This Week

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This WeekPolar Stratospheric Chemistry
READING Chapter 10 of text SPADE Data Analysis
Pt 1 Due Tuesday Nov 13
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TodayOzone Hole Part 1

• Characterization of Ozone Hole
• Finding the Smoking Gun With Human Fingerprints

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Polar Ozone Loss
Severe depletion of stratospheric O3 occurs every
spring (since 1980s) over the poles (especially
South Pole).

• Example of
• far reach of human activities
• environmental catastrophe and political blame
  game
• scientific process and ability of humans to
  correct

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Discovery of Antarctic Ozone Hole
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Antarctic O3 Hole A Springtime Event
Ozone Watch Web Page
2006 Ozone Hole Movie From Satellite Observations
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Vertical Extent of Ozone Hole
Chlorine from CFCs predicted to be most effective
40 km. So why is depletion at 15 20 km?
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Discovery of Antarctic Ozone Hole
The Conundrum

• Known catalytic reactions with chlorine not fast
  enough to explain near complete depletion in
  couple months.
• Why only in spring?
• Why only between 15 25 km?
• Why only in polar region?

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Debate Over Causes of Ozone Hole
There was also a real scientific debate over the
relative roles of chemistry and
meteorology. Turns out to be both (of course!)
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Chlorine The Smoking Gun?
ClO and O3 certainly anti-correlated. But how
is there so much ClO? What is the mechanism for
ClO to destroy ozone so fast? Cant be ClO O
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QUESTIONS
1. Where in the stratosphere (altitude) would the
ClO dimer mechanism be most important? 2. If
CFC concentrations were to double, how much
faster would ozone loss by the ClO dimer
mechanism occur?  
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Polar Stratospheric Clouds (PSCs)
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PSC Formation Starts on S.A.L
Stratosphere
Stratospheric Aerosol Layer
SO2
H2SO4
OCS
15-30 km
volcanoes
Troposphere
oceans

• Extends vertically from 15 to 35 km
• Aerosol composition primarily H2SO4-H2O
• Typical particle radius 700 nm
• Typical particle number density 10 cm-3

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Sunrise over Texas From Space Shuttle
Stratospheric Aerosol Layer
Tropospheric Clouds
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PSC Formation and Types
From stratospheric aerosol layer
H2SO4-H2O
HNO3
Crystalline HNO3-3H2O (NAT) containing particles
HNO3/H2O/ H2SO4 Type Ia PSC
197 K
T
Supercooled Ternary Solutions (STS) H2SO4/HNO3/H2O
Type Ib PSC
H2O
185 K
Mainly H2O- ice particles Type II PSC
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PSC Formation Requires Very Low T
Arctic T
Color shows westerly wind speed
Antarctic T
Polar Vortex cuts off warm, ozone/NOx rich
mid-latitude air
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Heterogeneous Chemistry on PSCs
PSCs convert inactive Cly to active ClOx
Turco, 1987
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Chlorine Activation in Polar Vortex
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Antarctic O3 Hole A Springtime Event
Ozone Watch Web Page
2006 Ozone Hole Movie From Satellite Observations
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Antarctic Ozone Hole Key Points

• Why only in spring?
• Why only between 15 25 km?
• Why only in (south) polar region?

Wintertime processing on PSCs needed for active
chlorine production and denitrification. Sunlight
required to generate Cl atoms from Cl2 and
ClOOCl.
Where PSCs form (on aerosol layer), and where the
ClOOCl mechanism is fastest.
PSCs are required and only form under cold
conditions achieved during polar winter (and
mainly only Antarctic winter).
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The Solution Montreal Protocol
The persistent observations of the ozone hole
from 1984 1987 led to legally binding
international agreements. The Montreal Protocol
and its amendments eventually called for a near
complete ban on the production and use of
CFCs. A suitable and easy replacement for CFCs,
known as HCFCs, made these acts easier to swallow.
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Ozone Recovery

• Computer model predictions
• Antarctic ozone will recover to pre-1980 values
  by 2040.
• Extra-polar ozone should recover by 2020-2040.
• Predictions assume strict adherence to Montreal
  Protocol.

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Questions

• Large volcanic eruptions can inject sulfur and
  particles into the stratosphere. How would that
  affect stratospheric ozone at present?
• How might eruptions in the distant past affect
  stratospheric ozone?
• 3. Increasing greenhouse gases in the troposphere
  lead to a cooling of the stratosphere. How might
  global warming affect stratospheric ozone levels?

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Effect of N2O5 Aerosols? 2HNO3
Only gas-phase chemistry
Including Heterogeneous Chemistry
HOx becomes most important catalyst in aerosol
layer!