Ozone Depletion in the Stratosphere
EVPP 111 Lecture
Dr. Largen
Ozone layer
A layer of ozone in lower stratosphere
keeps ~95% of the sun’s harmful UV radiation from reaching earth’s surface
Thinning of the ozone layer
Ozone concentration in stratosphere is determined by
measuring instruments on
- balloons
- aircraft
- satellites
Thinning of the ozone layer
Ozone concentration in stratosphere is
depleted seasonally over Antarctica and the Arctic
lower overall thinning everywhere except over the tropics
Thinning of the ozone layer
Ozone depletion in the stratosphere is considered to be
a serious long-term threat to
- humans
- many other animals
- the sunlight-driven primary producers (mostly plants)
that support the earth’s food chains and webs
What causes ozone depletion
Chlorofluorocarbons (CFCs) and other chlorine-containing compounds destroy the ozone in the stratosphere
What causes ozone depletion
Chlorofluorocarbons (CFCs)
were first discovered in 1930 by a General Motors chemist named Thomas Midgley, Jr.
other chemists went on to make similar compounds to create a family of highly useful CFCs
What causes ozone depletion
Chlorofluorocarbons (CFCs)
the two most widely used were known by their trade name as Freons
CFC-11 (trichloromethane, CCl3F)
CFC-12 (dichlorodifluoromethane, CCl2F2)
What causes ozone depletion
Chlorofluorocarbons (CFCs)
were originally considered by some to be "dream chemicals" because of their characteristics
chemically stable (nonreactive)
odorless
nonflammable
nontoxic
noncorrosive
What causes ozone depletion
Chlorofluorocarbons (CFCs)
because of their characteristics and the fact that they were cheap to make, they became popular for many uses
What causes ozone depletion
Chlorofluorocarbons (CFCs)
uses included
- coolants in air conditioners and refrigerators
- replacing toxic sulfur dioxide and ammonia that had been used previously
- propellants in aerosol spray cans
- cleaners for electronic parts such as computer chips
- sterilants for hospital instruments
- fumigants for granaries and ship cargo holds
- bubble sin plastic foam used for insulation and packaging
What causes ozone depletion
Chlorofluorocarbons (CFCs)
because of their characteristics, low cost and many uses
- production rose sharply between 1960 and the early 1990’s
What causes ozone depletion
Chlorofluorocarbons (CFCs)
in 1974, calculations made by two University of California-Irvine chemists, Sherwood Rowland and Mario Molina, indicated that
- CFCs were lowering the average concentration of ozone in the stratosphere
Rowland and Molina shocked the scientific community and the $28 billion per year CFC industry
- by calling for an immediate ban on CFCs in spray cans
- spray cans were singled out because substitutes were available at the time for these
What causes ozone depletion
Chlorofluorocarbons (CFCs)
Rowland and Molina concluded that
- large quantities of CFCs were being released into the troposphere mostly from
- use of CFCs as propellants in spray cans
- leaks from refrigeration and air conditioning equipment
- production and burning of plastic foam products
What causes ozone depletion
Chlorofluorocarbons (CFCs)
Rowland and Molina concluded that
- CFCs remain in the troposphere because they are insoluble in water and are chemically unreactive
- over a period of 11-20 years, CFCs rise into the stratosphere through
- convection
- random drift
- turbulent mixing in the troposphere
What causes ozone depletion
Chlorofluorocarbons (CFCs)
Rowland and Molina concluded that
- in the stratosphere, CFC molecules break down
- under the influence of high-energy UV radiation
- releasing highly reactive chlorine atoms
- which speed up the breakdown of reactive ozone into oxygen in a cyclic chain of chemical reactions
- causing ozone in some parts of stratosphere to be destroyed faster than it is formed
What causes ozone depletion
Chlorofluorocarbons (CFCs)
Rowland and Molina concluded that
- each CFC can last in the stratosphere for 65-385 years, depending on its type
- the most widely used CFCs lasting 75-111 years
- during which time each chlorine atom released from a CFC molecule can convert up to 100,000 molecules of ozone to oxygen
What causes ozone depletion
Chlorofluorocarbons (CFCs)
Rowland and Molina concluded that
- the "dream molecules" (CFCs) had turned into global ozone destroyers
What causes ozone depletion
Chlorofluorocarbons (CFCs)
CFC industry, led by Dupont Company
- attacked Rowland and Molina’s conclusion
- was powerful, well-funded with lots of profits asnd jobs at stake
What causes ozone depletion
Chlorofluorocarbons (CFCs)
Rowland and Molina held their ground against the industry
- explaining their calculations to other scientists, elected officials, the media
finally, in 1988, 14 years after Rowland and Molina’s study
- DuPont officials acknowledged that CFCs were depleting the ozone layer
- agreed to stop producing them once they found substitutes
What causes ozone depletion
Chlorofluorocarbons (CFCs)
