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STRATOSPHERIC OZONE DISTRIBUTION. Marion Marchand CNRS-UPMC-IPSL. SHAPE OF OZONE PROFILE. Altitude (km). UV. J O2 (s -1 ). Altitude (km). O 3 (molecules.cm -3 x 10 -12 ). Photolysis coefficient (s -1 ). O 2 (molecules.cm -3 x 10 -19 ).

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slide1

STRATOSPHERIC OZONE DISTRIBUTION

Marion Marchand CNRS-UPMC-IPSL

slide2

SHAPE OF OZONE PROFILE

Altitude (km)

UV

JO2 (s-1)

Altitude (km)

O3 (molecules.cm-3

x 10-12)

Photolysis coefficient (s-1)

O2 (molecules.cm-3

x 10-19)

O2 + UV-c -> O + O

O + O2 + M -> O3 + M

d[O3]/dt =2JO2 [O2]

slide3

O3

[O3]

[M]

[O3] concentration

(molec.cm-3 x 10-12)

[M] total air concentration

(molec.cm-3x 10-16)

VMR (Volume Mixing Ratio)=[O3]/[M]

(-> better indicator of chemistry )

slide4

O3 column (Dobson units)

O3 production from O2 photolysis (molec.cm-3)

tropical

maximum

latitude

tropical

minimum

Altitude (km)

latitude

Why?

month

Brewer (1949) quotes from Dobson et al. (1929): 'The only way in which we can reconcile the observed high ozone concentration in the Arctic in spring and the low concentration in the tropics, with the hypothesis that the ozone is formed by the action of the sunlight, would be to suppose a general slow poleward drift in the highest atmosphere with a slow descent of air near the poles. Such a current would carry the ozone formed in low latitudes to the poles and concentrate it there. If this were the case the ozone at the poles would be distributed through a moderate depth of atmosphere while that in low latitudes would all be high up.’ [SPARC]

slide7

Temperature (°K)

stratopause

Troposphere is humid but stratosphere is very dry. Why?

very dry statosphere

tropopause

humid troposphere

Brewer (1949) said `The observed distributions of water vapour can be explained by the existence of a circulation in which air enters the stratosphere at the equator, where it is dried by condensation, travels in the stratosphere to temperate and polar regions, and sinks into the troposphere.' [SPARC]

slide9

months

dO3/dt~f(dynamics)

dO3/dt = f(chemistry +

dynamics)

dO3/dt~f(chemistry)

hours

days

months

slide12

OXYGEN-ONLY CHEMISTRY

° First chemistry scheme proposed by Chapman in 1930, also called the ‘Chapman cycle’.

° The reactions are

O2 + h→ O + O jO2 < 242 nm

O + O2 + M → O3 + M kO+O2

O3 + h→ O + O2 jO3 < 336 nm

O + O3 → O2 + O2 kO+O3

slide13

O2 + h→ O + O

O + O2 + M → O3 + M

O3 + h→ O + O2

O + O3 → O2 + O2

d[O3]/dt = kO+O2.[O2].[O].[M] -JO3.[O3] -kO+O3.[O3].[O]

d[O]/dt = 2.JO2.[O2] +JO3.[O3] -kO+O2.[O2].[O].[M] -kO+O3.[O3].[O]

slide14

ODD OXYGEN CHEMICAL FAMILY

fast

° kO+O2 et JO3interconvert

O3 and O very rapidly, so

introduce new species

Ox[=O + O3] known as “odd-oxygen”

fast

slow

slow

d[Ox]/dt = d[O]/dt + d[O3]/dt

= 2.JO2.O2 - 2.kO+O3.O3.O

Production - Destruction

If Ox steady-state (i.e. d[Ox]/dt = 0),

JO2.[O2]= kO+O3.[O3].[O]

slide15

Mass balance for atomic oxygen O:

d[O]/dt = 2.JO2.[O2] +JO3.[O3] -kO+O2.[O2].[O].[M] -kO+O3.[O3].[O]

° if Ox steady-state (i.e. d[Ox]/dt = 0), JO2.[O2]= kO+O3.[O3].[O]

-> d[O]/dt = JO2.[O2] +JO3.[O3] -kO+O2.[O2].[O].[M]

