chapter 10: stratospheric chemistry

CHAPTER 10: STRATOSPHERIC CHEMISTRYCHAPTER 10: STRATOSPHERIC CHEMISTRY

THE MANY FACES OF ATMOSPHERIC OZONETHE MANY FACES OF ATMOSPHERIC OZONE

Troposphere

Stratosphere:90% of total

In stratosphere: UV shield

In middle/upper troposphere: greenhouse gas

In lower/middle troposphere: precursor of OH, main atmospheric oxidant

In surface air: toxic to humans and vegetation

STRATOSPHERIC OZONE HAS BEEN MEASURED STRATOSPHERIC OZONE HAS BEEN MEASURED FROM SPACE SINCE 1979FROM SPACE SINCE 1979

Method: UV solar backscatter

Scattering by Earth surface and atmosphere

Ozone layer

Ozoneabsorptionspectrum

Last Saturdays’s ozone layer…Notice the Antarctic ozone hole

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]=j [O ] 1

[O ]

[O ] [O ]

O a O

jR R 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 jj k

C nj k

k

Ox

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

O2+hv O3+hv

PHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCEPHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCE

0X+ ... ( ) ( )

X Xh j 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 j1nO2

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

Chapman got it almost right…

CATALYTIC CYCLES FOR OZONE LOSS: CATALYTIC CYCLES FOR OZONE LOSS: General Idea

O3 + X XO + O2

O + XO X + O2

Net: O3 + O 2 O2

X is a catalystThe catalyst is neither created nor destroyed…but the rate for the catalytic cycle [odd-O removal in this case] depends on catalyst concentrations

WATER VAPOR IN STRATOSPHEREWATER VAPOR IN STRATOSPHERE

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

HOHOxx-CATALYZED OZONE LOSS-CATALYZED OZONE LOSS

HOx H + OH + HO2 hydrogen oxide radical family

Initiation: 12H 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

NITROUS OXIDE IN THE STRATOSPHERENITROUS OXIDE IN THE STRATOSPHERE

Initiation N2O + O(1D) 2NO

O3 loss rate:

32

[O ]2 [NO ][O]

dk

dt

NONOxx-CATALYZED OZONE LOSS-CATALYZED OZONE LOSS

(NO(NOx x NO + NO NO + NO22))

Day

Night

NOy NOx + reservoirs (HNO3, N2O5, ..)

Also emitted

Termination RecyclingNO2 + OH + M HNO3 + M HNO3 + h NO2 + OH NO2 + O3 NO3 + O2 HNO3 + OH NO3 + H2ONO3 + NO2 + M N2O5 + M NO3 + h NO2 + ON2O5 + H2O 2HNO3 N2O5 + h NO2 + NO3

Propagation

NO + O3 NO2 + O2 NO + O3 NO2 + O2

NO2 + h NO + O NO2 + O NO + O2

O + O2 + M O3 + M

Null cycle Net O3 + O 2O2

ATMOSPHERIC CYCLING OF NOATMOSPHERIC CYCLING OF NOxx AND NO AND NOyy

Gas-phasechemistryonly

Source of Ox

Approximate closure!

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

Paul Crutzen shared 1995 Nobel Prize for his work on the NOx catalyzed destruction of ozone

STRATOSPHERIC DISTRIBUTION OF CFC-12STRATOSPHERIC DISTRIBUTION OF CFC-12

Initiation: Cl radical generation from non-radical precursors (e.g., CFC-12)

CF2Cl2 + h CF2Cl + Cl

Propagation:

Cl + O3 ClO + O2

ClO + O Cl + O2

Net: O3 + O 2O2

O3 loss rate:

3[O ]2 [ClO][O]

dk

dt

ClOClOxx-CATALYZED OZONE LOSS-CATALYZED OZONE LOSS

(ClO(ClOx x Cl + ClO) Cl + ClO)

Cly ClOx + reservoirs (HCl, ClNO3)

Termination: Recycling:Cl + CH4 HCl + CH3 HCl + OH Cl + H2O

ClO + NO2 + M ClNO3 + M ClNO3 + hv Cl + NO3

http://www.atmos.washington.edu/2004Q4/211/09_OzoneDep.swf

ATMOSPHERIC CYCLING OF ClOATMOSPHERIC CYCLING OF ClOxx AND Cl AND Clyy

Molina and Rowland shared 1995 Nobel Prize for their work on the ClOx catalyzed destruction of ozone