10. Stratospheric chemistry

Daniel J. Jacob, Atmospheric Chemistry, Harvard University, Spring 2017

The ozone layer

Latest satellite ozone data (March 12):http://www.temis.nl/protocols/O3total.html

Dobson unit: physical thickness (0.01 mm) of ozone layer if compressed to 1 atm, 0oC

1 DU = 2.69x1016 molecules cm-2

Structure of the natural ozone layer

Based on ozonesonde observations in the 1970s

Ozone number density, 1012 molecules cm-3

The natural ozone layer: chemical production (P) and loss (L)Ozone number density, 1012 molecules cm-3

P > L P > L

L > P

L > PL > P

Brewer-Dobson stratospheric circulation

Chapman mechanism for stratospheric ozone (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

ν λ

ν λ

+ →+ → +

+ → + <+ →

O O3O2slow

slow

fast

Odd oxygen family[Ox] = [O3] + [O]

R2

R3R4

R1

Calculation of photolysis rates

[ ]... [ ]d XX h k Xdt

ν+ → = −

k is the photolysis rate constant (also called photolysis frequency)

0

( ) ( ) ( )X Xk q I dλ σ λ λ λ∞

= ∫quantumyield absorption

x-section

actinic flux (omnidirectional)

photonis absorbed

Molecularcross-section A

Absorptioncross-section σ

photonis notabsorbed

Probability of absorption for incomingphotons = σ/A

Solar spectrum and absorption cross-sections

O2+hv O3+hv

Calculation of 3-body reaction rates

* (1)* (2)* * (3)

* heat (4

)Net:

A B ABAB A

A B

BAB M AB MM

M BM

A M

+ →→ ++ →

+ + →→ +

+

+

Low-pressure limit (Rate(2) >> Rate (3)):

A and B are reactants;AB* is the activated product;AB is the stable product;M is the “third body” (N2, O2 )

1 3

2 3

[ ][ ][ ][ ][ ]

k k A B Md ABdt k k M

=+

General solution:

1[ ] [ ][ ]d AB k A Bdt

=

1 3

2

[ ] [ ][ ][ ]k kd AB A B Mdt k

=

High-pressure limit (Rate(2) << Rate (3)):

Energy states of the O atom (1s22s22p4)

multiplicity total electronicorbital angularmomentum number

Multiplicity = 2S+1, where S is the spin. The spin of an electron is ±1/2.

Hund’s Rule: lowest-lying energy state is the one of maximum multiplicity

EnergyO(1 S)O(1D)O(3P)

determined by the arrangement of the four electrons in the 2p orbitals

O.

.

: :

.

.O(3P) is a diradical

Steady-state analysis of Chapman mechanismLifetime of O atoms:

O 22 2 4 3 2 O2

[O] 1 ~ 1 s [O][O ][M]+ [O ][O]

= ≈ak k k C n

τ

…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 kk C n

ττ

…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

τ = ≈

Steady state for Ox:1

321 2 2

3 O23

1 2 44

3 [O2 1 2 4 [O ] [O O] ]][ aR R k k k C nk k

k

=⇒

= = ⇒

τOx

Photolysis rate constants: dependence on altitude

0X+ ... ( ) ( )X Xh k q I dλν λ σ λ λ

∞→ = ∫

quantumyield

absorptionX-section

photonflux

2 2 3 3optical depth ( ( ) ( ))O O O Od n z n z dzδ σ σ= +

( )I z dz+

( )I z

2 2 3 3

( ) ( ) e

( ( ') ( ')) 'O O O Oz

I z I

n z n z dz

δ

δ σ σ

−

∞

= ∞

= +∫

Chapman mechanism vs. observations

-3

shapedeterminedby k1nO2

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

Radical 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 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

2O3O→

→ +→

Termination:2 2 2OH + HO H O + O→

OH HO2H2Oslow

slow

fast HOx radical family

Ozone loss catalyzed by hydrogen oxide (HOx ≡ H + OH + HO2) radicals

Questions

1. In the upper stratosphere, OH reacts with O atoms:OH + O → H + O2

followed by addition of O2 to H:H + O2 + M → HO2 + M

What is the effect on ozone?

1. A termination step for the HOx radical chain isHO2 + HO2 → H2O2 (hydrogen peroxide)Hydrogen peroxide can go on to either photolyze or react with OH:

H2O2 + hν → 2OHH2O2 + OH → H2O + HO2

Whether H2O2 photolyzes or reacts with OH has a large effect on HOx-catalyzed ozone loss, explain why.

Supersonic aircraft (Concorde) cruising at 60,000’

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.”

It is not necessarily the slowest reaction in the sequence!

NITROUS OXIDE IN THE STRATOSPHERE

H2O mixing ratio

N2O: LOW-YIELD PRODUCT OF BACTERIAL NITRIFICATION AND DENITRIFICATION

Important as• source of NOx radicals in stratosphere• greenhouse gas

IPCC[2014]

Main anthropogenic source: agriculture

ATMOSPHERIC CYCLING OF NOx AND NOy

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

Salawitch et al. [1989]

Chlorofluorocarbons (CFCs) in the atmosphere

Chlorofluorocarbons(CFCs)

(ppb)

ATMOSPHERIC CYCLING OF ClOx AND Cly

Chlorine partitioning in stratosphere

WMO [2014]

Source gas contributions to stratospheric chlorine

WMO [2014]

Decrease of Cl-containing gases following Montreal protocol

τ = 45 yearsτ = 100 years

τ = 26 yearsτ = 5 years

• Original Montreal protocol (1987): cap production rates at 1980s levels• London (1990), Copenhagen (1992) amendments: phase-out in developed world• Beijing (1999): worldwide ban on production

Loss of HNO3 by PSC sedimentationsuppresses conversion of ClO to ClNO3 in Antarctic spring

WMO [2014]

Loss of HNO3 by PSC sedimentationsuppresses conversion of ClO to ClNO3 in Antarctic spring

WMO [2014]

CHRONOLOGY OF ANTARCTIC OZONE HOLE

What about the Arctic?

PSCs in Kiruna, northern Sweden

PSC FORMATION AT COLD TEMPERATURES

PSC formation

Frost point of water

WMO [2014]

Ozone depletion can take place in Arctic stratosphere in spring

WMO [2014]Movie of 2015-2016 Arctic ozone season

http://ozonewatch.gsfc.nasa.gov/

1971-present trend in Arctic ozone column (March)

WMO [2014]

March 1979-2016 Arctic ozone movie

http://ozonewatch.gsfc.nasa.gov/

Arctic ozone watch, 2017

Embryo of ozone hole is occasionally seen in Arcticfollowing cold winters

Correlation of Arctic ozone loss with temperature

Rising CO2 warms the surface but cools the stratosphere

ground

troposphere

stratosphere

CO2

WARM

COLD

IPCC [2014]

Global ozone trend

WMO [2014]

Montreal Protocol as example of successful global environmental policy

Montreal Protocol and its amendments have reversed the stratospheric chlorine trend

Antarctic ozone hole is on its way to recovery,Arctic ozone hole appears to have been avoided

Model projections for future ozone recovery

WMO [2014]