° interconversion terms >> net chemical terms (chemical family approach): JO3.[O3] >> JO2.[O2]

-> d[O]/dt ~ JO3.[O3] -kO+O2.[O2].[O].[M]

° lifetime of O = [O]/loss = 1/(kO+O2.[O2].[M]) < 1 sec

-> O steady-state (i.e. d[O]/dt = 0),

[O]/[O3] = JO3 / (kO+O2.[O2].[M])

° as expected, interconversion terms determine partitioning within chemical family

slide16

Altitude (km)

O

O3

Volume mixing ratio

° Chapman’s model of [O]/[O3] validated against observations

° [O]/[O3] << 1 -> Ox[=O + O3] ~ O3 and d[Ox]/d= d[O3]/dt

slide17

° if Ox steady-state (i.e. d[Ox]/dt = 0),

JO2.[O2]= kO+O3.[O3].[O]

° if O steady-state (i.e. d[O]/dt = 0),

[O]/[O3] = JO3 / (kO+O2.[O2].[M])

° using [O]=f([O3]) expression in JO2.[O2]= kO+O3.[O3].[O],

[O3] = [O2] . (kO+O2.[M] / kO+O3)1/2 . (JO2 / JO3 )1/2

with [O2] =0.21 [M]

slide18

Calculated [O3] from Chapman’s model is much too high compared to observations. Why?

d[O3]/dt = JO2.[O2] - kO+O3.[O3].[O]

calculated

observed

slide19

OZONE DESTROYING CATALYTIC CYCLES

° Bates and Nicolet introduced in 1950 the idea of ozone being destroyed via the following catalytic cycle:

OH + O3→ HO2 + O2

HO2 + O → OH + O2

net: O + O3 → O2 + O2

° NO2 cycle in 1970 by Crutzen and also Johnston

NO + O3→ NO2 + O2

NO2 + O → NO + O2

net: O + O3 → O2 + O2

slide20

° ClO cycle in 1974 by Stolarski and Cicerone.

Cl + O3→ ClO + O2

ClO + O → Cl + O2

net: O + O3 → O2 + O2

° A general form:

X + O3→ XO + O2 fast

XO + O → X + O2slow

net: O + O3 → O2 + O2

with X, the catalyst, being radical H, OH, NO, Cl or Br

° slow reaction is the limiting step in the cycle,

d[O3]/dt ~ - 2.kXO+O.[XO].[O]

slide21

Catalyst: H, OH, NO, Cl, Br,..

d[O3]/dt ~ +2.JO2.[O2] -2.kO+O3.[O3]. [O] -2.kHO2+O3. [HO2]. [O3]

-2.kNO2+O. [NO2]. [O] -2.kHO2+O3. [ClO]. [O]

slide22

STRATOSPHERIC SOURCE GASES

The key stratospheric source gases are long-lived in the troposphere, and hence, once emitted at the surface, they can reach the stratosphere

° Stratospheric hydrogen radicals (OH, HO2) originate mostly from H2O injected from the troposphere and from the in-situ oxidation of (natural and anthropogenic) CH4 by,

O(1D) + H2O → OH + OH

O(1D) + CH4→ OH + CH3 --> more oxidation, more OH

° Most of the stratospheric nitrogen oxide radicals (NO2, NO) originates from N2O oxidised in the stratosphere via the following reaction,

O(1D) + N2O → NO + NO

slide23

STRATOSPHERIC SOURCE GASES

° Stratospheric chlorine radicals (Cl, ClO) originate mostly from CFCs that are photolysed by UV radiation following,

CFxCly + h→ Cl + CFxCl(y-1)--> more oxidation, more Cl

slide25

RESERVOIR SPECIES

° Up to now, we considered the different catalytic cycles independently. But radicals from one chemical family can interact with radicals from another family.

° Reactions between radicals lead to formation of species with longer lifetimes, much less reactive, called reservoirs, e.g.

