Chem. 253

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Chem. 253 2/5 Lecture
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Announcements I

• I have updated the homework set on the web site
• Now has solutions for non-collected subset 1.1
  problems
• Also has subset 1.2 (for next week)
• Last Weeks Group Assignment
• Trial, not graded
• This Weeks Group Assignment
• on stratospheric chemistry (O only, mostly
  covered last time)

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Announcements II

• Todays Lecture Topics
• More on O only chemistry (putting reactions in
  context and covering spatial variations)
• Catalytic destruction of ozone Mechanism I
• Catalytic destruction of ozone - reservoir
  species and Mechanism II
• The ozone hole

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Stratospheric ChemistryO only Chemistry

• Factors Affecting Ozone Formation
• Full set of Chapman Mechanism reactions
• O2 hn ? 2O
• O O2 M ? O3 M
• O3 hn ? O2 O
• O O3 ? 2O2
• Short l UV needed to photolyze O2
• see next slide
• Pressure O3 formation is slower at higher
  altitudes (lower O2 and M)
• Short UV also shortens O3 lifetime at higher
  altitudes

rxn 4 kinetic equation -dO3/dt kOO3
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Stratospheric ChemistryO only Chemistry

• Factors Affecting Ozone Formation
• Short l UV needed to photolyze O2
• more prevalent at higher altitudes (less removal
  from absorption above)
• and at lower latitudes (due to transmission
  through more atmosphere at higher latitudes)

Earth
longer pathlength
surface at high latitudes
surface in tropics
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Stratospheric ChemistryO only Chemistry

• Ozone Production Rate

Wayne, Chemistry of Atmospheres, 2nd Ed., p. 124
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Stratospheric ChemistryO only Chemistry
N

• However, despite production occurring mostly at
  high altitudes/ low latitudes, concentrations are
  higher at high latitudes/low altitudes
• This is due to transport

O3 Concentration Plot Wayne - Chemistry of
Atmospheres, 2nd Ed., p. 119
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Stratospheric ChemistryOzone Loss

• Ozone Destruction O only chemistry
• Reaction 4 (O O3 ? 2O2) is only real loss
  (ozone photolysis recycles odd O)
• Ozone Destruction observations
• models predict higher than observed
  concentrations
• Catalytic Mechanisms for loss
• a catalytic mechanism should result in the same
  net reaction as Chapman Rxn 4, but can involve
  another species

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Stratospheric ChemistryOzone Loss Mechanism I

• Mechanism I
• rxn 1 X O3 ? O2 XO
• rxn 2 XO O ? O2 X
• net rxn O O3 ? 2O2 same as Chapman 4
• X Catalytic Species
• X Cl, NO, OH
• sources from troposphere (will go into in more
  detail)

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Stratospheric ChemistryOzone Loss Mechanism I

• X NO
• NO source mainly N2O (both natural and
  anthropogenic)
• N2O (stable in troposphere, so can survive slow
  transport to stratosphere)
• reaction in stratosphere
• N2O O ? NO O2
• Catalytic Cycle
• rxn 1 NO O3 ? O2 NO2
• rxn 2 NO2 O ? O2 NO
• Complications
• NO2 hn ? NO O (null cycle reaction when
  teamed with rxn 1)

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Stratospheric ChemistryOzone Loss Mechanism I

• X Cl, Br
• Cl sources
• natural sources CH3Cl (a fraction is transported
  to stratosphere)
• anthropogenic sources CFCs
• very stable in troposphere (zero losses there)
• C-Cl bonds photolyzed in stratosphere
• Br sources halons (for extinguishing fires)
  CH3Br (fumigant)
• Both halogen reactions are very fast

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Stratospheric ChemistryOzone Loss Mechanism I

• X OH
• H sources
• tropospheric H2O (actually very little transport
  tropopause is a cold trap)
• CH4 O ? OH CH3 (CH3 produces more H radicals
  OH and HO2)

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Stratospheric ChemistryOzone Loss Mechanism I

• Mechanism I resulted in over prediction of O3
  loss, particularly from Cl cycle
• Reason missed reservoir species
• In 1970s, predictions changed frequently

