Ozone Depletion in the Stratosphere

Ozone Depletion in the Stratosphere

EVPP 111 Lecture

Dr. Largen

Ozone and ozone layer
Thinning of ozone layer
What causes ozone depletion
chlorofluorocarbons
other ozone depleting compounds
Seasonal thinning over the poles
Why should we care about ozone depletion
Solutions: protecting the ozone layer

Ozone and ozone layer

Ozone (O₃)

structure
forms and breaks down naturally in stratosphere

via reaction of O₂ with UV radiation
breaks O₂ into O which react with O₂ to reform O₃

process absorbs ~99% of UV

creates layer in lower stratosphere
altitude of ~10-16 miles

"good" ozone

stratospheric ozone

"bad" ozone

tropospheric ozone, "ground level" ozone
secondary air pollutant
component of photochemical smog

irritates respiratory tissue
causes permanent lung damage
damages plants
reduces agricultural yields

Thinning of the ozone layer

Ozone concentration in stratosphere

determination

balloons, aircraft, satellites

depleted seasonally over

Antarctica and Arctic

lower overall thinning of layer

everywhere except over tropics

Ozone depletion in stratosphere

considered a

serious long-term threat
humans
many other animals
primary producers

What causes ozone depletion

Certain chemicals
destroy ozone in stratosphere

primarily
chlorofluorocarbons (CFCs)
other chlorine-containing compounds

Chlorofluorocarbons (CFCs)

discovered in 1930

General Motors chemist, Thomas Midgley

other chemists

made similar compounds
creating family of highly useful CFCs

What causes ozone depletion

two most widely used

known by trade name - Freons
CFC-11 (trichloromethane, CCl₃F)
CFC-12 (dichlorodifluoromethane, CCl₂F₂)

What causes ozone depletion

originally considered "dream chemicals"

because of characteristics
chemically stable (nonreactive)
odorless
nonflammable
nontoxic
noncorrosive

became popular for many uses
uses included

coolants in air conditioners and refrigerators
replacing toxic sulfur dioxide and ammonia
propellants in aerosol spray cans
cleaners for electronic parts (computer chips)
sterilants for hospital instruments
fumigants for granaries and ship cargo holds
bubbles in plastic foam used for insulation and packaging
production rose sharply between 1960 and early 1990’s

in 1974

research by two University of California-Irvine chemists, Sherwood Rowland and Mario Molina
indicated that

CFCs were lowering average concentration of ozone in stratosphere

Rowland and Molina

shocked scientific community and $28 billion per year CFC industry
called for immediate ban on CFCs in spray cans
concluded that
large quantities of CFCs were being released into 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

CFCs remain in troposphere due to

insolubility in water
chemical unreactivity

over period of 11-20 years, CFCs rise into stratosphere through

convection
random drift
turbulent mixing in troposphere

in stratosphere, CFC molecules break down

under influence of high-energy UV radiation
releasing highly reactive chlorine atoms

each CFC can last in stratosphere for 65-385 years (most widely used, 75-111 years), depending on its type

each chlorine atom released from CFC molecule can convert up to 100,000 molecules of ozone to oxygen

"dream molecules" (CFCs) had turned into global ozone destroyers

CFC industry, led by Dupont Company

attacked Rowland and Molina’s conclusion
was powerful, well-funded with lots of profits and jobs at stake

Rowland and Molina held their ground against industry

explaining their calculations to other scientists, elected officials, media

in 1988, 14 years after Rowland and Molina’s study

DuPont officials acknowledged that CFCs were depleting ozone layer
agreed to stop producing them once they found substitutes

in 1995

Rowland and Molina won Nobel Prize in Chemistry for work on CFCs and ozone layer

Other ozone depleting compounds (ODC)

include

halons and HBFCs
used in fire extinguishers
methyl bromide(CH₃Br)
widely used fumigant
carbon tetrachloride (CCl₄)
cheap, highly toxic solvent
methyl chloroform (C₂H₃Cl₃)
cleaning solvent
propellant
hydrogen chloride (HCl)
emitted into stratosphere by US space shuttles

natural sources of

oceans and volcanic eruptions
release chlorine and bromine

Ozone depletion

Of observed ozone losses in stratosphere since 1976
~75-85% are attributed to compounds released into atmosphere by human activities beginning in 1950s

