What is Actually Important in Atmospheric Chemistry

1
What is Actually Important in Atmospheric
Chemistry?

• Ozone - The 'Ozone Problems'
• Oxidation Capacity - Removal of 'Pollutants'
• Free Radicals - The 'Driving Force'
• Greenhouse Gases - Chemistry Radiation Link
• Aerosol - 'Liquid - Solid'

2
Stratospheric ozone chemistry
Overview

• Stratospheric O3 Why do we care?
• Short historic overview
• Typical ozone profile
• The Chapman cycle
• Odd oxygen family
• Steady state solutions
• Global ozone distributions and implications for
  circulation
• Beyond Chapman Catalytic destruction cycles
• The polar ozone hole
• Ozone hole chemistry

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Ozone in the Atmosphere
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The solar spectrum
5
Stratospheric ozone Why do we care?
6
Penetration of short-wave UV radiation
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Solar radiation flux, absorption spectra of O2
and O3 and absorption in the atmosphere
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UV Penetration depth
350-315 nm
315-285 nm
lt285 nm
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The Dobson unit (DU)

• Bring all the ozone in the atmospheric column to
  the surface and measure the height of the layer
  at 1 atm and 273K
• 1DU equals 10-5 m of ozone
• Thus 100 DU amounts to 1 mm of O3
• Typical atmospheric values 250 400 DU

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Short historic review Discovery of O3
1840 Officially named as a chemical in 1840 by
Christian Schönbein, after he noted that it had a
smell that was similar to that of phosphorus when
exposed to air 1882 Chappuis discovers O3
absorption in the visible spectral range (ozone
liquifaction). Blue colour of the sky?? 1890 Sir
William Huggins discovers a new line group in the
spectrum of Sirius (long-wave UV absorption of
O3)
Absorption cross section of O3 Different parts of
the spectrum are named after the discoverers
Hartley-, Huggins- und Chappuis Bands.
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Short historic review Measurement of O3
1925 Gordon Dobson develops the
Dobson-Spectrophotometer to measure thickness of
ozone layer
Intensity measurement with a Dobson-spectrograph U
se several wavelength pairs
Absorption cross section (cm2)
wavelength (nm)
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Dobson-Animation
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Balloon flights by E. Regener, Stuttgart, 1934
First direct measurement of the ozone layer
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Ozone profiles
3,000 ppb
lt100 ppb
tropopause
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The ozone layer in different units
altitude (km)
concentration (molec/cm3)
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The Origin of Stratospheric Ozone
The Chapman Cycle, introduced in the 1920s by S.
Chapman, to explain the ozone layer. O2 h?
? O(3P) O(3P) Rate 1 JO2O2 O(3P) O2
M ? O3 M Rate 1 k2OO2M O3
h? ? O(3P) O2 (3?) Rate 3 JO3O3
? O(1D) O2 (1?) ..........O(1D) M ?
O(3P) M (quenching) O(3P) O3 ? O2
O2 Rate 4 k4OO2 O(3P) O(3P) M ?
O2 M Rate 5 k5OOM
llt242nm
llt1130nm
llt320nm
JO2 see figure k2 6 ? 10-34 ? (300/T)2.3 ?
M k4 8 ? 10-12 exp(-2060/T) cm3molec-1s-1 JO3
? 5 ? 10-4 s-1 (see figure, only slight
dependency on altitude) T absolute temperature
(K)
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SPECTRUM OF SOLAR RADIATION vs. ALTITUDE
O3 hn
O2 hn
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The O2- und O3- Photolysis frequencies (Jc2,
Jc3), s-1
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O3 formation figure
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Slow and fast reactions
The Chapman Cycle, introduced in the 1920s by S.
Chapman, to explain the ozone layer. O2 h?
? O O Rate 1 JO2O2 O O2 M ? O3
M Rate 2 k2OO2M O3 h? ? O O2
Rate 3 JO3O3 O O3 ? O2
O2 Rate 4 k4OO2
llt242nm
slow
fast
llt320nm
fast
slow
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The Chapman differential equations
1. Rate of change of O (short for
O(3P)) d/dtO 2?jO2?O2 jO3?O3
k2?O?O2?M k4?O?O3 2. Rate of change
of O3 d/dtO3 k2?O?O2?M JO3?O3
k4?O?O3 3. Rate of change of OX d/dtOX
2?jO2?O2 2?k4?O?O3
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Chemical steady state
Chemical steady-state assumed for species if
production and loss rate constant over
lifetime Lifetime of shortest-lived species ?O
O / k2OO2M 1 / k2O2M ? secs ?s.s.
for O between JO3 and R2 ( neglecting slow R1
and R4)
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Steady state for O, only fast reactions
d/dtO jO3?O k2?O?O2?M 0
O/O3 JO3/ k2MO2
O3
O/O3 ltlt1
Observations agree closely with Chapman
O
Ox O O3 ? O3
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Ox steady state
Effective O3 lifetime ? ?Ox ?Ox
Ox/2k4OO3 ? 1/ 2k4O
?Ox lt 1 day in the upper stratosphere ? steady
state assumption OK ?Ox years in lower
stratosphere
Steady state for Ox 2JO2 O2
2k4OO3 ?O3 (JO2k2/JO3k4)1/2 ?O2na3/2
?Ox
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Families in the Atmosphere
The "Odd Oxygen" Family Oodd OX O
O3 Formation of OX O2 h? ? O(3P) O(3P)
(Jc2) Destruction of OX O(3P) O3 ? O2
O2 (Rc3) O(3P) O(3P) M ?O2
M (Rc1)
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Steady-State solutions for Chapman ozone
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Analytical solution not trivial
O3 (JO2k2/JO3k4)1/2 ? O2na3/2