O3 Loss Rate Plot Wayne - Chemistry of
Atmospheres, 2nd Ed., p. 158
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Break for Group Activity
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Stratospheric ChemistryOzone Loss Reservoir
Species

• Example of reservoir species
• ClO NO ? ClONO2
• this removes both Cl and NO from catalytic loss
  cycles
• like many reservoir species, ClONO2 can be
  reactivated
• ClONO2 hn ? ClO NO
• Other reservoir species HCl, HBr, HNO3
• Inclusion of reservoir species tends to reduce
  predicted O3 loss rates (much of mechanism I X
  species in inactive forms)
• However, reactivation results in quick loss of
  ozone

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Stratospheric ChemistryOzone Loss Mechanism II

• Mechanism I reactions expects greater loss at
  high altitudes, while observations showed loss at
  lower altitudes
• Mechanism II losses
• X O3 ? XO O2
• X O3 ? XO O2 (note X or X must be Cl)
• XO XO ? XOOX ? (or ? ?) X X O2
• Net Reaction 2O3 ? 3O2
• Mechanism II does not involve O in reaction
• This is favored at lower altitudes/ lower
  temperatures

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Stratospheric ChemistryOzone Holes

• In the early 1980s, the main O3 loss expected was
  through gas phase mechanism I not focused on
  low altitude/high latitudes
• Ozone loss was observed through ground based and
  satellite measurements
• Ground Based measurement is Dobson Unit
  (equivalent thickness if a column is reduced to
  pure O3 at ground P, std T)
• Ground based measurements showed large loss in
  Antarctic Spring, but not observed in initial
  satellite measurements (very low concentrations
  were removed as not believeable)

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Stratospheric ChemistryOzone Holes

• Dobson unit O3 measures a column content, so it
  gives an indication for UV blockage
• More UV is blocked when Dobson Units are high
  but also depends on latitude
• Loss of ozone in Antarctic was not expected
• initial investigation was over cause (dynamic vs.
  chemical)
• special conditions occurs over Antarctica
• polar vortex isolating stratospheric air from low
  latitude air
• polar stratospheric clouds (PSCs) provide a
  surface for heterogeneous reactions

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Stratospheric ChemistryOzone Holes

• Since ozone loss occurred in spring, reactivation
  of reservoir species was thought to play a role
• PSC particles were found to help with
  reactivation reactions
• ClONO2 (g) H2O(aq) ? HOCl(aq) HNO3(aq)
• and HCl(g) ? H Cl-
• and Cl- HOCl(aq) ? Cl2 (g) OH

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Stratospheric ChemistryOzone Holes

• Upon the end of winter, sunlight converts
  unstable Cl species, HOCl and Cl2 into reactive
  Cl
• Measured ClO concentrations were found to be very
  high
• Also, NOx remains locked in PSC particles
• Ozone holes are transient and end when the vortex
  ends, warming air and releasing HNO3 from PSC
  particles allowing ClONO2 to reform

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Stratospheric ChemistryCauses and Effects

• While there are natural sources of catalytic
  species, for each type (of X), anthropogenic
  sources are significant
• Sources NOx species
• Anthropogenic N2O sources
• fertilizer use
• nylon production
• Air Transport (combustion produces NOx and planes
  can flow near or in the stratosphere)
• However, NOx also forms ClONO2 (not all bad)

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Stratospheric ChemistryCauses and Effects

• Sources HOx species
• Anthropogenic CH4 sources (rice farming,
  livestock, natural gas production)
• Besides affecting HOx catalytic reactions, H2O
  concentration and PSC occurrence is affected

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Stratospheric ChemistryCauses and Effects

• Sources Halogen species
• The greatest source of Cl is from CFCs
• Effects from this are dropping due to Montreal
  Protocol
• Br containing species are much greater at causing
  ozone loss because HBr and BrONO2 are poor
  reservoir species
• Br comes from halons (similar to CFCs but Br
  containing and used for fire extinguishers) and
  CH3Br (fumigant)

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Stratospheric ChemistryCauses and Effects

• Effects main worry is UV
• Higher UV flux leads to worse sunburns, greater
  incidence of skin cancer, and cataracts
• Ozone hole occurrence is in a low human density
  region (plus bad weather can limit worse
  problems), but high UV can affect other life
  forms and be transported to midlatitudes