stratospheric ozone distribution

STRATOSPHERIC OZONE DISTRIBUTION

Marion Marchand CNRS-UPMC-IPSL

O2 + UV-c -> O + O

O + O2 + M -> O3 + M

d[O3]/dt = 2 JO2 [O2]0 2 4 6 8 10 12 14

0

10

20

30

40

Altitude (km)

O

3

(ppmv)

O

3

(molecules cm

-3

x 10

-12

)

Air (molecules cm

-3

x 5 x 10

-19

)

O2 (molecules.cm-3

x 10-19)

Alt

itu

de

(km

)

Photolysis coefficient (s-1)

O3 (molecules.cm-3 x 10-12)

JO2 (s-1)

Altitude (km)SHAPE OF OZONE PROFILE

UV

VMR (Volume Mixing Ratio)=[O3]/[M] (-> better indicator of chemistry )

O3

[M] total air concentration (molec.cm-3x 10-16)

[O3] [M]

[O3] concentration

(molec.cm-3 x 10-12)

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]

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

tropical minimum

tropical maximum

latitude

lati

tude

month

Alt

itu

de (

km)

Why?

(=O3 concentration)

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]

Troposphere is humid but stratosphere is very dry. Why?

very dry statosphere

humid troposphere

Temperature (°K)

tropopause

stratopause

GLOBAL DISTRIBUTION OF CHEMICAL TRACERS

hours

days

months

dO3/dt~f(dynamics)

dO3/dt~f(chemistry)

months

dO3/dt = f(chemistry + dynamics)

CHEMISTRY OF OZONE:CONCEPTS

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

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]

° kO+O2 et JO3 interconvert

O3 and O very rapidly, so

introduce new species

Ox[=O + O3] known as “odd-oxygen”

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]

ODD OXYGEN CHEMICAL FAMILY

fast

fast

slowslow

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

Volume mixing ratio

Altitude (km)

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

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

O3

O

° 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]

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

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

° 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 + O2 slow

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]

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]

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

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

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

° Sources of stratospheric chlorine radicals (Cl, ClO)

natural

anthropogenic

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 ozone destroying radicals

CHEMICAL MODEL O3 BUDGET : Production - Destruction

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

NOx (N2O)

HOx (CH4,H2O)

ClOx (CFCs)

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

O/O3 cycle

O3=f(X, altitude)

ozone abundance (mPa)

° complete destruction between 14 and 22 km

° - d[O3]/dt ~ 2 %/day

PSC clouds formed at T < ~193 K by co-condensation of HNO3 and H2O

Heterogeneous chemistry: ° chlorine activation (reservoirs species -> into chlorine radicals) ° O3 destruction

-> speed up reactions that are are very slow or non-existent in the gas-phase

Ice or HNO3/H2O PSCs

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)

° 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

high chlorine loading andvery cold/ isolated polar vortex

INTERACTIONS OZONE-CLIMAT

Effet de Serre

=

Bilan au niveau de la Terre + effet de Serre:

Bilan au niveau de la couche (loi Kirchhoff):

a = 2

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

= Ts = 255 K seulement !

+ = _2

Stratospheric cooling / heating rate (K/day)

Heating: O3 + UV → O + O2

O + O2 + M → O3 + M (Q) -> dT/dz > 0

Cooling: mainly CO2 +/- IR H2O and O3 significant

Temperature

tropopause

stratopause

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

~ 45 kmRelation-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

Relation-1 O3 - T :O3 + UV → O + O2

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

total

Ozone changes

Greenhouse gas changes

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)

-0.15 Wm-2

COMPLEXITY OF INTERACTIONS

CTM + GCM = CCM (Chemistry-Climate Model) -> predict the future evolution of ozone layer

DYNAMICS(T, winds)

RADIATION

CHEMISTRY

CO2

O3, CH4

CCM

Future O3 = f(CFCs, greenhouse gases)

Montreal process: chlorine loading -> ozone layer

Kyoto process greenhouse gases -> climate