Transcript Introduction to Environmental Geochemistry

OZONE LAYER: NATURAL
DEPLETION
AND CURRENT STATUS
GLY 4241 - Lecture 8
Fall, 2014
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Mount Pinatubo Eruption, 1991
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1997 Montserrat Eruption
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Sulfur Dioxide Conversion
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Aerosol
Optical
Depth
Analyses
• BRW is Pt.
Barrow,
Alaska
• MLO is Mauna
Loa, Hawaii
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Pinatubo Aerosol Decay
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Surface Catalysis
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Brasseur and Granier Model
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Two-dimensional (latitude and altitude)
Extends from pole to pole
Altitude of 0 to 85 kilometers
Included 60 chemical species in 115 reactions
Time was not included in the model.
 Rather, specific situations at various times after the
eruption were modeled
 Using several assumptions concerning net heating in
the aerosol clouds, they predicted the ozone depletion
levels
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B-G Model Predictions
• Largest ozone depletions were predicted near the
poles, 42% depletion near the Arctic pole during
the Arctic winter, and 48% depletion near the
Antarctic pole during the Antarctic winter
• Mid-latitude reductions in the northern
hemisphere were predicted to be 10% during the
winter, and 6% during the following summer
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TOMS Results
• 1992 global average total ozone amounts were 2-3% less
than in any earlier year observed by TOMS (1979 to 1991)
• It has been suggested that transport-effects caused by
aerosol-induced radiative heating are responsible, but this
has not been proven
• Wilson et al. (1993) have shown that the Mount Pinatubo
eruption increased aerosol surface area concentrations by
factors of 30 or more
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1998 Prediction
• Observed (solid
lines, Cape Grim
data) and predicted
(dashed lines,
Montreal Protocol)
cumulative
atmospheric
concentrations (ppb
effective chlorine)
of CFCs,
chlorinated
solvents, HCFCs
and halons
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2010 Mass-Weighted Emissions
• Global mass-weighted
emissions expressed as
megatons per year
• The yellow dashed line
shows HCFC emissions
calculated without the
provisions of the 2007
accelerated HCFC phaseout under the Montreal
Protocol
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ODP-Weighted Emissions
• Global Ozone Depletion
Potential-weighted emissions
expressed as megatons of
CFC-11-equivalent per year
• The emissions of individual
gases are multiplied by their
respective ODPs (CFC-11 =
1) to obtain aggregate,
equivalent CFC-11 emissions
• Dashed line marks 1987, the
year of the Montreal Protocol
signing.
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Replacement of CFC’s
• HCFCs and HFCs are used to replace CFCs because their
chemical resistance to breakdown is less than CFC’s
• Carbon-chlorine and carbon-fluorine bonds are chemically
strong, and they are not easily broken
 CFCs have a long residence time in the atmosphere
 Carbon-hydrogen bonds are weaker, and HCFCs have
substantially shorter atmospheric residence times
 HFCs are advantageous because they contain no chlorine, and thus
cannot release chlorine to the stratosphere
 The 196 signatory nations to the Montreal protocol have
encouraged all nations to replace CFC and HCFC compounds with
HFCs
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Atmospheric Chlorine Levels
• Chlorine and bromine levels are starting to
decrease
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Ozone and Climate Change
• As greenhouse gases build up in the troposphere,
more radiation is trapped in the troposphere,
cooling the stratosphere
• Cooler stratospheric conditions, for a fixed
amount of chlorine and bromine, results in more
ozone destruction in Antarctica
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Efforts to Cut GHG’s
• It was suggested that ozone treaty personnel take over the
incineration and destruction of HFC-23, a potent
greenhouse gas (GHG)
• This is a breakdown product of a common Freon, HCFC22
• United Nations and the World Bank provide funding for
companies that capture and destroy HFC-23
• Funding is based on the greenhouse potential of the
compound, estimated at between 11,700 to 14,800 that of
carbon dioxide
• The residence time of HFC-23 is 270 years
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Global Warming Potential
Emissions
• Global GWP-weighted emissions
expressed as gigatons of CO2equivalent per year
• Emissions of individual gases are
multiplied by their respective
GWPs (direct, 100-year time
horizon; CO2 = 1) to obtain
aggregate, equivalent CO2
emissions
• Shown for reference are emissions for the range of CO2
scenarios from the Intergovernmental Panel on Climate
Change (IPCC) Special Report on Emission Scenarios
(SRES).
