EPS 133 28 March – 04 April 2011
Polar Stratospheric Clouds

ATMOSPHERIC ATTENUATION OF SOLAR RADIATION
Solar UV radiation reaching the top of the atmosphere is absorbed by ozone

THE NATURAL OZONE LAYER
Based on ozonesonde observations in the 1970s

1 Dobson Unit (DU) is defined to be 0.01 mm thickness at stp; the ozone layer over Labrador is
~300 DU.
Mean ratio, column O
3
: air = 5 x 10 -7

Ozone mixing ratio in parts per million

SOLAR SPECTRUM AND ABSORPTION X-SECTIONS
O
2
+ hv
O
3
+ hv

CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE
(1930)
(R1) O
2
 h
 
(R2) O + O
2

M

O
3

M
(R3) O
3
 h
 
O
2

O (
 
320 nm)
(R4) O
3
 
2O
2
Odd oxygen family
[O x
] = [O
3
] + [O]
O
2 slow
R1
R2
O O
3
R3
R4 slow

STEADY-STATE ANALYSIS OF CHAPMAN MECHANISM
Lifetime of O atoms:
 
O k
2
[O]
[O][O ][M]+ [O ][O]
2 k
4 3

1 k C n
2 O2 a
2
1 s
…is sufficiently short to assume steady state for O:
R 2

R 3
 k
2
[O][O ][M]= [O ]
2 k
3 3

[O]
[O ]
3

[O ] [O ] x

3
 k
3 k C n
2 O 2 a
2


O

O 3
…so the budget of O
3 is controlled by the budget of O x
.
1
Lifetime of O x
:

Ox

[O ] x
 k
4 3 k
4
1
Steady state for O x
:
τ
Ox
R

R
 k
1
[O ]
 k
4
[O
3
][ O]

[O
3
]


 k k
1 2
4

1
2
3 a
2

PHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCE
X+ h
 
... k
 
0
 q
X
  
X
 quantum absorption photon yield X-section flux
(
 dz ) optical depth d
 
(
 n ( )
O 2 O 2
  n ( ))
O 3 O 3
( )
   
   z

(

O 2 n
O 2
( ')
 
O 3 n
O 3
( ')) dz '

CHAPMAN MECHANISM vs. OBSERVATION shape determined by k
1 n
O2
-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 radical + radical photolysis thermolysis oxidation by O( 1 D) non-radical + radical bimolecular redox reactions
Termination: radical + radical radical + radical + M non-radical + non-radical radical redox reaction non-radical + M 3-body recombination

WATER VAPOR IN STRATOSPHERE
H
2
O mixing ratio
Source: transport from troposphere, oxidation of methane (CH
4
)

Ozone loss catalyzed by hydrogen oxide
(HO x
≡ H + OH + HO
2
) radicals
Initiation:
H O + O(
2
1
D )

2OH
Propagation:
OH + O
3

HO
2

O
2
HO +
2
O
3

OH + 2O
2
Net: 2O
3

3O
2
Termination: OH + HO
2

H O + O
2 2
H
2
O slow
OH HO
2 HO x radical family slow

Rate limiting step: Example
OH + O
3
-> HO
2+
+ O
2
HO
2
+ O
3
-> OH + O
2 k1 k2
HO
2
+ NO ->->-> OH + NO + O
3
{ + O
2
+ h
 … } k3 d[OH] / dt = -d[HO
2
] / dt = - k1[OH][O
3
] + k2[O
3
][HO
2
] + k3 * [NO][HO
2
] ≈ 0 A d[O
3
] / dt = -k1[OH][O
3
] – k2[HO
2
][O
3
] + k3*[NO][HO
2
] B
To B , add (-1)x A ≈ 0 d[O
3
] / dt = - 2 k2 [HO
2
][O
3
]
OH + O
3
HO
2
HO
2
+ O
3
+ NO
Rate limiting step for removal of ozone by Reactions 1, 2, 3

STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES
CONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONS

NITROUS OXIDE IN THE STRATOSPHERE
H
2
O mixing ratio

ATMOSPHERIC CYCLING OF NO x
AND NO y

Rate limiting step, NOx: Example
NO + O
3
-> NO
2+
+ O
2
NO
2
+ h ν -> NO + O
NO
2
+ O -> NO + O
2
-> O
3 k1 k2 k3 d[NO] / dt = -d[NO
2
] / dt = - k1[NO][O
3
] + k2[NO
2
] + k3[NO
2
][O] ≈ 0 A d[O
3
] / dt = -k1[NO][O
3
] + k2[NO
2
] - k3[NO
2
][O] B
To B , add (-1)x A ≈ 0 d[O
3
] / dt = - 2 k3 [NO
2
][O]
NO + O
3
NO
2
NO
2
+ O
+ hv
Rate limiting step for removal of ozone by Reactions 1, 2, 3

STRATOSPHERIC DISTRIBUTION OF CF
2
Cl
2
(CFC-12)

ATMOSPHERIC CYCLING OF ClO x
AND Cl y

SOURCE GAS CONTRIBUTIONS TO
STRATOSPHERIC CHLORINE (2004)

CHLORINE PARTITIONING IN STRATOSPHERE

WHAT IS A RATE-LIMITING STEP?
• From IUPAC: “A rate-controlling (rate-determining or rate-limiting) step in a reaction occurring by a composite reaction sequence is an elementary reaction the rate constant for which exerts a strong effect — stronger than that of any other rate constant — on the overall rate.”

Latitude Latitude http://ccmc.gsfc.nasa.gov/modelweb/atmos/msise.html
ftp://hanna.ccmc.gsfc.nasa.gov/pub/modelweb/atmospheric/msis/msise90/

Latitude Latitude

Prof. James R. Holton

OZONE TREND AT HALLEY BAY, ANTARCTICA (OCTOBER)
Farman et al. paper published in Nature
1 Dobson Unit (DU) = 0.01 mm O
3
STP = 2.69x10
16 molecules cm -2

SPATIAL EXTENT OF THE OZONE HOLE
Mean October data
Isolated concentric region around Antarctic continent is called the polar vortex.
Strong westerly winds, little meridional transport

THE POLAR VORTEX (Sep-Oct 2006)

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
O
3
ClO
O
3
Sep. 16
ClO
Sep. 2, 1987
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?
HNO
3
-H
2
O 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, 6575˚S
Increasing CO
2 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