ClO + HO2 → HOCl + O2

HO2 + NO2 + M → HO2NO2 + M

ClO + NO2 + M → ClONO2 + M

OH + NO2 + M → HNO3 + M

NO3 + NO2 + M → N2O5 + M

° Reservoir species can be dissociated back rather quickly to release ozonedestroying radicals

slide26

CHEMICAL MODEL O3 BUDGET : Production - Destruction

d[O3]/dt = 2 JO2 [O2]-2 k[XO][O]with X=O2, OH, NO, Cl

ClOx (CFCs)

O/O3 cycle

NOx (N2O)

HOx (CH4,H2O)

production/destruction rate (molec.cm-3.s-1)

O3=f(X, altitude)

slide27

° complete destruction

between 14 and 22 km

° - d[O3]/dt ~2 %/day

ozone abundance (mPa)

slide29

-> speed up reactions that are

are very slow or non-existent

in the gas-phase

Heterogeneous chemistry:

° chlorine activation

(reservoirs species ->

into chlorine radicals)

° O3 destruction

Ice or HNO3/H2O PSCs

slide30

CATALYTIC CYCLES OF POLAR OZONE DESTRUCTION

° BUT observed loss rates (- d[O3]/dt) ~2 %/day

-> [O] is much too low (not enough sunlight during early spring) for ClO+O cycle to account for observed loss rates.

° Cl2O2 cycle in 1987 by Molina and Molina.

ClO + ClO + M Cl2O2 + M equilibrium

Cl2O2 + h→ Cl + ClOO

ClOO + M → Cl + O2 + M

2 x ( Cl + O3→ ClO + O2 )

-> little sunlight is required (fast JCl2O2) and more efficient at low temperatures because it slows down thermal decomposition of Cl2O2 (Cl2O2 → ClO + ClO)

slide31

° ClO-BrO cycle

ClO + BrO → Cl + Br + O2 slow

Cl + O3→ ClO + O2 fast

Br + O3→ BrO + O2 fast

net: O + O3 → O2 + O2

-> little sunlight is required because the cycle does not involve atomic oxygen O

° Polar ozone loss rate:

d[O3]/dt ~ -2.JCl2O2. [Cl2O2] -2.kClO+BrO.ClO.BrO

slide32

high chlorine

loading

and

very cold/

isolated polar

vortex

slide34

Effet de Serre

Bilan au niveau de la Terre sans effet de Serre (loi de stefan):

=

Ts = 255 K seulement !

Bilan au niveau de la Terre + effet de Serre:

=

_

2

+

=

Bilan au niveau de la couche (loi Kirchhoff):

a

=

2

slide36

Stratospheric cooling / heating rate (K/day)

stratopause

tropopause

Temperature

Heating:

O3 + UV → O + O2

O + O2 + M → O3 + M (Q)

-> dT/dz > 0

Cooling:

mainly CO2 +/- IR

H2O and O3 significant

slide37

Relation-1 O3 - T :

O3 + UV → O + O2

O + O2 + M → O3 + M (Q) => d(O3)/dt > 0 -> d(T)/dt > 0

Relation-2 O3-T:

O + O3 → O2 + O2

k = 2.e-11 *exp( -2350 / T)

Quand T diminue => ralentissement destruction O3

=>d(T)/dt > 0 -> d(O3)/dt < 0

~ 45 km

O3 and T strongly coupled:

d(O3)/dt > 0 -> d(T)/dt > 0 -> attenue le d(O3)/dt > 0

d(T)/dt > 0 -> d(O3)/dt < 0 -> attenue le d(T)/dt > 0

slide38

Greenhouse gas changes

Ozone changes

total

slide39

-0.15 Wm-2

  • Diminution O3-strato modifie l’équilibre radiatif à la tropopause
  • effet sur la T au sol :
  • Augmentation de la pénétration des UV dans le système surface-troposphère
  • (Forçage positif)
  • Sans ajustement de T: réduction des émissions IR de corp noir vers la surface
  • (Forçage négatif)
  • Avec ajustement de T: diminution de l’absorption du rayonnement UV par l’O3 => refroidissement de la stratosphère => réduction de l’émission des corps noirs vers le sol (Forçage négatif)
slide40

COMPLEXITY OF INTERACTIONS

DYNAMICS

(T, winds)

CHEMISTRY

CCM

O3, CH4

RADIATION

CO2

CTM + GCM = CCM (Chemistry-Climate Model)

-> predict the future evolution of ozone layer

slide41

Future O3 = f(CFCs,

greenhouse gases)

Montreal process:

chlorine loading

-> ozone layer

Kyoto process

greenhouse gases

-> climate