in 1995, Rowland and Molina won the Nobel Prize in Chemistry for their work on CFCs and the ozone layer
Other ozone depleting compounds
Other ozone depleting compounds (ODC) include
halons and HBFCs
used in fire extinguishers
methyl bromide(CH3Br)
a widely used funigant
carbon tetrachloride (CCl4)
a cheap, highly toxic solvent
methyl chloroform (C2H3Cl3)
a cleaning solvent for clothes and metals
a propellant in more than 160 consumer products including correction fluid, dry-cleaning sprays, spray adhesives, other aerosols
hydrogen chloride (HCl)
emitted into the stratosphere by US space shuttles
Other ozone depleting compounds
natural sources of their ozone depleting compounds
oceans and volcanic eruptions
- release chlorine and bromine into the troposphere
- most of these chlorine compounds don’t make it to the stratosphere
- they dissolve in water and are washed out in the rain
- bromine compounds are less likely to be washed out of the atmosphere
- more study is needed to understand their fate
Ozone depletion
Of the observed ozone losses in the stratosphere since 1976
~75-85% are attributed to compounds released into the atmosphere by human activities beginning in the 1950s
Figure 54.27b Erosion of Earth’s ozone shield: Thickness of the ozone layer
Seasonal ozone thinning over the poles
In the mid-1980s, researchers discovered that ~40-50% of the ozone in stratosphere over Antarctica was being destroyed during
Antarctic spring and summer (September-December)
Figure 54.27a Erosion of Earth’s ozone shield: The ozone hole over the Antarctic
Seasonal ozone thinning over the poles
This seasonal loss of ozone over Antarctica was incorrectly dubbed the ozone hole
it is more accurately referred to as ozone thinning
- the degree of depletion varies with altitude and location
the total area of atmosphere above Antarctica that suffers from ozone thinning varies from year to year
in 2000, the seasonal thinning above Antarctica was the largest ever and covered an area ~3X the size of the continental US
Seasonal ozone thinning over the poles
Why is the loss of ozone over Antarctica seasonal
during winter
- its sunless
- steady winds blow in circular pattern over earth’s poles
- creates polar vortex
- a huge swirling mass of very cold air that is isolated from rest of the atmosphere
- until the sun returns a few months later
Seasonal ozone thinning over the poles
Why is the loss of ozone over Antarctica seasonal
during winter
- when water droplets in clouds enter the polar vortex (a mass of extremely frigid air)
- they form tiny ice crystals which collects CFCs and other ODCs on their surfaces
- serve as catalysts for speeding up the chemical reactions that release Cl & ClO
- Cl and ClO react with each other to form Cl2O2
- in the dark of winter the Cl2O2 molecules can’t react with ozone so they accumulate in the polar vortex
Seasonal ozone thinning over the poles
Why is the loss of ozone over Antarctica seasonal
during spring
- when sunlight returns (October)
- Cl2O2 molecules are broken apart by UV light
- releasing large numbers of Cl atoms that can begin reacting with ozone in the cyclic chain of chemical reactions that destroy ozone
the sunlight
gradually melts the ice crystals
breaks up the vortex of trapped polar air
allows trapped air to begin mixing with the rest of the atmosphere
Seasonal ozone thinning over the poles
Why is the loss of ozone over Antarctica seasonal
during spring
40-50% of the ozone above Antarctica is destroyed
Seasonal ozone thinning over the poles
Why is the loss of ozone over Antarctica seasonal
during spring
- when the vortex breaks up
- huge masses of ozone depleted air above Antarctica flows northward
- lingers for a few weeks over Australia, New Zealand, South America, South Africa
- resulting in increases of 3-20% levels of biologically damaging UV-B radiation
Seasonal ozone thinning over the poles
Ozone thinning over the Arctic
in 1988, scientists discovered a similar but less severe ozone thinning over the Arctic during Arctic spring/summer (February-June)
producing a seasonal loss of 11-38% (compared with ~50% loss in Antarctic)
Why should we care about ozone loss
Less ozone in stratosphere results in more biologically damaging UV-A and UV-B radiation reaching the earth’s surface
impact on humans
- worse sunburns
- more eye cataracts
- which can lead to blindness if not corrected
more skin cancers
Why should we care about ozone loss
According to UNEP estimates, the additional UV-B radiation that would reach the earth’s surface as a result in a 10% annual loss of global ozone would lead to
300,000 aditional cases of squamous cell and basal cell cancer
4500-9000 additional cases of potentially fatal malignant melanoma
1.5 million new cases of cataracts
Why should we care about ozone loss
Other effects of increased UV exposure include
immune system suppression
increase in acid deposition
increase in photochemical smog
lower yields of key crops (corn, rice, soybeans, wheat, etc.)