• JO2(z) and JO3(z) are photolysis rate constants
  (not reaction rate constants)
• J ?? qX(?)?X (?)I?d ?
• I? I?,? e-?/cos?
• ?? ?(?O2 O2 ?O3 O3)dz

Actinic flux depends on O2 O3 above
Solar zenith angle
Optical depth
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Energy distribution in the stratosphere(Chapman
function)
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Chapman and measurements
Good qualitative agreement
Upper stratosphere flaw in theory
Maximum vertical dependence of Ox production
2JO2O2 O2 decreases with z JO2 increases
with z
Lower stratosphere s.s. not expected because of
long ?Ox ALSO DYNAMICS
What is missing? Source or sink?
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Ozone production rate and distribution
Production rate in molec cm-3 s-1, calculated
based on O2 photolysis rate
N-S asymmetry
Distribution in 1012 molec cm-3
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Distribution and annual cycle of O3 column
density (in DU)
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Brewer-Dobson circulation

• Observation
• O3 columns are smallest in tropics despite this
  being the main stratospheric O3 production region

• Explanation
• Rising tropospheric air with low ozone
• B-D circulation transports O3 from tropics to
  mid-high latitudes

• Other comments
• O3 maxima occur toward high latitudes in late
  winter/early spring - the result of the
  descending branch of the B-D circulation
• Virtually no seasonal change in the tropics
• More accurate data has led to improvements in our
  understanding of this simple circulation pattern.

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Atmospheric motion and mean CH4 mixing ratio
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Age of air in the stratosphere
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Turbulent diffusion constant KM, cm2s-1
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Age of air in the stratosphere
Waugh and Hall 2002
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Stratosphere-troposphere exchange
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Energy balance
Radiative heating and cooling in the stratosphere
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Catalytic O3 destruction cycles
Good qualitative agreement between Chapman theory
and measurements, but factor of 2 difference
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Radical reactions in the atmosphere
Does that also happen in the atmosphere?
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CATALYTIC CYCLES FOR OZONE LOSS
Chapman got it almost right
General Idea
O3 X ? XO O2 O XO ? X O2 Net O3
O ? 2 O2 X is a catalyst
The catalyst is neither created nor destroyedbut
the rate for the catalytic cycle odd-o removal
in this case depends on catalyst concentrations
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Hydrogen oxide (HOx) radicals (HOx OH HO2)
( H??)
Source from troposphere

• Initiation H2O O(1D) ? 2OH
• Propagation through cycling of HOx radical family
  (example)
• OH O3 ? HO2 O2
• HO2 O3 ? OH 2O2
  Net 2O3 ? 3O2
• Termination (example)
• OH HO2 ? H2O O2

HOx is a catalyst for O3 loss but not the only
one
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Nitrogen oxide (NOx)radicals (NOx NO NO2)

• Initiation N2O O(1D) ?2NO
• 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
• Termination Recycling
• NO2 OH M ? HNO3 M HNO3 h? ? NO2
  OH
• NO2 O3 ? NO3 O2 HNO3
  OH ?NO3 H2O
• NO3 NO2 M ? N2O5 M NO3 h? ? NO2 O
• N2O5 H2O ? 2HNO3 N2O5
  h? ?NO2 NO3

O3 loss rate
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ATMOSPHERIC CYCLING OF NOx AND NOy
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DESTRUCTION OF O3 BY NOX

• Production Natural NOx by
• N2O O(1D) - well understood source
• Loss Dominant sink is deposition to troposphere.
  Residence time for air is 1-2 years. Loss rate
  well constrained
• Cycling O3 loss related to NOy/NOx ratio. Under
  most conditions s.s. between NOy is a good
  approximation

NOx catalytic cycle reconciled Chapman theory
with observations1995 Nobel Prize (Paul Crutzen)
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STRATOSPHERIC DISTRIBUTION OF CFC-12
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NOMENCLATURE OF CFCs

