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STRATOSPHERIC CHEMISTRY
TOPICS FOR TODAY
1.
Review of stratospheric chemistry
2.
Recent trends in stratospheric ozone and forcing
3.
How will stratospheric chemistry be affected by climate
change?
REVIEWING STRATOSPHERIC CHEMISTRY…
BASIC MECHANISM
Chapman Mechanism:
Source of ozone (O2 + hv)
Sink of ozone (O+O3)
 predicts too much ozone!
Other ozone sinks: catalytic loss cycles:
1. HOx
2. NOx
3. ClOx
H2O
slow
OH
fast
HO2
slow
Closes the overall stratospheric ozone budget: sinks balance source
REVIEWING STRATOSPHERIC CHEMISTRY…
WHY DOESN’T 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?
REVIEWING STRATOSPHERIC CHEMISTRY…
THE 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
PSC
depletion
formation
Key to polar O3 depletion
is cold T + sunlight
Frost point
of water
REVIEWING STRATOSPHERIC CHEMISTRY…
N2O5 HYDROLYSIS: INCREASING SENSITIVITY TO Cl
N2O5 + H2O –aerosol 2HNO3
Effects on catalytic cycles (for ozone loss):
1. NOx cycle: moves from one reservoir (N2O5) to a longer-lived reservoir
(HNO3)  longer for NOx to be recycled
↓ importance of NOx-catalysis
2. 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
3. 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)
BREWER-DOBSON CIRCULATION
Observation
Explanation
O3 columns are smallest in
tropics despite this being the
main stratospheric O3
production region
Rising tropospheric air with low
ozone
B-D circulation transports O3
from tropics to mid-high
latitudes
TOPICS FOR TODAY
1.
Review of stratospheric chemistry
2.
Recent trends in stratospheric ozone and forcing
3.
How will stratospheric chemistry be affected by climate
change (and vice versa)?
MORE CULPRITS ON THE RADIATIVE FORCING FIGURE….
RF from strat O3 depletion:
-0.05 W/m2
 But note degree of spatial
variability (polar vs. midlatitude)
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]
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]
TRENDS IN GLOBAL OZONE
Mt. Pinatubo
LONG-TERM COOLING OF THE STRATOSPHERE
Sep 21-30, 25 km, 65-75˚S
Increasing CO2 is expected to cool the stratosphere
TRENDS IN POLAR OZONE
Could greenhouse-induced cooling of stratosphere
produce an Arctic ozone hole over the next decade?
Race between chlorine decrease and climate change
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.
Increasing
trend in late
20th century
satellite
(Arctic, 16-18 km)
[Randel et al., 2004]
TOPICS FOR TODAY
1.
Review of stratospheric chemistry
2.
Recent trends in stratospheric ozone and forcing
3.
How will stratospheric chemistry be affected by
climate change (and vice versa)?
WILL WATER VAPOUR INCREASE IN THE
STRATOSPHERE?
?
H2O mixing ratio
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
INTERACTIONS BETWEEN STRATOSPHERIC OZONE
AND CLIMATE
1. Cooling stratosphere (from either CO2 or UV heating from O3), leads
to more PSC formation, O3 depletion
2. Ozone itself is a GHG
3. Changes in stratospheric T (from either CO2 or UV heating from O3)
alter the Brewer-Dobson circulation and rate of cross-tropopause
transport
4. Increases in B-D circulation increases T in the polar regions and
decrease T in the tropics
5. 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
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)
SKIN CANCER
EPIDEMIOLOGY
PREDICTIONS