REVIEWING STRATOSPHERIC CHEMISTRY BASIC MECHANISM

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REVIEWING STRATOSPHERIC CHEMISTRYBASIC MECHANISM
Chapman Mechanism Source of ozone (O2
hv) Sink of ozone (OO3) ? predicts too much
ozone!
Other ozone sinks catalytic loss cycles 1.
HOx 2. NOx 3. ClOx
Closes the overall stratospheric ozone budget
sinks balance source
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REVIEWING STRATOSPHERIC CHEMISTRYWHY DOESNT
THIS EXPLAIN THE OZONE HOLE?
Antarctic ozone hole observed in austral
SPRING! Catalytic NOx and ClOx cycles all depend
on O ? The source of O is photolysis (peaks in
summer not spring!)
Need another mechanism to explain ozone depletion
in spring
ClO ClO M ? ClOOCl M ClOOCl hv ? ClOO
Cl ClOO M ? Cl O2 2 x Cl O3 ? ClO
O2 NET 2O3 ? 3O2
Perfect! Key here is high ClO Where from?
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REVIEWING STRATOSPHERIC CHEMISTRYTHE IMPORTANCE
OF PSCs
Polar stratospheric clouds conversion of ClOx
reservoirs to Cl2 ClNO3 HCl PSC?Cl2
HNO3 Cl2 hv ? 2Cl

• Once sun comes up the Cl goes on to react with O3
• At first ClO builds up (remember not enough O),
  eventually ClO ClO cycle takes over
• ClOOCl photolyzed back to Cl ? more ozone
  depletion
• Then stratosphere warms up, no more PSCs, no more
  conversion of reservoir species, slows down
  depletion

PSC formation
Key to polar O3 depletion is cold T sunlight
Frost point of water
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CHLORINE CYCLING THE OZONE HOLE
HCl
ClNO3
Standard ClOx cycle
PSC
Antarctica early Winter
Antarctica as Winter progresses
Cl2
NO2
OH
hv
CH4
Antarctica Spring!
hv
O3
ClO
Cl
ClOOCl
O
hv
ClOO
M
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REVIEWING STRATOSPHERIC CHEMISTRYN2O5
HYDROLYSIS INCREASING SENSITIVITY TO Cl
N2O5 H2O aerosol? 2HNO3

• Effects on catalytic cycles (for ozone loss)
• NOx cycle moves from one reservoir (N2O5) to a
  longer-lived reservoir (HNO3) ? longer for NOx to
  be recycled
• ? importance of NOx-catalysis
• ClOx cycle NOx important for termination of ClOx
  cycling. A reduction in NOx reduces efficiency
  of ClO NO2 M ? ClNO3 M
• ? importance of ClOx-catalysis
• HOx cycle When HNO3 eventually returned to NOx
  will also produce HOx (HNO3 h? ? NO2 OH)
• ? importance of HOx-catalysis

Overall generally has little effect on TOTAL
ozone loss, but does make ozone loss more
sensitive to Cl levels Also will enhance ozone
loss in the presence of aerosols (eg. volcano)