Stratospheric Chemistry EPS 133 28 March – 04 April 2011

Stratospheric ChemistryStratospheric ChemistryEPS 133 28 March – 04 April 2011EPS 133 28 March – 04 April 2011

Polar Stratospheric Clouds

ATMOSPHERIC ATTENUATION OF SOLAR RADIATIONATMOSPHERIC ATTENUATION OF SOLAR RADIATION

Solar UV radiation reaching the top of the atmosphere is absorbed by ozone

THE NATURAL OZONE LAYERTHE NATURAL OZONE LAYERBased 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 O3: air = 5 x 10-7

Ozone mixing ratio in parts per million

SOLAR SPECTRUM AND ABSORPTION X-SECTIONSSOLAR SPECTRUM AND ABSORPTION X-SECTIONS

O2+hv O3+hv

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]= [O ] 1

[O ]

[O ] [O ]O a O

kR R k 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 kk k

C nk k

k

τOx

PHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCEPHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCE

0X+ ... ( ) ( )X Xh k 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 k1nO2

Chapman mechanism reproduces shape, but is too high by factor 2-3missing sink!

RADICAL REACTION CHAINS IN THE ATMOSPHERERADICAL REACTION CHAINS IN THE ATMOSPHERE

non-radical radical + radicalInitiation:photolysisthermolysisoxidation by O(1D)

radical + non-radical non-radical + radicalPropagation: bimolecularredox reactions

non-radical + non-radicalTermination: radical redox reaction

radical + radical

non-radical + M radical + radical + M 3-body recombination

WATER VAPOR IN STRATOSPHEREWATER VAPOR IN STRATOSPHERE

Source: transport from troposphere, oxidation of methane (CH4)

H2O mixing ratio

Initiation:1

2H 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

Ozone loss catalyzed by hydrogen oxide Ozone loss catalyzed by hydrogen oxide (HO(HOxx ≡ H + OH + HO ≡ H + OH + HO22) radicals) radicals

Rate limiting step: Example

OH + O3 -> HO2+ + O2 k1

HO2 + O3 -> OH + O2 k2

HO2 + NO ->->-> OH + NO + O3 k3

{ + O2 + h … }

d[OH] / dt = -d[HO2] / dt = - k1[OH][O3] + k2[O3][HO2] + k3*[NO][HO2] ≈ 0 A

d[O3] / dt = -k1[OH][O3] – k2[HO2][O3] + k3*[NO][HO2] B

To B, add (-1)xA ≈ 0

d[O3] / dt = - 2 k2 [HO2][O3]

Rate limiting step for removal of ozone by Reactions 1, 2, 3

OH + O3

HO2 + O3

HO2 + NO

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

NITROUS OXIDE IN THE STRATOSPHERENITROUS OXIDE IN THE STRATOSPHERE

H2O mixing ratio

ATMOSPHERIC CYCLING OF NOATMOSPHERIC CYCLING OF NOxx AND NO AND NOyy

Rate limiting step, NOx: Example

NO + O3 -> NO2+ + O2 k1

NO2 + hν -> NO + O -> O3 k2

NO2 + O -> NO + O2 k3

d[NO] / dt = -d[NO2] / dt = - k1[NO][O3] + k2[NO2] + k3[NO2][O] ≈ 0 A

d[O3] / dt = -k1[NO][O3] + k2[NO2] - k3[NO2][O] B

To B, add (-1)xA ≈ 0

d[O3] / dt = - 2 k3 [NO2][O]

Rate limiting step for removal of ozone by Reactions 1, 2, 3

NO + O3

NO2 + ONO2 + hv

STRATOSPHERIC DISTRIBUTION OF CFSTRATOSPHERIC DISTRIBUTION OF CF22ClCl22 (CFC-12) (CFC-12)

ATMOSPHERIC CYCLING OF ClOATMOSPHERIC CYCLING OF ClOxx AND Cl AND Clyy

SOURCE GAS CONTRIBUTIONS TOSOURCE GAS CONTRIBUTIONS TOSTRATOSPHERIC CHLORINE (2004)STRATOSPHERIC CHLORINE (2004)