- with estimated losses totaling ~2.5 billion/ year
decline in forest productivitiy
increased degradation and breakdown of matierals such as plastics, paints
Why should we care about ozone loss
Other effects of increased UV exposure include
reduction in productivity of surface-dwelling phytoplankton resulting in
- disruption of aquatic food chains
- decrease in yields of seafood eaten by humans
- possible acceleration of global warming by decreasing oceanic uptake of carbon dioxide
Solutions: Protecting the ozone layer
- The scientific consensus for researchers
immediately stop producing all ozone-depleting chemicals
there are substitutes available for most CFCs
additional substitutes are being developed
Solutions: Protecting the ozone layer
Is there a possibility of a quick fix from technology that would allow us to keep using CFCs?
Some strange proposals have been floated
Solutions: Protecting the ozone layer
strange "technofix" proposals
blimps
- fleet of radio-controlled, football-field long blimps launched into stratosphere over Antarctica
- curtains of electrical wires hanging from blimps would inject electrons into stratosphere
- which would react with and remove chlorine atoms
Solutions: Protecting the ozone layer
strange "technofix" proposals
lasers
- tens of thousands of lasers would "blast" CFCs out of the atmosphere before they could reach the stratosphere
Solutions: Protecting the ozone layer
Efforts to reduce ozone depletion
the Montreal Protocol, a treaty
- developed in 1987 at a meeting in Montreal of representatives of 36 nations
- to cut emissions of CFCs by ~35% -50% between 1989 and 2000
- other ozone-depleting chemicals were not addressed
Solutions: Protecting the ozone layer
Efforts to reduce ozone depletion
a new protocol was adopted following meeting in 1990 & 1992 of representative from 93 countries
- in response to news in 1989 about the seasonal thinning of ozone layer over Antarctica
Solutions: Protecting the ozone layer
Efforts to reduce ozone depletion
to date, landmark international agreements
- signed by 175 nations
- illustrate global response to a serious global environmental problem
Solutions: Protecting the ozone layer
Efforts to reduce ozone depletion
according to a 1998 study by the World Meteorological Organization (WMO), the ozone layer
- will continue to be depleted for several decades
- will return to 1980 levels by about 2050 and to 1950 level by about 2100 if certain assumptions hold
- depletion has resulted in a cooling of the troposphere
Solutions: Protecting the ozone layer
Efforts to reduce ozone depletion
according to 1998 WMO study, ozone layer
- will continue to be depleted for several decades because
- there’s an 11-20 year time lag between the release of ODCs and their arrival in stratosphere
- ODCs persist in the stratosphere for decades
Solutions: Protecting the ozone layer
Efforts to reduce ozone depletion
according to 1998 WMO study, ozone layer
- will return to 1980 levels by about 2050 and to 1950 levels by about 2100, assuming
- the international agreements are followed
- there are no major volcanic eruptions
or, to rephrase
if all ozone use was stopped today, it would take 47 years for concentrations to return to 1980 levels and 97 years to return to "safe" levels of the 1950s
Solutions: Protecting the ozone layer
Efforts to reduce ozone depletion
according to 1998 WMO study
- depletion of ozone in stratosphere has resulted in
- a cooling of the troposphere
- which has perhaps offset or disguised as much as 30% of the global warming caused by greenhouse gas emissions
- restoration of ozone layer could lead to an increase in global warming
Solutions: Protecting the ozone layer
As a result of the Montreal Protocol and other international agreements
CFC emissions dropped ~87% from their peak in 1988
in 1991, DuPont announced development of new refrigerants that don’t harm ozone layer
in 1996, US stopped producing CFCs
Solutions: Protecting the ozone layer
Some substitutes/replacements include
new coolant for air conditioners
soapy water and hot air for circuit boards
sound waves for cooling
helium gas for refrigeration
liquid nitrogen (-196°C) and supercooled CO2 (-60°C; dry ice) for shipping