Transcript Document

CHAPTER 10: STRATOSPHERIC CHEMISTRY
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
STRATOSPHERIC OZONE HAS BEEN MEASURED
FROM SPACE SINCE 1979
Last Saturdays’s ozone layer…
Notice the Antarctic ozone hole
Method: UV solar backscatter
l1
Ozone layer
Scattering by
Earth surface
and atmosphere
Ozone
absorption
spectrum
l1 l2
l2
CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE
(1930)
(R1)
O 2  h  O + O
(l < 240 nm)
(R2)
O + O 2  M  O3  M
(R3)
O3  h  O 2  O
(R4)
O3  O  2O 2
(l  320 nm)
Odd oxygen family
[Ox] = [O3] + [O]
slow
O2
R1
R2
O
fast
R3
R4
slow
O3
STEADY-STATE ANALYSIS OF CHAPMAN MECHANISM
Lifetime of O atoms:
[O]
1
O 

k2 [O][O2 ][M]+k4 [O3 ][O] k2CO2 na2
1s
…is sufficiently short to assume steady state for O:
j3
O
[O]
R2  R3  k2 [O][O2 ][M]=j3[O3 ] 


2
[O3 ] k2CO 2 na  O3
1
 [Ox ]  [O3 ] …so the budget of O3 is controlled by the budget of Ox.
Lifetime of Ox:
 Ox
[Ox ]
1


2k4 [O3 ][O] 2k4 [O]
Steady state for Ox:
Ox
1
2
3
 j1k2 
2
2R1  2R4  j1[O2 ]  k4 [O3 ][O]  [O3 ]  
 CO2 na
 j3k4 
SOLAR SPECTRUM AND ABSORPTION X-SECTIONS
O2+hv
O3+hv
PHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCE
X+h  ...

j   qX (l ) X (l ) I l d l
I ( z  dz )
0
quantum
yield
optical depth d  ( O2nO2 ( z)   O3nO3 ( z))dz
I ( z)
I ( z )  I () e

    O 2 nO 2 ( z ')   O3nO3 ( z ') dz '
z
absorption
X-section
photon
flux
CHAPMAN MECHANISM vs. OBSERVATION
shape
determined
by j1nO2
-3
Chapman mechanism reproduces shape, but is too high by factor 2-3
e missing sink!
Chapman got it almost right…
CATALYTIC CYCLES FOR OZONE LOSS:
General Idea
O3 + X  XO + O2
O + XO  X + O2
Net:
O 3 + O  2 O2
X is a catalyst
The 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 STRATOSPHERE
H2O mixing ratio
Source: transport from troposphere, oxidation of methane (CH4)
HOx-CATALYZED OZONE LOSS
HOx  H + OH + HO2 hydrogen oxide radical family
H2O + O(1D)  2OH
Initiation:
OH + O3  HO 2  O 2
Propagation:
HO2 + O3  OH + 2O2
Net:
Termination:
2O3  3O2
OH + HO2  H2O + O2
slow
H2O
OH fast HO2
slow
HOx radical family
NITROUS OXIDE IN THE STRATOSPHERE
H2O mixing ratio
NOx-CATALYZED OZONE LOSS
(NOx  NO + NO2)
Initiation
N 2O +
O(1D)
Propagation
NO + O3  NO2 + O2
NO2 + h  NO + O
O + O2 + M  O 3 + M
Null cycle
2NO
Also emitted
NO + O3  NO2 + O2
NO2 + O  NO + O2
Net O3 + O  2O2
Termination
Day NO2 + OH + M  HNO3 + M
NO2 + O3  NO3 + O2
Night
NO3 + NO2 + M  N2O5 + M
N2O5 + H2O  2HNO3
O3 loss rate:

d [O3 ]
 2k[NO 2 ][O]
dt
Recycling
HNO3 + h  NO2 + OH
HNO3 + OH NO3 + H2O
NO3 + h  NO2 + O
N2O5 + h NO2 + NO3
NOy  NOx + reservoirs (HNO3, N2O5, ..)
ATMOSPHERIC CYCLING OF NOx AND NOy
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
STRATOSPHERIC DISTRIBUTION OF CFC-12
ClOx-CATALYZED OZONE LOSS
(ClOx  Cl + ClO)
Initiation: Cl radical generation from non-radical precursors (e.g.,
CFC-12)
CF2Cl2 + h  CF2Cl + Cl
O3 loss rate:
Propagation:
Cl + O3  ClO + O2
ClO + O  Cl + O2
Net: O3 + O  2O2
Termination:
Cl + CH4  HCl + CH3
ClO + NO2 + M  ClNO3 + M
d [O3 ]

 2k[ClO][O]
dt
Recycling:
HCl + OH  Cl + H2O
ClNO3 + hv  Cl + NO3
Cly  ClOx + reservoirs (HCl, ClNO3)
http://www.atmos.washington.edu/2004Q4/211/09_OzoneDep.swf
ATMOSPHERIC CYCLING OF ClOx AND Cly
Molina and Rowland shared 1995 Nobel Prize for their
work on the ClOx catalyzed destruction of ozone