CHLORINE PARTITIONING IN STRATOSPHERECHLORINE PARTITIONING IN STRATOSPHERE

WHAT IS A RATE-LIMITING STEP?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

alti

tud

e

http://ccmc.gsfc.nasa.gov/modelweb/atmos/msise.html

ftp://hanna.ccmc.gsfc.nasa.gov/pub/modelweb/atmospheric/msis/msise90/

Latitude Latitude

alti

tud

e

Prof. James R. Holton

Stratospheric Circulation

OZONE TREND AT HALLEY BAY, ANTARCTICA (OCTOBER)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 HOLESPATIAL EXTENT OF THE OZONE HOLE

Isolated concentric region around Antarctic continent is called the polar vortex.Strong westerly winds, little meridional transport

Mean Octoberdata

THE POLAR VORTEX (Sep-Oct 2006)THE POLAR VORTEX (Sep-Oct 2006)

THE OZONE HOLE IS A SPRINGTIME PHENOMENONTHE OZONE HOLE IS A SPRINGTIME PHENOMENON

VERTICAL STRUCTURE OF THE OZONE HOLE:VERTICAL STRUCTURE OF THE OZONE HOLE:near-total depletion in lower stratospherenear-total depletion in lower stratosphere

Argentine Antarctic station southern tip of S. America

Sep. 2, 1987

Sep. 16

20 km altitude

ASSOCIATION OF ANTARCTIC OZONE HOLEASSOCIATION OF ANTARCTIC OZONE HOLEWITH HIGH LEVELS OF CLOWITH HIGH LEVELS OF CLO

Sept. 1987 ER-2 aircraft measurements at 20 km altitude south of Punta Arenas

ClO

ClO

O3

O3

Edge ofPolar vortex

Measurements by Jim Anderson’s group (Harvard)

SATELLITE OBSERVATIONS OF ClO SATELLITE OBSERVATIONS OF ClO IN THE SOUTHERN HEMISPHERE STRATOSPHERE IN THE SOUTHERN HEMISPHERE STRATOSPHERE

WHY THE HIGH ClO IN ANTARCTIC VORTEX?WHY THE HIGH ClO IN ANTARCTIC VORTEX?Release of chlorine radicals from reactions of reservoir species in Release of chlorine radicals from reactions of reservoir species in

polar stratospheric clouds (PSCs)polar stratospheric clouds (PSCs)

PSC FORMATION AT COLD TEMPERATURESPSC FORMATION AT COLD TEMPERATURES

PSC formation

Frost point of water

HOW DO PSCs START FORMING AT 195K?HOW DO PSCs START FORMING AT 195K?HNOHNO33-H-H22O PHASE DIAGRAMO PHASE DIAGRAM

Antarcticvortexconditions

PSCs are not water but nitric acid trihydrate (NAT) clouds

DENITRIFICATION IN THE POLAR VORTEX:DENITRIFICATION IN THE POLAR VORTEX:SEDIMENTATION OF PSCsSEDIMENTATION OF PSCs

CHRONOLOGY OF ANTARCTIC OZONE HOLECHRONOLOGY OF ANTARCTIC OZONE HOLE

TRENDS IN GLOBAL OZONETRENDS IN GLOBAL OZONE

Mt. Pinatubo

LONG-TERM COOLING OF THE STRATOSPHERELONG-TERM COOLING OF THE STRATOSPHERE

Sep 21-30, 25 km, 65-75˚S

Increasing CO2 is expected to cool the stratosphere

TRENDS IN POLAR OZONETRENDS IN POLAR OZONECould greenhouse-induced cooling of stratosphereCould greenhouse-induced cooling of stratosphereproduce an Arctic ozone hole over the next decade?produce an Arctic ozone hole over the next decade?

Race between chlorine decrease and climate change

SKIN CANCER SKIN CANCER EPIDEMIOLOGY EPIDEMIOLOGY PREDICTIONSPREDICTIONS