stratospheric chemistry eps 133 28 march – 04 april 2011
DESCRIPTION
Stratospheric Chemistry 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. Based on ozonesonde observations in the 1970s. THE NATURAL OZONE LAYER. - PowerPoint PPT PresentationTRANSCRIPT
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
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
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
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
SOURCE GAS CONTRIBUTIONS TOSOURCE GAS CONTRIBUTIONS TOSTRATOSPHERIC CHLORINE (2004)STRATOSPHERIC CHLORINE (2004)
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/
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
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
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