THE NATURAL OZONE LAYER

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THE NATURAL OZONE LAYER
Based on ozonesonde observations in the 1970s
Ozone concentrations in units of 1012
molecules cm-3
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ATMOSPHERIC ATTENUATION OF SOLAR RADIATION
Solar UV radiation reaching the top of the
atmosphere is absorbed by ozone
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SOLAR SPECTRUM AND ABSORPTION X-SECTIONS
O3hv
O2hv
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CALCULATION OF PHOTOLYSIS RATES
k is the photolysis rate constant (also called
photolysis frequency or J-value)
actinic flux
quantum yield
absorption x-section
Probability of absorption for incoming photons
s/A
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CALCULATION OF 3-BODY REACTION RATES
A and B are reactants AB is the activated
product AB is the stable product M is the
third body (N2, O2 )
General solution
Low-pressure limit (Rate(2) gtgt Rate (3))
High-pressure limit (Rate(2) ltlt Rate (3))
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ENERGY STATES OF THE O ATOM (1s22s22p4)
determined by the arrangement of the four
electrons in the 2p orbitals
total electronic orbital angular momentum number
multiplicity
Multiplicity 2S1, where S is the spin. The
spin of an electron is (/-) 1/2.
Hunds Rule lowest-lying energy state is the one
of maximum multiplicity
O(1 S) O(1D) O(3P)
Energy
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CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE (1930)
Odd oxygen family Ox O3 O
slow
R2
R1
O2
O O3
fast
R3
R4
slow
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STEADY-STATE ANALYSIS OF CHAPMAN MECHANISM
Lifetime of O atoms
is sufficiently short to assume steady state for
O
so the budget of O3 is controlled by the budget
of Ox.
Lifetime of Ox
?Ox
Steady state for Ox
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PHOTOLYSIS RATE CONSTANTS VERTICAL DEPENDENCE
quantum yield
absorption X-section
actinic flux
In simplest case of overhead Sun and no scatter
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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 -gt 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?
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CHAPMAN MECHANISM vs. OBSERVATION
-3
Chapman mechanism reproduces shape, but is too
high by factor 2-3 e missing sink!
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RADICAL REACTION CHAINS IN THE ATMOSPHERE
photolysis thermolysis oxidation by O(1D)
Initiation
bimolecular redox reactions
radical non-radical
non-radical radical
Propagation
radical redox reaction
Termination
non-radical non-radical
radical radical
non-radical M
radical radical M
3-body recombination
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WATER VAPOR IN STRATOSPHERE
Source transport from troposphere, oxidation of
methane (CH4)
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Ozone loss catalyzed by hydrogen oxide (HOx H
OH HO2) radicals
Initiation
Propagation
Termination
slow
OH HO2
H2O
fast
HOx radical family
slow
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STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES
CONSTRAINED FROM 1980s SPACE SHUTTLE
OBSERVATIONS
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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?
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METHANE AS TRACER OF STRATOSPHERIC TRANSPORT
Methane originates from the surface, has an
atmospheric lifetime 10 years against oxidation
(ppm)
Satellite Climatology (CLAES HALOE)
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BREWER-DOBSON CIRCULATION OF STRATOSPHERE
Net loss
Net loss
Net production
Ozone number density
Ozone mixing ratio
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NITROUS OXIDE IN THE STRATOSPHERE
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ATMOSPHERIC CYCLING OF NOx AND NOy
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NOy NO NO2 HNO3 ClNO3
Keim et al., JGR 102 13193, 1997
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N2O LOW-YIELD PRODUCT OF BACTERIAL NITRIFICATION
AND DENITRIFICATION
IPCC 2007
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PRESENT-DAY GLOBAL BUDGET OF ATMOSPHERIC N2O
IPCC 2001
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)
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QUESTIONS
1. A minor branch for NO3 photolysis is
What is its effect on stratospheric ozone?
2. What is the effect on stratospheric ozone of
the reaction
3. What is the effect on stratospheric ozone of
the reaction
4. What is the effect on stratospheric ozone of
production and loss of peroxynitric acid,
5. Show that N2O5 hydrolysis is a source of HOx
.
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STRATOSPHERIC DISTRIBUTION OF CF2Cl2 (CFC-12)
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ATMOSPHERIC CYCLING OF ClOx AND Cly
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SOURCE GAS CONTRIBUTIONS TOSTRATOSPHERIC
CHLORINE (2004)
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CHLORINE PARTITIONING IN STRATOSPHERE
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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
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SPATIAL EXTENT OF THE OZONE HOLE
Mean October data
Isolated concentric region around Antarctic
continent is called the polar vortex. Strong
westerly winds, little meridional transport
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THE POLAR VORTEX (Sep-Oct 2006)
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THE OZONE HOLE IS A SPRINGTIME PHENOMENON
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THE 2010 OZONE HOLE SEASON
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VERTICAL STRUCTURE OF THE OZONE HOLEnear-total
depletion in lower stratosphere
Argentine Antarctic station southern tip of S.
America
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ASSOCIATION OF ANTARCTIC OZONE HOLEWITH HIGH
LEVELS OF CLO
Sept. 1987 ER-2 aircraft measurements at 20 km
altitude south of Punta Arenas
O3
ClO
O3
Sep. 16
Edge of Polar vortex
ClO
Sep. 2, 1987
20 km altitude
Measurements by Jim Andersons group (Harvard)
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SATELLITE OBSERVATIONS OF ClO IN THE SOUTHERN
HEMISPHERE STRATOSPHERE
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WHY THE HIGH ClO IN ANTARCTIC VORTEX?Release of
chlorine radicals from reactions of reservoir
species in polar stratospheric clouds (PSCs)
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QUESTIONS

• 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 isnt it 33?
• 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?
• However, a supersonic aircraft fleet could also
  aggravate the Antarctic ozone hole. Explain why.

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PSC FORMATION AT COLD TEMPERATURES
PSC formation
Frost point of water
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HOW DO PSCs START FORMING AT 195K?HNO3-H2O PHASE
DIAGRAM
Antarctic vortex conditions
PSCs are not water but nitric acid trihydrate
(NAT) clouds
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DENITRIFICATION IN THE POLAR VORTEXSEDIMENTATION
OF PSCs
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CHRONOLOGY OF ANTARCTIC OZONE HOLE
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TRENDS IN GLOBAL OZONE
Mt. Pinatubo
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STRATOSPHERIC SULFATE AEROSOL
condensation
OH, O(1D)
OH
H2SO4H2O
H2SO4
COS
SO2
aerosol
stratosphere troposphere
OH
COS
volcanic eruptions
vegetation
oceans
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SKIN CANCER EPIDEMIOLOGY PREDICTIONS
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LONG-TERM COOLING OF THE STRATOSPHERE
Sep 21-30, 25 km, 65-75S
Increasing CO2 is expected to cool the
stratosphere
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TRENDS IN POLAR OZONECould greenhouse-induced
cooling of stratosphereproduce an Arctic ozone
hole over the next decade?
Race between chlorine decrease and climate change
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OZONE LOSS IN ARCTIC STRATOSPHERE vs. PSC
PROCESSING
Rex et al. GRL 2006
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VERY-SHORT-LIVED SPECIES (VSLS) AND
STRATOSPHERIC OZONE
WMO 2006
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OZONE LOSS BUDGET IN LOWER STRATOSPHERE
WMO 2006