STRATOSPHERIC CHEMISTRY

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• STRATOSPHERIC CHEMISTRY

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TOPICS FOR TODAY

• Review of stratospheric chemistry
• Recent trends in stratospheric ozone and forcing
• How will stratospheric chemistry be affected by
  climate change?

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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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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)
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BREWER-DOBSON CIRCULATION
Explanation Rising tropospheric air with low
ozone B-D circulation transports O3 from tropics
to mid-high latitudes
Observation O3 columns are smallest in tropics
despite this being the main stratospheric O3
production region
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TOPICS FOR TODAY

• Review of stratospheric chemistry
• Recent trends in stratospheric ozone and forcing
• How will stratospheric chemistry be affected by
  climate change (and vice versa)?

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MORE CULPRITS ON THE RADIATIVE FORCING FIGURE.
RF from strat O3 depletion -0.05 W/m2 ? But note
degree of spatial variability (polar vs.
mid-latitude)
RF from strat water vapour 0.07 W/m2 ? ONLY
from increased methane (not feedbacks)
RF from halocarbons 0.32 W/m2 ? forcing here is
direct (as LLGHGs)
IPCC, 2007
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TREND IN HALOCARBONS
Halocarbons are regulated by the Montreal
Protocol. Long lifetimes means it takes some
time for strict emission controls to slow down
growth.
Not just a catalyst for stratospheric ozone
depletion, also make up 12 of GHG forcing!
IPCC, 2007
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TRENDS IN GLOBAL OZONE
Mt. Pinatubo
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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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TRENDS IN WATER VAPOUR COMPLEX AND CHANGING
satellite (global)
Recent decreases linked to changes in
circulation?
balloon (16-18 km)
Water vapour trends difficult to interpret, can
differ vertically and may be strong dynamical
link.
satellite (Arctic, 16-18 km)
Increasing trend in late 20th century
Randel et al., 2004
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TOPICS FOR TODAY

• Review of stratospheric chemistry
• Recent trends in stratospheric ozone and forcing
• How will stratospheric chemistry be affected by
  climate change (and vice versa)?

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WILL WATER VAPOUR INCREASE IN THE STRATOSPHERE?
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UNCLEAR If so (1) modeling studies predict
increase in HOx ozone depletion (2) in polar
regions this would raise the T threshold for PSC
formation (phase diagram), potentially increasing
ozone depletion
H2O mixing ratio
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INTERACTIONS BETWEEN STRATOSPHERIC OZONE AND
CLIMATE

• Cooling stratosphere (from either CO2 or UV
  heating from O3), leads to more PSC formation, O3
  depletion
• Ozone itself is a GHG
• Changes in stratospheric T (from either CO2 or UV
  heating from O3) alter the Brewer-Dobson
  circulation and rate of cross-tropopause
  transport
• Increases in B-D circulation increases T in the
  polar regions and decrease T in the tropics
• Increases in UV radiation (from depletion of the
  O3 layer) affect the biosphere, biogenic
  emissions, increase OH production ? affecting CH4
  and O3 in the troposphere

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CCM VALIDATION ACTIVITY FOR STRATOSPHERIC
PROCESSES AND THEIR ROLE IN CLIMATE(SPARC CCMVal)
Models that integrate chemical changes, transport
changes and other changes to the climate
(chemistry-climate models)
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SKIN CANCER EPIDEMIOLOGY PREDICTIONS