Figure: Erosion of Earth’s ozone shield: Thickness of the ozone layer

Seasonal ozone thinning over poles

In mid-1980s
researchers discovered that ~40-50% of ozone over Antarctica was being destroyed during

Antarctic spring and summer (September-December)

seasonal loss of ozone over Antarctica was incorrectly dubbed ozone hole

actually ozone thinning

degree of depletion varies with altitude and location

total area of atmosphere above Antarctica that suffers from ozone thinning varies from year to year
in 2000, seasonal thinning above Antarctica was largest ever

Why is loss of ozone over Antarctica seasonal

during winter

its sunless
steady winds blow in circular pattern over earth’s poles
creates polar vortex
swirling mass of very cold air that is isolated from rest of atmosphere

until sun returns a few months later

water droplets in clouds enter polar vortex
form tiny ice crystals which collect CFCs and other ODCs on their surfaces

serve as catalysts for speeding up chemical reactions that release Cl & ClO
Cl and ClO react with each other to form Cl₂O₂
in dark of winter Cl₂O₂molecules can’t react with ozone so they accumulate in polar vortex

during spring

when sunlight returns (October)
Cl₂O₂molecules are broken apart by UV light
releasing large numbers of Cl atoms

which begin reacting with ozone

sunlight
gradually melts ice crystals
breaks up vortex of trapped polar air
allows trapped air to begin mixing with rest of atmosphere
within weeks
40-50% of ozone above Antarctica is destroyed
when vortex breaks up
huge masses of ozone depleted air above Antarctica flows northward

lingers for few weeks over Australia, New Zealand, South America, South Africa
resulting in increases of 3-20% levels of biologically damaging UV-B radiation

Ozone thinning over the Arctic

in 1988

scientists discovered similar but less severe ozone thinning over 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 surface

impact on humans
worse sunburns
more eye cataracts
more skin cancers

According to UNEP estimates
additional UV-B radiation reaching surface would cause 10% annual loss of global ozone leading 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

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.)

estimated losses totaling ~2.5 billion/ year

decline in forest productivity
increased degradation and breakdown of materials such as plastics, paints
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

scientific consensus of researchers

immediately stop producing all ozone-depleting chemicals

substitutes available for most CFCs
additional substitutes are being developed

Is there a possibility of a quick fix from technology that would allow us to keep using CFCs?
Some strange proposals have been floated

blimps
lasers

strange "technofix" proposals
blimps

inject electrons into stratosphere
which would react with and remove chlorine atoms

lasers

"blast" CFCs out of atmosphere before they could reach stratosphere

Efforts to reduce ozone depletion
Montreal Protocol, a treaty

developed in 1987
cut emissions of CFCs by ~35% -50% between 1989 and 2000

new protocol was adopted following meetings in 1990 & 1992 of representative from 93 countries

in response to news in 1989 about seasonal thinning of ozone layer over Antarctica

to date, landmark international agreements

signed by 175 nations
illustrate global response to a serious global environmental problem

according to 1998 study by World Meteorological Organization (WMO)

ozone layer
will continue to be depleted for several decades
will return to 1980 levels by ~2050 and to 1950 level by ~2100 if certain assumptions hold
depletion has resulted in cooling of troposphere
will continue to be depleted for several decades because
11-20 year time lag between release of ODCs and their arrival in stratosphere
ODCs persist in stratosphere for decades
will return to 1980 levels by ~2050 and to 1950 levels by ~2100, assuming
international agreements are followed
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 1950s
depletion of ozone in stratosphere has resulted in
cooling of troposphere

possibly offset or disguised as much as 30% of global warming caused by greenhouse gas emissions

restoration of ozone layer could lead to an increase in global warming

As result of 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

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

The End