THE NATURAL OZONE LAYERBased on ozonesonde observations in the 1970s

Ozoneconcentrations in units of 1012 molecules cm-3

ATMOSPHERIC ATTENUATION OF SOLAR RADIATION

Solar UV radiation reaching the top of the atmosphere is absorbed by ozone

SOLAR SPECTRUM AND ABSORPTION X-SECTIONS

O2+hv O3+hv

CALCULATION OF PHOTOLYSIS RATES

[ ]... [ ]

d XX h k X

dt

k is the photolysis rate constant (also called photolysis frequency or J-value)

0

( ) ( ) ( )X Xk q I d

quantumyield absorption

x-section

actinic flux

photonis absorbed

Molecularcross-section A

Absorptioncross-section s

photonis notabsorbed

Probability of absorption for incomingphotons = σ/A

CALCULATION OF 3-BODY REACTION RATES

* (1)

* (2)

* * (3)

* heat (4

)

Net:

A B AB

AB A

A B

B

AB M AB M

M

M B

M

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 AB

dt k k M

General solution:

1

[ ][ ][ ]

d ABk A B

dt

1 3

2

[ ][ ][ ][ ]

k kd ABA B M

dt 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

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

O O3O2

slow

slow

fast

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

R2

R3

R4

R1

STEADY-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

Ox

PHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCE

0X+ ... ( ) ( )X Xh k q I d

quantumyield

absorptionX-section

actinicflux

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

In simplest caseof overhead Sunand no scatter:

QUESTIONS

1. Consider harmful UV radiation for which the ozone layer has an optical depth of 10. The ozone layer has thinned by 6% since 1970. What is the resulting percent increase in the flux of this UV radiation at the surface of the Earth?

2.  The original Chapman mechanism included a fifth reaction:                                                                O + O + M -> O2 + MWhat is the effect of this reaction on ozone? Is it more important in the lower or in the upper stratosphere?

3. Based on the Chapman mechanism, would you expect O and O3 concentrations in the stratosphere to vary with time of day, and if so how?

CHAPMAN MECHANISM vs. OBSERVATION

-3

shapedeterminedby k1nO2

Chapman mechanism reproduces shape, but is too high by factor 2-3e 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

2O

3O

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

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

QUESTIONS

1. A persistent mystery in atmospheric chemistry is why the stratosphere is so dry (3-5 ppmv H2O). Based on water vapor concentrations observed just below the tropopause, one would expect the air entering the stratosphere to be moister, One theory is that very strong thunderstorms piercing through the tropopause can act as a “cold finger” for condensation of water and thereby remove water from the lower stratosphere. Explain how this would work.

2. We saw that a chain termination pathway for HOx- -catalyzed ozone loss is the self-reaction of HO2 radicals producing H2O2 (hydrogen peroxide). But H2O2 has a lifetime of only about 1 day against photolysis (50%) and oxidation by OH (50%). How will each of these two sinks of H2O2 affect further HOx -catalyzed ozone loss?

METHANE AS TRACER OF STRATOSPHERIC TRANSPORT

Methane originates from the surface, has an atmospheric lifetime ~ 10 years against oxidation

SatelliteClimatology(CLAES +HALOE)

(ppm)

BREWER-DOBSON CIRCULATION OF STRATOSPHERE

Ozone number density Ozone mixing ratio

Net production

Net loss Net loss

NITROUS OXIDE IN THE STRATOSPHERE

                             

H2O mixing ratio

ATMOSPHERIC CYCLING OF NOx AND NOy

Keim et al., JGR 102 13193, 1997

NOy = NO + NO2 + HNO3 + ClNO3 + …

N2O: LOW-YIELD PRODUCT OF BACTERIAL NITRIFICATION AND DENITRIFICATION

IPCC[2007]

NH4

++3/2O2 NO2

+ H2O + 2 H+

NO3

+ Org-C N2 + …

N2O

PRESENT-DAY GLOBAL BUDGET OF ATMOSPHERIC N2O

SOURCES (Tg N yr-1) 18 (7 – 37)

Natural 10 (5 – 16)

Ocean 3 (1 - 5)

Tropical soils 4 (3 – 6)

Temperate soils 2 (1 – 4)

