittance
Absorbance
Transmittance
Absorbance
Beer’s Law
dP(x)/dx = -κ P(x)
P(x) = exp(-κ x)
λ ≥ 280 nm
© 2012 W. H. Freeman & Co.
© 2012 W. H. Freeman & Co.
Mean vertical distribution of the ozone concentration according to
observations at different latitudes. Note the variations in total column
abundance.
1 Dobson unit (DU) is:
0.01 mm of pure O3 at 1 atm. and 0 oC
NB: what does 300 DU expand to at 10-6 atm?
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O2
λ ≤ 240 nm
O
O2
Ozone production
O
O2
λ ≤ 280 nm
O3
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X = HOx (H, OH, HOO)
NOx (NO, NO2)
ClOx (Cl, ClO)
Catalytic destruction of ozone
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Chlorine reservoirs
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Chlorine reservoirs
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Atomic Cl and Br as X:
Cl can destroy tens of thousands of O3 molecules each, but
is mainly in inactive forms (HCl, ClONO2) in stratosphere.
RESERVOIRS:
ClO + NO2  ClONO2
Cl + CH4  HCl + CH3
CH3 does not operate as an X catalyst, since it combines
with O2 to give CO2.
Recipe for ozone loss:
1. POLAR WINTER
 Polar vortex
 Isolates air
2. LOW TEMPERATURE
 PSC
(=Polar Stratospheric Clouds)
(=Nitric acid – HNO3)
3. SURFACE CHEMISTRY
 Cl2
(+denoxification)
4. LIGHT RETUNS
 Cl*
 Catalytic destruction of O3
CHAPTER 02: Figure 2.1
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CHAPTER 02: Figure 2.2
© 2012 W. H. Freeman & Co.
NASA projections of stratospheric ozone
concentrations if chlorofluorocarbons had
not been banned.
Mount manitoba, June 1991
CHAPTER 02: Figure 2.10
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Prefix
Meaning
Atoms in the Molecule
CFC
chlorofluorocarbon
Cl, F, C
HCFC
hydrochlorofluorocarbon H, Cl, F, C
HBFC
hydrobromofluorocarbon H, Br, F, C
HFC
hydrofluorocarbon
H, F, C
HC
hydrocarbon
H, C
PFC
perfluorocarbon
F, C
N/A
Br, Cl (in some but not
all), F, H (in some but not
all), C
Halon
© 2012 W. H. Freeman & Co.
© 2012 W. H. Freeman & Co.
© 2012 W. H. Freeman & Co.