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Ozone Hole Data, 2003 - 2013
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2012 South Pole Ozone Update
• NOAA video update dated 10-1-12
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2013 Southern Hemisphere
Observations
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ODGI Index
• In response to amendments to the Clean Air Act in 1990,
NASA and NOAA were required to monitor stratospheric
ozone and ozone-depleting substances
• NOAA has developed an index, the Ozone Depleting Gas
Index, ODGI, in order to make data concerning the ozone
layer more accessible and more easily understood by the
public
• The ODGI index combines data from atmospheric
measurements of chemicals that contain chlorine and
bromine at multiple remote surface sites across the globe.
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Source of ODGI Data
•
ODGI estimates come from observations at Earth’s
surface of the most abundant long-lived, chlorine and
bromine containing gases regulated by the Montreal
Protocol
• The surface-based observations provide a measure of the
total number of chlorine and bromine atoms in the
atmosphere likely to reach the stratosphere and contribute
to ozone depletion in the near future
• Antarctic stratosphere vs. mid-latitude statrosphere
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ECI & EECI
• When the enhanced efficiency of bromine to destroy ozone
compared to chlorine is also considered, this total halogen
amount is called the Equivalent Chlorine (ECl) burden of
the atmosphere
• The Equivalent Effective Chlorine (EECl) can be derived
to represent how the burden of ozone-depleting
halogenated gases is changing in the mid-latitude
stratosphere, based on compound-dependent degradation
rates in the stratosphere, a younger mean stratospheric air
age, and the enhanced efficiency for bromine to destroy
ozone compared to chlorine
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Ozone Depleting Gas
Concentrations
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ODGI
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ECl Contributions
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EECl Contributions
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Stratospheric Water Vapor
• Measured over Boulder, Colorado utilizing balloonborne frost-point hygrometers
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Water Vapor Profiles,
1981 and 2013
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Arctic Low Ozone Areas, 1979 2006
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2003 Northern Hemisphere
Observations
• Anomalously high total ozone values predominated over
the Arctic region
 Portions of the Arctic region where average values of total ozone
were greater than 45 percent higher than comparable values during
the early 1980s
 At the same time, total ozone values over middle latitudes had
much lower than average values
• Lower Stratospheric minimum temperatures rarely fell
below minus 78 ̊ C, producing a weak polar vortex
• Amounts of Ozone Destroying Substances (ODS) reached
peak values in 1997-98, and have remained high
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Which part of the Stratosphere?
• NOAA work focuses on the lower stratosphere
• Core Randall and colleagues found that strong solar storms in
October 2003 indirectly affected the ozone levels in the upper
stratosphere
 Energetic electrons and protons were blasted into the Earth's upper
atmosphere, where they boosted production of nitrogen oxides by a
factor of four
 Inside the polar stratospheric vortex, which was exceptionally
powerful during the 2003-2004 winter transported nitrogen oxides
deeper into the atmosphere
 At around 40 kilometers' height, the nitrogen oxides mixed with, and
attacked, the ozone layer
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Natural vs. Anthropogenic Effects
• Cora Randall said, “"No one predicted the
dramatic loss of ozone in the upper
stratosphere of the northern hemisphere in
the spring of 2004. That we can still be
surprised illustrates the difficulties in
separating atmospheric effects due to
natural and human-induced causes."
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2004 Northern Hemisphere
Observations
• Anomalously low total ozone values predominated over
the Arctic region
 From 12/04 to 2/05, large portions of the Arctic region had average
values of total ozone 30 to 45% lower than comparable values
during the early 1980s
 Size of the Arctic area of anomalously low total ozone was among
the largest of any year on record since 1979
• Lower stratospheric minimum temperatures observed in
portions of the Arctic region were near record low values
throughout the winter, and through February remained
below minus 78 ̊ C
• A very strong polar vortex was associated with the regions
of dominance of low ozone in the Arctic
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2006 Arctic Ozone
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2006 Ozone vs. 1979-1986 Average
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Will Ozone Levels Return?
• “Although recent data suggest that total column ozone abundances
have at least not decreased over the past eight years for most of the
world, it is still uncertain whether this improvement is actually
attributable to the observed decline in the amount of ozonedepleting substances in the Earth’s atmosphere. The high natural
variability in ozone abundances, due in part to the solar cycle as
well as changes in transport and temperature, could override the
relatively small changes expected from the recent decrease in
ozone-depleting substances. Whatever the benefits of the Montreal
agreement, recovery of ozone is likely to occur in a different
atmospheric environment, with changes expected in atmospheric
transport, temperature and important trace gases. It is therefore
unlikely that ozone will stabilize at levels observed before 1980,
when a decline in ozone concentrations was first observed.”
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