CHAPTER 10: STRATOSPHERIC CHEMISTRY
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Transcript of CHAPTER 10: STRATOSPHERIC CHEMISTRY
CHAPTER 10: STRATOSPHERIC CHEMISTRYCHAPTER 10: STRATOSPHERIC CHEMISTRY
THE MANY FACES OF ATMOSPHERIC OZONETHE MANY FACES OF ATMOSPHERIC OZONE
Troposphere
Stratosphere:90% of total
In stratosphere: UV shield
In middle/upper troposphere: greenhouse gas
In lower/middle troposphere: precursor of OH, main atmospheric oxidant
In surface air: toxic to humans and vegetation
STRATOSPHERIC OZONE HAS BEEN MEASURED STRATOSPHERIC OZONE HAS BEEN MEASURED FROM SPACE SINCE 1979FROM SPACE SINCE 1979
Method: UV solar backscatter
Scattering by Earth surface and atmosphere
Ozone layer
Ozoneabsorptionspectrum
Last Saturdays’s ozone layer…Notice the Antarctic ozone hole
CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE (1930)(1930)
2
2 3
3 2
3 2
(R1) O O + O ( < 240 nm)
(R2) O + O M O M
(R3) O O O ( 320 nm)
(R4) O O 2O
h
h
O O3O2
slow
slow
fast
Odd oxygen family [Ox] = [O3] + [O]
R2
R3
R4
R1
STEADY-STATE ANALYSIS OF CHAPMAN MECHANISMSTEADY-STATE ANALYSIS OF CHAPMAN MECHANISMLifetime of O atoms:
O 22 2 4 3 2 O2
[O] 11 s
[O][O ][M]+ [O ][O] ak k k C n
…is sufficiently short to assume steady state for O:
3 O2 2 3 3 2
3 2 2 3
x 3
[O]2 3 [O][O ][M]=j [O ] 1
[O ]
[O ] [O ]
O a O
jR R k
k C n
…so the budget of O3 is controlled by the budget of Ox.
Lifetime of Ox:
xOx
4 3 4
[O ] 1
2 [O ][O] 2 [O]k k
Steady state for Ox:1
321 2 2
3 O23
1 2 44
3 [O2 1 2 4 [O ] [O ] O ][ ]
aR R jj k
C nj k
k
Ox
SOLAR SPECTRUM AND ABSORPTION X-SECTIONSSOLAR SPECTRUM AND ABSORPTION X-SECTIONS
O2+hv O3+hv
PHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCEPHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCE
0X+ ... ( ) ( )
X Xh j q I d
quantumyield
absorptionX-section
photonflux
2 2 3 3optical depth ( ( ) ( ))O O O Od n z n z dz
( )I z dz
( )I z
2 2 3 3
( ) ( ) e
( ') ( ') 'O O O Oz
I z I
n z n z dz
CHAPMAN MECHANISM vs. OBSERVATIONCHAPMAN MECHANISM vs. OBSERVATION
-3
shapedeterminedby j1nO2
Chapman mechanism reproduces shape, but is too high by factor 2-3missing sink!
Chapman got it almost right…
CATALYTIC CYCLES FOR OZONE LOSS: CATALYTIC CYCLES FOR OZONE LOSS: General Idea
O3 + X XO + O2
O + XO X + O2
Net: O3 + O 2 O2
X is a catalystThe catalyst is neither created nor destroyed…but the rate for the catalytic cycle [odd-O removal in this case] depends on catalyst concentrations
WATER VAPOR IN STRATOSPHEREWATER VAPOR IN STRATOSPHERE
Source: transport from troposphere, oxidation of methane (CH4)
HOHOxx-CATALYZED OZONE LOSS-CATALYZED OZONE LOSS
HOx H + OH + HO2 hydrogen oxide radical family
Initiation: 12H O + O( ) 2OHD
Propagation:3 2 2
2 3
3
2
2
OH + O HO O
HO +
Net:
O OH +
2O
2O
3O
Termination:2 2 2OH + HO H O + O
OH HO2H2Oslow
slow
fast HOx radical family
NITROUS OXIDE IN THE STRATOSPHERENITROUS OXIDE IN THE STRATOSPHERE
Initiation N2O + O(1D) 2NO
O3 loss rate:
32
[O ]2 [NO ][O]
dk
dt
NONOxx-CATALYZED OZONE LOSS-CATALYZED OZONE LOSS
(NO(NOx x NO + NO NO + NO22))
Day
Night
NOy NOx + reservoirs (HNO3, N2O5, ..)
Also emitted
Termination RecyclingNO2 + OH + M HNO3 + M HNO3 + h NO2 + OH NO2 + O3 NO3 + O2 HNO3 + OH NO3 + H2ONO3 + NO2 + M N2O5 + M NO3 + h NO2 + ON2O5 + H2O 2HNO3 N2O5 + h NO2 + NO3
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
ATMOSPHERIC CYCLING OF NOATMOSPHERIC CYCLING OF NOxx AND NO AND NOyy
Gas-phasechemistryonly
Source of Ox
Approximate closure!
STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES CONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONSCONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONS
Paul Crutzen shared 1995 Nobel Prize for his work on the NOx catalyzed destruction of ozone
STRATOSPHERIC DISTRIBUTION OF CFC-12STRATOSPHERIC DISTRIBUTION OF CFC-12
Initiation: Cl radical generation from non-radical precursors (e.g., CFC-12)
CF2Cl2 + h CF2Cl + Cl
Propagation:
Cl + O3 ClO + O2
ClO + O Cl + O2
Net: O3 + O 2O2
O3 loss rate:
3[O ]2 [ClO][O]
dk
dt
ClOClOxx-CATALYZED OZONE LOSS-CATALYZED OZONE LOSS
(ClO(ClOx x Cl + ClO) Cl + ClO)
Cly ClOx + reservoirs (HCl, ClNO3)
Termination: Recycling:Cl + CH4 HCl + CH3 HCl + OH Cl + H2O
ClO + NO2 + M ClNO3 + M ClNO3 + hv Cl + NO3
http://www.atmos.washington.edu/2004Q4/211/09_OzoneDep.swf
ATMOSPHERIC CYCLING OF ClOATMOSPHERIC CYCLING OF ClOxx AND Cl AND Clyy
Molina and Rowland shared 1995 Nobel Prize for their work on the ClOx catalyzed destruction of ozone