CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE
(1930)
(R1)
O 2  h  O + O
(R2)
O + O 2  M  O3  M
(R3)
O3  h  O 2  O
(R4)
O3  O  2O 2
slow
O2
R1
fast
R3
R4
slow
(  320 nm)
Odd oxygen family
[Ox] = [O3] + [O]
R2
O
( < 240 nm)
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:
k3
O
[O]
R 2  R3  k2 [O][O2 ][M]=k3[O3 ] 


2
[O3 ] k2CO 2 na  O3
 [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
 k1k2 
2
2R1  2R4  k1[O2 ]  k4 [O3 ][O]  [O3 ]  
 CO2 na
 k3k4 
1
SOLAR SPECTRUM AND ABSORPTION X-SECTIONS
O2+hv
O3+hv
PHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCE

X+h  ...
k   qX ( ) X ( ) I  d 
0
I ( z  dz )
optical depth d  ( O2nO2 ( z)   O3nO3 ( z))dz
I ( z)
I ( z)  I () e

   ( O 2 nO 2 ( z ')  O3nO3 ( z '))dz '
z
quantum
yield
absorption
X-section
photon
flux
CHAPMAN MECHANISM vs. OBSERVATION
shape
determined
by k1nO2
-3
Chapman mechanism reproduces shape, but is too high by factor 2-3
e missing sink!
RADICAL REACTION CHAINS IN THE ATMOSPHERE
Initiation:
non-radical
Propagation: radical + non-radical
Termination:
radical + radical
radical + radical + M
radical + radical
photolysis
thermolysis
oxidation by O(1D)
non-radical + radical bimolecular
redox reactions
non-radical + non-radical radical redox
reaction
non-radical + M 3-body recombination
WATER VAPOR IN STRATOSPHERE
H2O mixing ratio
Source: transport from troposphere, oxidation of methane (CH4)
Ozone loss catalyzed by hydrogen oxide (HOx ≡ H + OH + HO2)
radicals
H2O + O( D)  2OH
1
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
Ozone loss catalyzed by nitrogen oxide (NOx ≡ NO + NO2)
radicals
• 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
O3 loss rate:
Null cycle
Net O3 + O  2O2  d [O3 ]  2k[NO ][O]
2
dt
• 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
ATMOSPHERIC CYCLING OF NOx AND NOy
STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES
CONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONS
Gas-phase
chemistry
only
STRATOSPHERIC DISTRIBUTION OF CF2Cl2 (CFC-12)
Ozone loss catalyzed by chlorine (ClOx ≡ Cl + ClO) radicals
•
Initiation: Cl radical generation from non-radical precursors (e.g., CFC-12)
CF2Cl2 + h g CF2Cl + Cl
•
Propagation:
Cl + O3 g ClO + O2
ClO + O g Cl + O2
Net: O3 + O g 2O2
•
Termination:
Cl + CH4 g HCl + CH3
ClO + NO2 + M g ClNO3 + M
d [O3 ]
 2k[ClO][O]
O3 loss rate: 
dt
Recycling:
HCl + OH gCl + H2O
ClNO3 + h gCl + NO3
ATMOSPHERIC CYCLING OF ClOx AND Cly
SOURCE GAS CONTRIBUTIONS TO
STRATOSPHERIC CHLORINE (2004)
CHLORINE PARTITIONING IN STRATOSPHERE
OZONE TREND AT HALLEY BAY, ANTARCTICA (OCTOBER)
Farman et al. paper
published in Nature
1 Dobson Unit (DU) = 0.01 mm O3 STP = 2.69x1016 molecules cm-2
SPATIAL EXTENT OF THE OZONE HOLE
Isolated concentric region around Antarctic continent is called the polar vortex.
Strong westerly winds, little meridional transport
THE OZONE HOLE IS A SPRINGTIME PHENOMENON
VERTICAL STRUCTURE OF THE OZONE HOLE:
near-total depletion in lower stratosphere
Argentine Antarctic station
southern tip of S. America
ASSOCIATION OF ANTARCTIC OZONE HOLE
WITH HIGH LEVELS OF CLO
Sept. 1987 ER-2 aircraft measurements at 20 km altitude south of Punta Arenas
O3
ClO
O3
Sep. 16
Sep. 2, 1987
ClO
20 km
altitude
Measurements by Jim Anderson’s group (Harvard)
Edge of
Polar
vortex
SATELLITE OBSERVATIONS OF ClO
IN THE SOUTHERN HEMISPHERE STRATOSPHERE
WHY THE HIGH ClO IN ANTARCTIC VORTEX?
Release of chlorine radicals from reactions of reservoir species in
polar stratospheric clouds (PSCs)
PSC FORMATION AT COLD TEMPERATURES
PSC formation
Frost point of water
HOW DO PSCs START FORMING AT 195K?
HNO3-H2O PHASE DIAGRAM
Antarctic
vortex
conditions
PSCs are not water but nitric acid trihydrate (NAT) clouds
DENITRIFICATION IN THE POLAR VORTEX:
SEDIMENTATION OF PSCs
CHRONOLOGY OF ANTARCTIC OZONE HOLE
TRENDS IN GLOBAL OZONE
Mt. Pinatubo
LONG-TERM COOLING OF THE STRATOSPHERE
Sep 21-30, 25 km, 65-75˚S
Increasing CO2 is expected to cool the stratosphere
TRENDS IN POLAR OZONE
Could greenhouse-induced cooling of stratosphere
produce an Arctic ozone hole over the next decade?
Race between chlorine decrease and climate change
SKIN CANCER
EPIDEMIOLOGY
PREDICTIONS