• CFC-01234a where 0 number of double bonds (
  omitted if zero )
• 1 Carbon atoms - 1 ( omitted
  if 0 )
• 2 Hydrogen atoms 1
• 3 Fluorine atoms
• 4 Chlorine atoms replaced by
  Bromine ("B" prefix added )
• a letter added to identify
  isomers, the "normal" isomer
• in any number has the
  smallest mass difference on each
• carbon, and a, b, or c are
  added as the masses diverge
• from normal.
• CFC-11 CCl3F trichlorofluoromethane
  
• CFC-12 CCl2F2 dichlorodifluoromethane
  
• CFC-113 CCl2F-CClF2 1,1,2-trichlorotrifluoro
  ethane
• HCFC-22 CHClF2 chlorodifluoromethane
• HCFC-123 CHCl2-CF3 2,2-dichloro-1,1,1-trifl
  uoroethane
• HCFC-123a CHClF-CClF2 1,2-dichloro-1,1,2-trifl
  uoroethane
• HFC-23 CHF3 trifluoromethane
  
• R-20 CHCl3 chloroform
• R-22B1 CHBrF2 bromodifluoromethane
• R-1120 CHClCCl2 trichloroethylene
  

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CATALYTIC CYCLES FOR OZONE LOSSChlorine (ClOx
Cl ClO) radicals

• Initiation
• CF2Cl2 hn g CF2Cl Cl
• Propagation
• Cl O3 g ClO O2
• ClO O g Cl O2
• Net O3 O g 2O2
• Termination Recycling
• Cl CH4 g HCl CH3 HCl OH g Cl H2O
• ClO NO2 M g ClNO3 M ClNO3 hn g ClO NO2

O3 loss rate
Discovery of the halogen catalytic cycles 1995
Nobel Prize (Mario Molina and Sherwood Rowland)
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ATMOSPHERIC CYCLING OF ClOx AND Cly
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STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES
BASED ON 1980s SPACE SHUTTLE OBSERVATIONS
Gas-phase chemistry only
What is the effect of increasing stratospheric
NOx on the rate of ClOx-catalyzed ozone loss?
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ATMOSPHERIC TREND OF CFC-11
CFC production Is banned
Montreal protocol
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OZONE COLUMN TREND, 60oS-60oN WMO, 1998
Stabilization of chlorine?
Chlorine Or dynamics?
Pinatubo
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RECENT RISE IN HFCs (CFC REPLACEMENT PRODUCTS)
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OZONE TREND AT HALLEY BAY, ANTARCTICA
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THE ANTARCTIC OZONE HOLE
Southern hemisphere ozone column seen from TOMS,
October
DU
1 Dobson Unit (DU) 0.01 mm O3 STP 2.69x1016
molecules cm-2
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OZONE HOLE IS A SPRING PHENOMENON
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Vertical structure of the Ozone hole
Ozone profiles in August (Antarctic winter) of
1996 and October (Antarctic spring) of 1997,
Nardi et al. 1999
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What is missing in O3 chemistry?

• Problem at the low zenith angles in polar spring
  there is not enough O2 photolysis to produce
  sufficient O atoms
• But O atoms needed for most catalytic cycles

Solution ClO dimer cycle ClO ClO M g
ClOOCl M ClOOCl h?? g ClOO Cl ClOO
?? g Cl OO M Cl O3 g ClO O2 (2?)
Net 2O3 g 3O2
No O atoms needed!!! ClOOCl breaks at Cl-O
bond 70 of total O3 loss in O3 hole!
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O3 and ClO
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SEASONAL TREND OF ClO IN THE POLAR VORTEX
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Additional O3 destruction cycle

• Cl O3 g? ClO O2
• Br O3 g? BrO O2
• ClO BrO g? Cl Br O2
• Net 2O3 g 3O2

30 of O3 destruction in O3 hole conditions
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Why is there so much ClO over Antarctica?
???
ClNO3 HCl g Cl2 HNO3 Cl2 h? g?2 Cl
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PSC BILD2
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PSCs OVER KIRUNA, SWEDEN (SOLVE MISSION)
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PSC FORMATION vs. TEMPERATURE
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ACTIVATION OF CHLORINE IN THE ANTARCTIC VORTEX
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HNO3-H2O PHASE DIAGRAM SHOWING STABLE CONDENSED
PHASES AT EQUILIBRUM WITH VAPOR
Antarctic vortex conditions
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DENITRIFICATION IN THE POLAR VORTEX
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CHRONOLOGY OF ANTARCTIC OZONE HOLE
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Ozone at mid latitudes
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The Gradual Decrease of Global Stratospheric O3
(W. Attmannspacher in Graedel and Crutzen, 1994)