Anthropogenic 8 (2 – 21)

Agricultural soils 4 (1 – 15)

Livestock 2 (1 – 3)

Industrial 1 (1 – 2)

SINK (Tg N yr-1) Photolysis and oxidation in stratosphere

12 (9 – 16)

ACCUMULATION (Tg N yr-1) 4 (3 – 5)

Although a closed budget can be constructed, uncertainties in sources are large! (N2O atm mass = 5.13 1018 kg x 3.1 10-7 x28/29 = 1535 Tg N)

IPCC[2001]

QUESTIONS

1. A minor branch for NO3 photolysis is3 2NO h NO O

What is its effect on stratospheric ozone?

2. What is the effect on stratospheric ozone of the reaction

2 2HO NO OH NO

3. What is the effect on stratospheric ozone of the reaction

2ClO NO Cl NO

4. What is the effect on stratospheric ozone of production and loss of peroxynitric acid,

2 2 4

4 2 2 2

HO NO M HNO M

HNO OH NO H O O

5. Show that N2O5 hydrolysis is a source of HOx .

STRATOSPHERIC DISTRIBUTION OF CF2Cl2 (CFC-12)

ATMOSPHERIC CYCLING OF ClOx AND Cly

SOURCE GAS CONTRIBUTIONS TOSTRATOSPHERIC CHLORINE (2004)

CHLORINE PARTITIONING IN STRATOSPHERE

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 HOLE

Isolated concentric region around Antarctic continent is called the polar vortex.Strong westerly winds, little meridional transport

Mean Octoberdata

THE POLAR VORTEX (Sep-Oct 2006)

THE OZONE HOLE IS A SPRINGTIME PHENOMENON

THE 2010 OZONE HOLE SEASON

VERTICAL STRUCTURE OF THE OZONE HOLE:near-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 HOLEWITH 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 IN THE SOUTHERN HEMISPHERE STRATOSPHERE

WHY THE HIGH ClO IN ANTARCTIC VORTEX?Release of chlorine radicals from reactions of reservoir species in

polar stratospheric clouds (PSCs)

QUESTIONS

1. We saw that the Chapman mechanism overestimates the amount of ozone in the stratosphere by a factor of 3. Yet the diagram of the contribution of different mechanisms to ozone loss (“stratospheric ozone budget”) shows the Chapman mechanism accounting for only about 10% of total ozone loss. Why isn’t it 33%?

2. It has been suggested that “now is the time” for a supersonic aircraft fleet as it would help slow down the standard mechanism for chlorine-catalyzed ozone loss. How is that?

3. However, a supersonic aircraft fleet could also aggravate the Antarctic ozone hole. Explain why.

PSC FORMATION AT COLD TEMPERATURES

PSC formation

Frost point of water

HOW DO PSCs START FORMING AT 195K?HNO3-H2O PHASE DIAGRAM

Antarcticvortexconditions

PSCs are not water but nitric acid trihydrate (NAT) clouds

DENITRIFICATION IN THE POLAR VORTEX:SEDIMENTATION OF PSCs

CHRONOLOGY OF ANTARCTIC OZONE HOLE

TRENDS IN GLOBAL OZONE

Mt. Pinatubo

STRATOSPHERIC SULFATE AEROSOL

COSOH

COS SO2

OHOH, O(1D)H2SO4 H2SO4•H2O

condensation

aerosol

oceans vegetation volcaniceruptions

stratospheretroposphere

SKIN CANCER EPIDEMIOLOGY PREDICTIONS

LONG-TERM COOLING OF THE STRATOSPHERE

Sep 21-30, 25 km, 65-75˚S

Increasing CO2 is expected to cool the stratosphere

TRENDS IN POLAR OZONECould greenhouse-induced cooling of stratosphereproduce an Arctic ozone hole over the next decade?

Race between chlorine decrease and climate change

OZONE LOSS IN ARCTIC STRATOSPHERE vs. PSC PROCESSING

Rex et al. [GRL 2006]

VERY-SHORT-LIVED SPECIES (VSLS) AND STRATOSPHERIC OZONE

WMO [2006]

OZONE LOSS BUDGET IN LOWER STRATOSPHERE

WMO [2006]